science universe

farsightedness See HYPEROPIA.

flashes Visual phantoms that appear as spots of light. An ophthalmologist should evaluate occurrences of flashes, as they can be symptoms of RETI- NAL DETACHMENT or other conditions affecting the RETINA. Flashes represent stimulation of the rods and cones, the cells of vision carpeting the retina, that occurs when the gelatinous fluid holding the retina in place (vitreous humor) moves across them. Because the NERVE signals these cells send to the BRAIN encode patterns of light, the brain interprets messages from them as light. A blow to the head that causes a person to “see stars” has similar effect when it is forceful enough to jostle the vitreous humor against the retina. Flashes may appear as multiple spots of light, “light showers,” or lightning-like streaks.

The ophthalmologist typically performs a full OPHTHALMIC EXAMINATION to assess the integrity of the retina. Prompt treatment is necessary to intervene with a retinal tear or retinal detachment, to preserve vision. Isolated flashes of light generally are harmless and may occur for various reasons. Lines or waves of light that last 20 to 60 minutes are common with migraine headaches and have no significance for vision or the health of the eye.

See also FLOATERS; HEADACHE; VISION IMPAIRMENT; VITREOUS DETACHMENT.

floaters Fragments of inner EYE material that float through the vitreous humor, casting shadows against the RETINA as entering light strikes them.

Floaters may take various shapes and sizes, and typically move around the VISUAL FIELD, changing position with blinking or eye movement. Most floaters are harmless, though large floaters may interfere with vision. Holding the eye still may allow the floater to settle to the bottom of the eye, out of the visual field. A sudden increase in the number of floaters, or floaters that occur in combination with FLASHES, can signal a retinal tear or RETINAL DETACHMENT. Prompt intervention is necessary to prevent further retinal damage and pre- serve vision. Floaters may also indicate UVEITIS. Large floaters may remain indefinitely; small floaters may eventually break apart and become absorbed into the vitreous humor. There is no treatment for floaters.

See also VISION IMPAIRMENT; VITREOUS DETACHMENT.

fluorescein staining A simple procedure for diagnosing CORNEAL INJURY or foreign objects in the EYE. The ophthalmologist places a strip of paper containing fluorescein at the edge of the eye. The dye rapidly leaches into the tears. Some people experience a slight burning sensation when the dye washes across the eye for the first time. Blinking disperses the tears across the CORNEA. With the regular room lights turned off, the ophthalmologist shines a cobalt blue light on the eye. Any breach in the eye’s surface shows as bright green. The tears wash the fluorescein from the eye within a few minutes.

See also OPHTHALMIC EXAMINATION; SLIT LAMP EXAMINATION; TRAUMA TO THE EYE.

farsightedness See HYPEROPIA.

flashes Visual phantoms that appear as spots of light. An ophthalmologist should evaluate occurrences of flashes, as they can be symptoms of RETI- NAL DETACHMENT or other conditions affecting the RETINA. Flashes represent stimulation of the rods and cones, the cells of vision carpeting the retina, that occurs when the gelatinous fluid holding the retina in place (vitreous humor) moves across them. Because the NERVE signals these cells send to the BRAIN encode patterns of light, the brain interprets messages from them as light. A blow to the head that causes a person to “see stars” has similar effect when it is forceful enough to jostle the vitreous humor against the retina. Flashes may appear as multiple spots of light, “light showers,” or lightning-like streaks.

The ophthalmologist typically performs a full OPHTHALMIC EXAMINATION to assess the integrity of the retina. Prompt treatment is necessary to intervene with a retinal tear or retinal detachment, to preserve vision. Isolated flashes of light generally are harmless and may occur for various reasons. Lines or waves of light that last 20 to 60 minutes are common with migraine headaches and have no significance for vision or the health of the eye.

See also FLOATERS; HEADACHE; VISION IMPAIRMENT; VITREOUS DETACHMENT.

floaters Fragments of inner EYE material that float through the vitreous humor, casting shadows against the RETINA as entering light strikes them.

Floaters may take various shapes and sizes, and typically move around the VISUAL FIELD, changing position with blinking or eye movement. Most floaters are harmless, though large floaters may interfere with vision. Holding the eye still may allow the floater to settle to the bottom of the eye, out of the visual field. A sudden increase in the number of floaters, or floaters that occur in combination with FLASHES, can signal a retinal tear or RETINAL DETACHMENT. Prompt intervention is necessary to prevent further retinal damage and pre- serve vision. Floaters may also indicate UVEITIS. Large floaters may remain indefinitely; small floaters may eventually break apart and become absorbed into the vitreous humor. There is no treatment for floaters.

See also VISION IMPAIRMENT; VITREOUS DETACHMENT.

fluorescein staining A simple procedure for diagnosing CORNEAL INJURY or foreign objects in the EYE. The ophthalmologist places a strip of paper containing fluorescein at the edge of the eye. The dye rapidly leaches into the tears. Some people experience a slight burning sensation when the dye washes across the eye for the first time. Blinking disperses the tears across the CORNEA. With the regular room lights turned off, the ophthalmologist shines a cobalt blue light on the eye. Any breach in the eye’s surface shows as bright green. The tears wash the fluorescein from the eye within a few minutes.

See also OPHTHALMIC EXAMINATION; SLIT LAMP EXAMINATION; TRAUMA TO THE EYE.

ectropion Loss of elasticity or control of the eyelid, usually the lower eyelid, that causes it to sag away from the EYE. Ectropion allows tears to over- flow the lid rather than remaining in the eye. It also fails to protect the eye, and especially the CORNEA, permitting dryness and exposure to environmental particles that create irritation and possibly injury to the cornea and sclera (“white” of the eye). Common causes of ectropion include

• aging

• damage to the nerves that control the eyelids

• CICATRICIAL PEMPHIGOID

Ectropion is a common symptom of BELL’S PALSY, a temporary paralysis of one side of the face that results from INFLAMMATION of the seventh cranial NERVE (facial nerve), and also may accompany neurologic disorders such as PARKINSON’S DISEASE and MULTIPLE SCLEROSIS.

With ectropion the eye feels irritated and scratchy. Tear production becomes excessive as the eye attempts to lubricate and protect itself, and tears typically run over the lip of the lid and onto the cheeks. The doctor can diagnose ectropion based on its appearance. Treatment is typically surgery to tighten the lid structure to permit the lid to stay against the eye. Whether the ectropion recurs depends on the underlying cause. Untreated ectropion may result in extensive dam- age to the surface of the eye and cornea, including INFECTION, that interferes with vision and the health of the eye.

See also AGING, VISION AND EYE CHANGES THAT OCCUR WITH; CONJUNCTIVITIS; CORNEAL INJURY; CRANIAL NERVES; ENTROPION; KERATITIS.

electroretinography A test that measures the electrical activity of the RETINA’s rods and cones in response to light stimulation. The ophthalmologist places anesthetic drops in the EYE to numb it, then attaches an electrode to the surface of the CORNEA. The electrode detects electrical impulses on the retina when the ophthalmologist flashes a beam of light onto the retina, and sends signals to the electroretinograph machine. An electroretinogram is the recording the machine makes of the retina’s responses. Electroretinography helps diagnose dis- orders of the retina such as RETINAL DETACHMENT and RETINITIS PIGMENTOSA.

See also DARK ADAPTATION TEST; RETINOPATHY; SLIT LAMP EXAMINATION.

entropion Deformity of the eyelid in which the lip of the lid, including the eyelashes, curls inward toward the EYE. Scarring that results from CICATRI- CIAL PEMPHIGOID (an AUTOIMMUNE DISORDER in which painful blisters repeatedly form on the insides of the eyelids) or recurrent CONJUNCTIVITIS (INFLAMMA- TION or INFECTION of the inner lining of the eyelids) is a common cause of entropion. Entropion may also develop for unknown reasons (idiopathic). The ophthalmologist can diagnose entropion by its presentation. The irritation of the lid and lashes against the surface of the eye is painful and can cause significant damage to the CORNEA, resulting in VISION IMPAIRMENT and perhaps the need for CORNEAL TRANSPLANTATION. Treatment seeks to relieve the irritation. In mild entropion, lubricating eye drops may be sufficient to protect the eye. Moderate to severe entropion requires surgery to restore the eyelid to its appropriate structure. Once corrected, entropion usually does not recur. See also CORNEAL INJURY; ECTROPION; KERATITIS.

enucleation Surgical removal of a cancerous EYE or a severely diseased or damaged eye. The OPERA- TION, performed under general ANESTHESIA, takes about an hour. After removing the eye, the surgeon places an implant to fill the shape of the socket and provide a means of attaching a PROSTHETIC EYE. A pressure dressing stays in place over the eye orbit for one to two days to minimize swelling and allow the implant to become firmly rooted in the conjunctival tissue. During this time it is common as well as frightening for people to have difficulty opening the other eye, as the eyes are accustomed to functioning together. Once the bandage comes off and the eyelid of the operated eye is free to move, the eyelid for the unoperated eye resumes normal functioning. Complete HEALING takes about six weeks, during which time it is necessary to place anti-inflammatory and antibiotic drops in the operated eye socket to keep swelling and the risk for INFECTION to a minimum.

Though the operation is uncomplicated and the body quickly heals following the surgery, enucleation can be a difficult procedure for people to accommodate emotionally. Even when the eye has been visionless for a long time, the prospect of losing the eye troubles many people. The modern prosthetic eye is typically such a close match for the remaining eye that it is unapparent to other people. Once the operative site heals, the eye orbit (socket) and implant require little care or attention beyond cleaning the external eyelid area for hygienic purposes.

See also ANTIBIOTIC MEDICATIONS; RETINOBLASTOMA; SURGERY BENEFIT AND RISK MANAGEMENT; VISION IMPAIRMENT.

episcleritis INFLAMMATION of the episclera, the membrane that covers the sclera (fibrous outer layer, the “white,” of the EYE). Most episcleritis is idiopathic (occurs for unknown reasons), though the condition sometimes accompanies AUTOIMMUNE DISORDERS such as RHEUMATOID ARTHRITIS and

REITER’S SYNDROME. Episodes are self-limiting though may recur over time, with each episode of inflammation generally lasting 7 to 10 days. Symptoms may include mild irritation and red- ness, and occasionally a nodule (bump) on the surface of the sclera. The doctor can diagnose episcleritis by its appearance. Lubricating eye drops

help relieve the irritation until the inflammation subsides. This is usually the only treatment necessary. Some studies suggest a correlation between episcleritis and hormonal shifts such as occur with the MENSTRUAL CYCLE or MENOPAUSE. Episcleritis is three times more common in women than men. Episcleritis does not affect vision or result in any long-term effects on the health of the eye.

See also CONJUNCTIVITIS; KERATITIS; SCLERITIS.

exophthalmos Bulging outward of the EYE, sometimes called poptosis. Most exophthalmos results from Graves’s disease and is a classic symptom of this form of HYPERTHYROIDISM. Thyroid- related exophthalmos results from swelling of the tissues around the eye and within the orbit that develops in reaction to the high levels of thyroid HORMONE present in the circulation. Other causes of exophthalmos include ORBITAL CELLULITIS, the autoimmune disorder Wegener’s granulomatosis, and FRACTURE of the facial or orbital bones that push the eye out of place. Less common causes of exophthalmos include tumors of the eye, OPTIC NERVE, or BRAIN that protrude into the orbital socket and ANEURYSM (ballooning of the arterial wall) of the internal carotid ARTERY, a branch of which runs behind the eye. Exophthalmos can affect one eye (unilateral) or both eyes (bilateral), and when bilateral can affect one eye more prominently than the other.

Exophthalmos can cause significant and permanent vision impairment, and requires prompt treatment.

The diagnostic path begins with an OPHTHALMO- LOGIC EXAMINATION and blood tests to assess thyroid function. When Graves’s disease or hyperthyroidism is the cause, treatment to restore appropriate levels of thyroid hormones often though not always returns the eye to its normal position. Persistent exophthalmos may prevent the eyelids from closing over the eye, exposing the CORNEA to excessive dryness and potential trauma. Untreated exophthalmos results in VISION IMPAIRMENT that can progress to blindness.

See also AUTOIMMUNE DISORDERS; GRAVES’S OPH- THALMOPATHY.

eye The organ of vision. The paired eyes work in coordination to present NERVE impulses the BRAIN interprets as dimensional (stereovisual) images. The function of sight requires close integration among the structures of the eye, the neurologic sys- tem, and the muscular system. Each eye is a fluid-detect color (cones) and brightness (rods). These cells convert the light to nerve impulses that con- verge at the back of the retina at the optic disk, their portal to the optic nerve. The optic nerve conveys the signals to the brain, which interprets them as images.

filled, elongated globe of fibrous tissue, about 1⁄4 inch from front to back and 1 inch from top to bot- tom and side to side, contained within the protective cavity of the orbital socket in the skull. The OPTIC NERVE, the second cranial nerve, provides a direct pathway from the back of the eye to the brain. Six muscles move each eye up and down, from side to side, and in rotation. These muscles direct the eye toward objects within the VISUAL FIELD and hold the eyes steady.

The process of vision begins when lightwaves enter the eye through the CORNEA, a transparent portion of the eye’s tough outer layer, the sclera. The cornea’s convex front surface initially refracts the lightwaves for preliminary focusing. The cornea is soft and flexible but fixed; it does not adjust or move. The LENS, a transparent and flexible convex disk behind the cornea, further refracts the lightwaves. Tiny muscles at the edge of the lens, the ciliary muscles, cause the lens to thicken or flatten to adjust the degree of refraction for optimal focus. The resulting light pattern strikes the RETINA, activating the specialized cells that

detect color (cones) and brightness (rods). These cells convert the light to nerve impulses that converge at the back of the retina at the optic disk, their portal to the optic nerve. The optic nerve conveys the signals to the brain, which interprets them as images.

For further discussion of the eye within the context of ophthalmologic structure and function please see the overview section “The Eyes.” See also AGING, VISION AND EYE CHANGES THAT OCCUR WITH; CRANIAL NERVES.

eye pain

Sensations discomfort involving the EYE and its supporting structures. Eye PAIN may vary

from scratchy irritation to intense and debilitating pain. Much eye pain in the form of burning and itching arises from minor and treatable causes that affect the structures around the eye rather than the eye itself. Eye pain that is throbbing, stabbing, deep, or accompanies visual disturbances may suggest conditions such as GLAUCOMA.

Eye pain that is sudden and severe, accompanies partial or complete loss of vision, prevents movement of the eye, or follows TRAUMA TO THE EYE or face requires emergency medical attention. When there is the possibility of penetrating eye injury, loosely patch both eyes to minimize movement.

The diagnostic path begins with careful examination of both eyes, which may include OPHTHAL- MOSCOPY, FLUORESCEIN STAINING when the doctor suspects CORNEAL INJURY, TONOMETRY to measure the pressure inside the eye, and SLIT LAMP EXAMINATION for further assessment of the RETINA and other structures of the inner eye. The doctor may place an anesthetic medication (numbing eye drops) in the eye to determine whether the pain is coming from the surface of the eye, in which case the pain will go away, or from within the eye, in which case the pain will persist. Often the doctor will also conduct basic tests of VISUAL ACUITY such as a SNELLEN CHART reading.

People who wear contact lenses should remove them at the first sign of discomfort. Treatment for eye pain targets the underlying cause. Most minor causes resolve without complications or permanent VISION IMPAIRMENT. Causes such as severe corneal injury (BURNS, lacerations), glaucoma, and ORBITAL CELLULITIS seriously threaten vision and can result in permanent and complete vision loss without urgent and appropriate treatment.

See also RETINAL DETACHMENT.

eye strain The sensation of tiredness and irritation of the eyes, often accompanying long periods of time involved in performing the same task such as reading, computer work, watching television, playing video games, and assembly work. EYE strain generally results from overuse of the muscles that move the eyes. The overuse tires the muscles, which become less responsive to the focusing needs of the eyes. The difficulty generates temporary vision disturbances such as blurring, and may also cause muscle tension headaches. Insufficient blinking, which causes the eyes to become dry and irritated, often accompanies the overuse.

These measures can help relieve eye strain:

• Blink frequently.

• Use artificial tears to improve the moisture con- tent of the eyes.

• Make sure lighting is of the appropriate intensity and placement.

• Reduce glare and reflection.

• Look away from close tasks every 10 to 15 minutes to focus on objects in the distance.

• Wear reading glasses or CORRECTIVE LENSES to accommodate PRESBYOPIA.

• Wear eye protection when in environments that are dusty or windy, and when in the sun.

Contrary to popular belief, eye strain (such as reading in dim light) does not cause permanent VISION IMPAIRMENT. However, eye strain may result from undetected vision impairment, such as ASTIG- MATISM and HYPEROPIA, that affect the eye’s ability to focus on near objects. An ophthalmologist or optometrist should evaluate eye strain that persists despite efforts to improve the visual environment.

See also ERGONOMICS; HEADACHE; MUSCLE; OCCUPA- TIONAL HEALTH AND SAFETY; VISION HEALTH.

ectropion Loss of elasticity or control of the eyelid, usually the lower eyelid, that causes it to sag away from the EYE. Ectropion allows tears to over- flow the lid rather than remaining in the eye. It also fails to protect the eye, and especially the CORNEA, permitting dryness and exposure to environmental particles that create irritation and possibly injury to the cornea and sclera (“white” of the eye). Common causes of ectropion include

• aging

• damage to the nerves that control the eyelids

• CICATRICIAL PEMPHIGOID

Ectropion is a common symptom of BELL’S PALSY, a temporary paralysis of one side of the face that results from INFLAMMATION of the seventh cranial NERVE (facial nerve), and also may accompany neurologic disorders such as PARKINSON’S DISEASE and MULTIPLE SCLEROSIS.

With ectropion the eye feels irritated and scratchy. Tear production becomes excessive as the eye attempts to lubricate and protect itself, and tears typically run over the lip of the lid and onto the cheeks. The doctor can diagnose ectropion based on its appearance. Treatment is typically surgery to tighten the lid structure to permit the lid to stay against the eye. Whether the ectropion recurs depends on the underlying cause. Untreated ectropion may result in extensive dam- age to the surface of the eye and cornea, including INFECTION, that interferes with vision and the health of the eye.

See also AGING, VISION AND EYE CHANGES THAT OCCUR WITH; CONJUNCTIVITIS; CORNEAL INJURY; CRANIAL NERVES; ENTROPION; KERATITIS.

electroretinography A test that measures the electrical activity of the RETINA’s rods and cones in response to light stimulation. The ophthalmologist places anesthetic drops in the EYE to numb it, then attaches an electrode to the surface of the CORNEA. The electrode detects electrical impulses on the retina when the ophthalmologist flashes a beam of light onto the retina, and sends signals to the electroretinograph machine. An electroretinogram is the recording the machine makes of the retina’s responses. Electroretinography helps diagnose dis- orders of the retina such as RETINAL DETACHMENT and RETINITIS PIGMENTOSA.

See also DARK ADAPTATION TEST; RETINOPATHY; SLIT LAMP EXAMINATION.

entropion Deformity of the eyelid in which the lip of the lid, including the eyelashes, curls inward toward the EYE. Scarring that results from CICATRI- CIAL PEMPHIGOID (an AUTOIMMUNE DISORDER in which painful blisters repeatedly form on the insides of the eyelids) or recurrent CONJUNCTIVITIS (INFLAMMA- TION or INFECTION of the inner lining of the eyelids) is a common cause of entropion. Entropion may also develop for unknown reasons (idiopathic). The ophthalmologist can diagnose entropion by its presentation. The irritation of the lid and lashes against the surface of the eye is painful and can cause significant damage to the CORNEA, resulting in VISION IMPAIRMENT and perhaps the need for CORNEAL TRANSPLANTATION. Treatment seeks to relieve the irritation. In mild entropion, lubricating eye drops may be sufficient to protect the eye. Moderate to severe entropion requires surgery to restore the eyelid to its appropriate structure. Once corrected, entropion usually does not recur. See also CORNEAL INJURY; ECTROPION; KERATITIS.

enucleation Surgical removal of a cancerous EYE or a severely diseased or damaged eye. The OPERA- TION, performed under general ANESTHESIA, takes about an hour. After removing the eye, the surgeon places an implant to fill the shape of the socket and provide a means of attaching a PROSTHETIC EYE. A pressure dressing stays in place over the eye orbit for one to two days to minimize swelling and allow the implant to become firmly rooted in the conjunctival tissue. During this time it is common as well as frightening for people to have difficulty opening the other eye, as the eyes are accustomed to functioning together. Once the bandage comes off and the eyelid of the operated eye is free to move, the eyelid for the unoperated eye resumes normal functioning. Complete HEALING takes about six weeks, during which time it is necessary to place anti-inflammatory and antibiotic drops in the operated eye socket to keep swelling and the risk for INFECTION to a minimum.

Though the operation is uncomplicated and the body quickly heals following the surgery, enucleation can be a difficult procedure for people to accommodate emotionally. Even when the eye has been visionless for a long time, the prospect of losing the eye troubles many people. The modern prosthetic eye is typically such a close match for the remaining eye that it is unapparent to other people. Once the operative site heals, the eye orbit (socket) and implant require little care or attention beyond cleaning the external eyelid area for hygienic purposes.

See also ANTIBIOTIC MEDICATIONS; RETINOBLASTOMA; SURGERY BENEFIT AND RISK MANAGEMENT; VISION IMPAIRMENT.

episcleritis INFLAMMATION of the episclera, the membrane that covers the sclera (fibrous outer layer, the “white,” of the EYE). Most episcleritis is idiopathic (occurs for unknown reasons), though the condition sometimes accompanies AUTOIMMUNE DISORDERS such as RHEUMATOID ARTHRITIS and

REITER’S SYNDROME. Episodes are self-limiting though may recur over time, with each episode of inflammation generally lasting 7 to 10 days. Symptoms may include mild irritation and red- ness, and occasionally a nodule (bump) on the surface of the sclera. The doctor can diagnose episcleritis by its appearance. Lubricating eye drops

help relieve the irritation until the inflammation subsides. This is usually the only treatment necessary. Some studies suggest a correlation between episcleritis and hormonal shifts such as occur with the MENSTRUAL CYCLE or MENOPAUSE. Episcleritis is three times more common in women than men. Episcleritis does not affect vision or result in any long-term effects on the health of the eye.

See also CONJUNCTIVITIS; KERATITIS; SCLERITIS.

exophthalmos Bulging outward of the EYE, sometimes called poptosis. Most exophthalmos results from Graves’s disease and is a classic symptom of this form of HYPERTHYROIDISM. Thyroid- related exophthalmos results from swelling of the tissues around the eye and within the orbit that develops in reaction to the high levels of thyroid HORMONE present in the circulation. Other causes of exophthalmos include ORBITAL CELLULITIS, the autoimmune disorder Wegener’s granulomatosis, and FRACTURE of the facial or orbital bones that push the eye out of place. Less common causes of exophthalmos include tumors of the eye, OPTIC NERVE, or BRAIN that protrude into the orbital socket and ANEURYSM (ballooning of the arterial wall) of the internal carotid ARTERY, a branch of which runs behind the eye. Exophthalmos can affect one eye (unilateral) or both eyes (bilateral), and when bilateral can affect one eye more prominently than the other.

Exophthalmos can cause significant and permanent vision impairment, and requires prompt treatment.

The diagnostic path begins with an OPHTHALMO- LOGIC EXAMINATION and blood tests to assess thyroid function. When Graves’s disease or hyperthyroidism is the cause, treatment to restore appropriate levels of thyroid hormones often though not always returns the eye to its normal position. Persistent exophthalmos may prevent the eyelids from closing over the eye, exposing the CORNEA to excessive dryness and potential trauma. Untreated exophthalmos results in VISION IMPAIRMENT that can progress to blindness.

See also AUTOIMMUNE DISORDERS; GRAVES’S OPH- THALMOPATHY.

eye The organ of vision. The paired eyes work in coordination to present NERVE impulses the BRAIN interprets as dimensional (stereovisual) images. The function of sight requires close integration among the structures of the eye, the neurologic sys- tem, and the muscular system. Each eye is a fluid-detect color (cones) and brightness (rods). These cells convert the light to nerve impulses that con- verge at the back of the retina at the optic disk, their portal to the optic nerve. The optic nerve conveys the signals to the brain, which interprets them as images.

filled, elongated globe of fibrous tissue, about 1⁄4 inch from front to back and 1 inch from top to bot- tom and side to side, contained within the protective cavity of the orbital socket in the skull. The OPTIC NERVE, the second cranial nerve, provides a direct pathway from the back of the eye to the brain. Six muscles move each eye up and down, from side to side, and in rotation. These muscles direct the eye toward objects within the VISUAL FIELD and hold the eyes steady.

The process of vision begins when lightwaves enter the eye through the CORNEA, a transparent portion of the eye’s tough outer layer, the sclera. The cornea’s convex front surface initially refracts the lightwaves for preliminary focusing. The cornea is soft and flexible but fixed; it does not adjust or move. The LENS, a transparent and flexible convex disk behind the cornea, further refracts the lightwaves. Tiny muscles at the edge of the lens, the ciliary muscles, cause the lens to thicken or flatten to adjust the degree of refraction for optimal focus. The resulting light pattern strikes the RETINA, activating the specialized cells that

detect color (cones) and brightness (rods). These cells convert the light to nerve impulses that converge at the back of the retina at the optic disk, their portal to the optic nerve. The optic nerve conveys the signals to the brain, which interprets them as images.

For further discussion of the eye within the context of ophthalmologic structure and function please see the overview section “The Eyes.” See also AGING, VISION AND EYE CHANGES THAT OCCUR WITH; CRANIAL NERVES.

eye pain

Sensations discomfort involving the EYE and its supporting structures. Eye PAIN may vary

from scratchy irritation to intense and debilitating pain. Much eye pain in the form of burning and itching arises from minor and treatable causes that affect the structures around the eye rather than the eye itself. Eye pain that is throbbing, stabbing, deep, or accompanies visual disturbances may suggest conditions such as GLAUCOMA.

Eye pain that is sudden and severe, accompanies partial or complete loss of vision, prevents movement of the eye, or follows TRAUMA TO THE EYE or face requires emergency medical attention. When there is the possibility of penetrating eye injury, loosely patch both eyes to minimize movement.

The diagnostic path begins with careful examination of both eyes, which may include OPHTHAL- MOSCOPY, FLUORESCEIN STAINING when the doctor suspects CORNEAL INJURY, TONOMETRY to measure the pressure inside the eye, and SLIT LAMP EXAMINATION for further assessment of the RETINA and other structures of the inner eye. The doctor may place an anesthetic medication (numbing eye drops) in the eye to determine whether the pain is coming from the surface of the eye, in which case the pain will go away, or from within the eye, in which case the pain will persist. Often the doctor will also conduct basic tests of VISUAL ACUITY such as a SNELLEN CHART reading.

People who wear contact lenses should remove them at the first sign of discomfort. Treatment for eye pain targets the underlying cause. Most minor causes resolve without complications or permanent VISION IMPAIRMENT. Causes such as severe corneal injury (BURNS, lacerations), glaucoma, and ORBITAL CELLULITIS seriously threaten vision and can result in permanent and complete vision loss without urgent and appropriate treatment.

See also RETINAL DETACHMENT.

eye strain The sensation of tiredness and irritation of the eyes, often accompanying long periods of time involved in performing the same task such as reading, computer work, watching television, playing video games, and assembly work. EYE strain generally results from overuse of the muscles that move the eyes. The overuse tires the muscles, which become less responsive to the focusing needs of the eyes. The difficulty generates temporary vision disturbances such as blurring, and may also cause muscle tension headaches. Insufficient blinking, which causes the eyes to become dry and irritated, often accompanies the overuse.

These measures can help relieve eye strain:

• Blink frequently.

• Use artificial tears to improve the moisture con- tent of the eyes.

• Make sure lighting is of the appropriate intensity and placement.

• Reduce glare and reflection.

• Look away from close tasks every 10 to 15 minutes to focus on objects in the distance.

• Wear reading glasses or CORRECTIVE LENSES to accommodate PRESBYOPIA.

• Wear eye protection when in environments that are dusty or windy, and when in the sun.

Contrary to popular belief, eye strain (such as reading in dim light) does not cause permanent VISION IMPAIRMENT. However, eye strain may result from undetected vision impairment, such as ASTIG- MATISM and HYPEROPIA, that affect the eye’s ability to focus on near objects. An ophthalmologist or optometrist should evaluate eye strain that persists despite efforts to improve the visual environment.

See also ERGONOMICS; HEADACHE; MUSCLE; OCCUPA- TIONAL HEALTH AND SAFETY; VISION HEALTH.

dacryocystitis INFLAMMATION of the lacrimal (tear) ducts, typically the nasolacrimal ducts in the corners of the EYE near the NOSE. Dacryocystitis develops when there is a blockage of the lacrimal duct, which may result from DACRYOSTENOSIS (narrowing of the lacrimal duct), INFECTION, or chronic irritation such as might occur with ALLERGIC RHINI- TIS or ALLERGIC CONJUNCTIVITIS. Dacryocystitis can be acute (of sudden onset) or chronic (recurrent or long-standing). It also can be congenital (the result of defects of the lacrimal gland and duct structures) or acquired. Most people who have acquired dacryocystitis are over age 65.

Common symptoms include

• redness and swelling between the eye and the bridge of the nose

• rhinitis (runny nose)

• PAIN

• overflowing tears

• FEVER when an infection is present

The doctor can typically diagnose dacryocystitis based on its presentation. Dye tests, in which the doctor places a special dye in the eye and watches to see whether the dye discolors nasal discharge, help identify the extent of blockage causing the inflammation. Treatment includes ANTIBIOTIC MEDICATIONS when there is an infection, or procedures to dilate the lacrimal duct when there is no infection. Sometimes surgery is necessary to correct dacryostenosis or other structural defects. Appropriate treatment resolves the dacryocystitis.

See also BLEPHARITIS; EYE PAIN; OPERATION; ORBITAL CELLULITIS.

dacryostenosis Narrowing of the lacrimal (tear) duct, usually congenital, that blocks the flow of tears. An infant does not produce a great volume of tears during the first few weeks to months after birth, so the doctor may not suspect or diagnose dacryostenosis until the infant is three to four months of age. The most common symptom is tears that overflow the eye and run down the face (epiphora). Most infants outgrow dacryostenosis by age six months, so doctors tend to take an approach of watchful waiting. When dacryosteno- sis persists, the doctor may dilate the lacrimal duct (under anesthetic) to gently stretch and enlarge the opening for tears to pass unimpeded. Untreated dacryostenosis can result in frequent episodes of DACRYOCYSTITIS (infected lacrimal ducts) in adulthood. Appropriate treatment can com- pletely resolve dacryostenosis.

See also INFECTION; ORBITAL CELLULITIS.

dark adaptation test A test that assesses the ability to see in a dimly lighted environment. There are several ways to perform a dark adaptation test. One of the most common is to have the person sit in a dimly lit room. The examiner shines a light into the EYE, gradually increasing the light’s intensity until the person reports seeing the light. The examiner notes the light’s intensity and the length of time it takes for the light to become noticeable. Depending on the reason for the test, the examiner may direct the light to different parts of the RETINA to test the responsiveness of the rods (the cells responsible for low-light vision). A decrease in dark adaptation response is normal with aging as the photochemical reactions in the eye slow.

See also AGING, VISION AND EYE CHANGES THAT OCCUR WITH; ELECTRORETINOGRAPHY; NIGHT BLINDNESS; RETINITIS PIGMENTOSA; RETINOPATHY.

diplopia The medical term for double vision, a circumstance in which a person perceives a single object as a two distinct images. Diplopia can be vertical (images one above the other) or horizontal (images beside each other). Diplopia that is present when using both eyes and goes away when covering one EYE is binocular; diplopia that persists even when one eye is covered is monocular. Each has different clinical implications. Numerous health conditions can cause diplopia or have diplopia among their symptoms.

The diagnostic path begins with basic OPHTHALMIC EXAMINATION and NEUROLOGIC EXAMINATION. The findings of these exams determine the direction and nature of further testing. As diplopia is a symptom rather than a condition, treatment targets the underlying cause. In degenerative disorders such as MULTIPLE SCLEROSIS and MYASTHENIA GRAVIS, diplopia may persist or worsen as the condition progresses. For monocular diplopia, patching the affected eye may alleviate the double image.

See also AMBLYOPIA; CRANIAL NERVES; STRABISMUS; VISION IMPAIRMENT.

dry eye syndrome A condition in which the lacrimal (tear) glands do not produce enough tears or the tears evaporate too quickly, causing the EYE to become dry and irritated. Dry eye syndrome has numerous causes, the most common of which are aging, medication side effects, and extended exposure to a dry or dusty environment. People who work in occupations that require close focus, such as with computers or assembly-line tasks, also may develop dry eyes as a result of insufficient blinking. Dry eyes also may accompany autoimmune conditions such as SYSTEMIC LUPUS ERYTHEMATOSUS (SLE), RHEUMATOID ARTHRITIS, and SJÖGREN’S SYNDROME. Cigarette smoking exacerbates dry eye syndrome.

The symptoms of dry eye syndrome include redness, itching, and the sensation of grit in the eyes. The diagnostic path targets identifying the underlying cause when possible. ANTIHISTAMINE MEDICATIONS, antihypertensive medications, ANTIDE- PRESSANT MEDICATIONS, and medications to treat PARKINSON’S DISEASE commonly cause dry eyes as a SIDE EFFECT; sometimes switching to a different medication reduces eye dryness and irritation.

Treatment is frequent use of artificial tears or restasis drops and remedying any identifiable cause when possible. The ophthalmologist may treat persistent dry eye syndrome with lacrimal plugs (also called punctal plugs), tiny segments of acrylic that become soft and gelatinous when inserted into the lacrimal ducts. These plugs slow the drainage of tears from the eye. Some recent studies suggest that increasing dietary intake of essential fatty acids, notably linoleic and gamma- linolenic acids, improves the eye’s ability to pro- duce tears.

See also AGING, VISION AND EYE CHANGES THAT OCCUR WITH; ALLERGIC CONJUNCTIVITIS; ALLERGIC RHINI- TIS; BLEPHARITIS; CONJUNCTIVITIS.

dacryocystitis INFLAMMATION of the lacrimal (tear) ducts, typically the nasolacrimal ducts in the corners of the EYE near the NOSE. Dacryocystitis develops when there is a blockage of the lacrimal duct, which may result from DACRYOSTENOSIS (narrowing of the lacrimal duct), INFECTION, or chronic irritation such as might occur with ALLERGIC RHINI- TIS or ALLERGIC CONJUNCTIVITIS. Dacryocystitis can be acute (of sudden onset) or chronic (recurrent or long-standing). It also can be congenital (the result of defects of the lacrimal gland and duct structures) or acquired. Most people who have acquired dacryocystitis are over age 65.

Common symptoms include

• redness and swelling between the eye and the bridge of the nose

• rhinitis (runny nose)

• PAIN

• overflowing tears

• FEVER when an infection is present

The doctor can typically diagnose dacryocystitis based on its presentation. Dye tests, in which the doctor places a special dye in the eye and watches to see whether the dye discolors nasal discharge, help identify the extent of blockage causing the inflammation. Treatment includes ANTIBIOTIC MEDICATIONS when there is an infection, or procedures to dilate the lacrimal duct when there is no infection. Sometimes surgery is necessary to correct dacryostenosis or other structural defects. Appropriate treatment resolves the dacryocystitis.

See also BLEPHARITIS; EYE PAIN; OPERATION; ORBITAL CELLULITIS.

dacryostenosis Narrowing of the lacrimal (tear) duct, usually congenital, that blocks the flow of tears. An infant does not produce a great volume of tears during the first few weeks to months after birth, so the doctor may not suspect or diagnose dacryostenosis until the infant is three to four months of age. The most common symptom is tears that overflow the eye and run down the face (epiphora). Most infants outgrow dacryostenosis by age six months, so doctors tend to take an approach of watchful waiting. When dacryosteno- sis persists, the doctor may dilate the lacrimal duct (under anesthetic) to gently stretch and enlarge the opening for tears to pass unimpeded. Untreated dacryostenosis can result in frequent episodes of DACRYOCYSTITIS (infected lacrimal ducts) in adulthood. Appropriate treatment can com- pletely resolve dacryostenosis.

See also INFECTION; ORBITAL CELLULITIS.

dark adaptation test A test that assesses the ability to see in a dimly lighted environment. There are several ways to perform a dark adaptation test. One of the most common is to have the person sit in a dimly lit room. The examiner shines a light into the EYE, gradually increasing the light’s intensity until the person reports seeing the light. The examiner notes the light’s intensity and the length of time it takes for the light to become noticeable. Depending on the reason for the test, the examiner may direct the light to different parts of the RETINA to test the responsiveness of the rods (the cells responsible for low-light vision). A decrease in dark adaptation response is normal with aging as the photochemical reactions in the eye slow.

See also AGING, VISION AND EYE CHANGES THAT OCCUR WITH; ELECTRORETINOGRAPHY; NIGHT BLINDNESS; RETINITIS PIGMENTOSA; RETINOPATHY.

diplopia The medical term for double vision, a circumstance in which a person perceives a single object as a two distinct images. Diplopia can be vertical (images one above the other) or horizontal (images beside each other). Diplopia that is present when using both eyes and goes away when covering one EYE is binocular; diplopia that persists even when one eye is covered is monocular. Each has different clinical implications. Numerous health conditions can cause diplopia or have diplopia among their symptoms.

The diagnostic path begins with basic OPHTHALMIC EXAMINATION and NEUROLOGIC EXAMINATION. The findings of these exams determine the direction and nature of further testing. As diplopia is a symptom rather than a condition, treatment targets the underlying cause. In degenerative disorders such as MULTIPLE SCLEROSIS and MYASTHENIA GRAVIS, diplopia may persist or worsen as the condition progresses. For monocular diplopia, patching the affected eye may alleviate the double image.

See also AMBLYOPIA; CRANIAL NERVES; STRABISMUS; VISION IMPAIRMENT.

dry eye syndrome A condition in which the lacrimal (tear) glands do not produce enough tears or the tears evaporate too quickly, causing the EYE to become dry and irritated. Dry eye syndrome has numerous causes, the most common of which are aging, medication side effects, and extended exposure to a dry or dusty environment. People who work in occupations that require close focus, such as with computers or assembly-line tasks, also may develop dry eyes as a result of insufficient blinking. Dry eyes also may accompany autoimmune conditions such as SYSTEMIC LUPUS ERYTHEMATOSUS (SLE), RHEUMATOID ARTHRITIS, and SJÖGREN’S SYNDROME. Cigarette smoking exacerbates dry eye syndrome.

The symptoms of dry eye syndrome include redness, itching, and the sensation of grit in the eyes. The diagnostic path targets identifying the underlying cause when possible. ANTIHISTAMINE MEDICATIONS, antihypertensive medications, ANTIDE- PRESSANT MEDICATIONS, and medications to treat PARKINSON’S DISEASE commonly cause dry eyes as a SIDE EFFECT; sometimes switching to a different medication reduces eye dryness and irritation.

Treatment is frequent use of artificial tears or restasis drops and remedying any identifiable cause when possible. The ophthalmologist may treat persistent dry eye syndrome with lacrimal plugs (also called punctal plugs), tiny segments of acrylic that become soft and gelatinous when inserted into the lacrimal ducts. These plugs slow the drainage of tears from the eye. Some recent studies suggest that increasing dietary intake of essential fatty acids, notably linoleic and gamma- linolenic acids, improves the eye’s ability to pro- duce tears.

See also AGING, VISION AND EYE CHANGES THAT OCCUR WITH; ALLERGIC CONJUNCTIVITIS; ALLERGIC RHINI- TIS; BLEPHARITIS; CONJUNCTIVITIS.

cataract Cloudiness and discoloration of the LENS. Cataracts become increasingly common with advancing age, affecting half of all people age 80 and older. Cataracts were once a leading cause of age-related blindness. Today ophthalmologists surgically remove cataracts and replace the lens with a prosthetic intraocular lens (IOL) that restores vision.

Cataracts result from protein deposits that accumulate within the lens. These deposits disperse light in much the same way cracks in a window might splinter sunlight shining through. The fragmented light creates areas of accentuated bright- ness, causing the halos and sensitivity to lights at night. The opacity of the cataract interferes with the refractive function of the lens, causing blurry or hazy vision. The yellow or gray discoloration of the lens common with mature or “ripe” cataracts filters the lightwaves that enter the EYE, particularly affecting those in the spectrum of blue. The location of the cataract on the lens determines the nature and extent of VISION IMPAIRMENT.

Age-related cataracts Most cataracts develop as a function of aging. Protein structures within the body, including the lens of the eye, begin to change. The lens becomes less resilient. Such changes make it easier for proteins to clump together, forming areas of opacity that eventually form cataracts. Nuclear cataracts form in the nucleus (gelatinous center) of the lens and are the most common type of age-related cataract. Corti- cal cataracts form in the cortex, or outer layer, of the lens and often do not affect vision.

Congenital cataracts Infants may be born with cataracts. A congenital cataract affecting only one eye typically is idiopathic (without identifiable cause); congenital cataracts affecting both eyes

often suggest genetic disorders such as DOWN SYN- DROME. A congenital cataract that is in the line of vision (on the visual axis) can cause significant vision impairment or blindness because the path- ways for vision develop in the infant’s first few months of life. Ophthalmologists usually remove such cataracts as soon as possible. Other congeni- tal cataracts may be small and located so they are inconsequential to vision; ophthalmologists generally take an approach of watchful waiting with these.

Cataracts of diabetes GLUCOSE, which can be present in high blood levels with DIABETES, inter- acts with the protein structure of the lens, causing protein clumping. People who have type 1 (INSULIN-dependent) diabetes are at greatest risk for cataracts of diabetes, which often develop at a young age. People who have type 2 diabetes or insulin resistance also are at increased risk. Developing cataracts account in part for the vision disturbances that are among the symptoms of diabetes. Treatment for cataracts of diabetes is the same as for age-related cataracts.

Symptoms and Diagnostic Path

Because cataracts develop slowly, symptoms become gradually noticeable. Symptoms usually affect only one eye (though cataracts may develop concurrently in both eyes) and may include

• blurry or hazy vision

• double vision

• halos around lights at night

• difficulty seeing at night

• colors appearing faded or dull, or difficulty perceiving shades of blue and purple

Gradual loss of vision at middle age and beyond may be a symptom of AGE- RELATED MACULAR DEGENERATION (ARMD) or GLAUCOMA. Untreated, these conditions result in significant and permanent vision impairments. Any decrease in vision requires an ophthalmologist’s or optometrist’s prompt evaluation.

The ophthalmologist can see cataracts during OPHTHALMOSCOPY, a painless procedure for examining the interior of the eye.

Treatment Options and Outlook

CATARACT EXTRACTION AND LENS REPLACEMENT is the treatment of choice for nearly all cataracts. There is no element of time-sensitivity for the surgery. Though VISUAL ACUITY will progressively deteriorate as the cataract enlarges, there is no permanent harm to vision by waiting to extract the cataract. Following cataract surgery, more than 90 percent of people experience vastly improved vision. Some people who are unable to receive an IOL because of other eye conditions will need to wear a special contact lens or eyeglasses to carry out the refractive functions of the extracted lens. Nearly every- one will still need reading glasses to accommodate PRESBYOPIA.

Risk Factors and Preventive Measures Cataracts are primarily a consequence of aging. Cataracts also can develop as a SIDE EFFECT of long- term STEROID use (therapeutic or performance enhancing). Cigarette smoking, excessive ALCOHOL consumption, and extended exposure to sunlight (ultraviolet rays) are among the lifestyle factors associated with early or accelerated cataract development. There are no known methods for pre- venting cataracts.  See also AGING, EYE AND VISION CHANGES THAT OCCUR WITH; ANABOLIC STEROIDS AND STEROID PRECUR- SORS; CORTICOSTEROID MEDICATIONS; SMOKING AND HEALTH.

stage of its development. The vast majority of people who undergo cataract extraction fully recover without complications and experience VISUAL ACU- ITY correctable to 20/40 or better.

Surgical Procedure

Cataract extraction is nearly always an outpatient surgery performed under local anesthetic and a mild general sedative for comfort. There are three surgical procedures for cataract extraction. Each takes 20 to 30 minutes for the ophthalmologist to complete. Many variables influence the ophthalmologist’s choice for which to use.

Phacoemulsification The most commonly per- formed cataract extraction procedure is phacoemulsification, which requires a tiny incision into the capsule containing the lens. The ophthalmologist first uses ULTRASOUND to liquefy the central nucleus (inner, gelatinous portion of the lens) and then uses aspiration to remove it. Last the ophthalmologist removes the cortex (outer layer of the lens) from the capsule in multiple segments.

Extracapsular cataract extraction The extra- capsular cataract extraction procedure requires a slightly larger incision in the capsule, through which the ophthalmologist removes the central nucleus of the lens intact, then removes the cortex in multiple segments.

Lens replacement After extracting the cataract, the ophthalmologist inserts either a monofocal or multifocal IOL to give the eye the ability to focus. Contemporary lens designs allow the ophthalmologist to fold the lens, insert it into the lens capsule through the tiny incision used to extract the cataract, and unfold the IOL to place it in position.

Risks and Complications

Most ophthalmologists prescribe antibiotic and anti-inflammatory eye drops applied to the eye for four to six weeks following surgery, and recommend wearing dark glasses in bright light to help protect the eye from light sensitivity. Swelling and irritation of the tissues around the operated eye is normal in the first few weeks following surgery. Clear vision may take four to six weeks, though many people experience dramatic improvement immediately. Though the short-term risks of cataract extraction and lens replacement are minor, RETINAL DETACHMENT can occur months to years following surgery.

Cataract extraction is a permanent solution for cataracts. Once removed, cataracts cannot grow back. Some people do develop a complication called posterior capsule opacity, in which the membrane behind the IOL becomes cloudy (opaque). This is a complication that results when residual cells that remain after removal of the lens begin to grow across the membrane, causing the membrane to thicken. A follow-up procedure, either yttrium-aluminum-garnet (YAG) laser cap- sulotomy or conventional surgery, is necessary to remove the membrane.

Outlook and Lifestyle Modifications

About 90 percent of people experience vastly improved vision after cataract extraction. How- ever, other eye problems or underlying conditions (such as RETINOPATHY of diabetes) can affect the quality of vision. Many people do need eyeglasses after cataract extraction, as the IOL does not adjust for focus as does a natural lens. It is important to see the ophthalmologist for follow-up and routine eye care as recommended.

See also AGE-RELATED MACULAR DEGENERATION (ARMD); BULLOUS KERATOPATHY; HYPEROPIA; MYOPIA; PRESBYOPIA; SMOKING AND HEALTH; SURGERY BENEFIT AND RISK ASSESSMENT.

chalazion A painless, hard nodule that arises from a gland (meibomian or sebaceous) along the edge of the eyelid, the result of glandular secretions that granulate. A chalazion may extend deep into the structure of the eyelid. A chalazion some- times forms at the site of a recurrent HORDEOLUM (an infected eyelid SEBACEOUS GLAND, also called a stye). Often a small chalazion will go away on its own, without treatment. Moist heat applied to the eyelid helps dissolve the granulated material and draw it from the gland. Because of the risk of scar-

ring and pain, the ophthalmologist may recommend excising (surgically removing) a chalazion that does not go away or that recurs. The procedure, with local anesthetic to numb the eyelid, takes only a few minutes in the doctor’s office. The wound typically heals within two weeks and leaves no scarring. Inflammatory skin conditions such as DERMATITIS or ROSACEA can block the eye- lid’s glands, causing a chalazion to develop. Care- ful eyelid hygiene helps keep secretions from accumulating.

See also BLEPHARITIS; CONJUNCTIVITIS; OPERATION.

cicatricial pemphigoid An autoimmune disorder in which painful blisters form on the inner surfaces of the eyelids (and may form on other mucus membranes, such as in the MOUTH and NOSE). SCAR tissue that forms after the blisters heal continues to irritate the inner eyelids as well as the outer surface of the EYE (sclera and CORNEA). The blisters commonly involve the lacrimal (tear) glands and ducts, reducing tear production and causing DRY EYE SYNDROME. Cicatricial pemphigoid occurs when the body’s IMMUNE SYSTEM produces antibodies that attack the cells that form the mucus membranes. Trauma appears to activate the eruptions of blisters and may be as inconsequential as rubbing the eye or the irritation such as occurs with exposure to environmental particulates such as pollen and dust. Some people first experience outbreaks of cicatricial pemphigoid following eye operations such as CATARACT EXTRACTION AND LENS REPLACEMENT or BLEPHAROPLASTY.

The diagnostic path includes laboratory tests to assess the levels of antibodies in the blood, particularly IMMUNOGLOBULIN G (IGG) and IMMUNOGLOBIN A

(IGA), the antibodies most closely associated with cicatricial pemphigoid. Treatment focuses on reducing BLISTER formation and minimizing scar- ring, typically by taking oral CORTICOSTEROID MED- ICATIONS or IMMUNOSUPPRESSIVE MEDICATIONS. As with

other AUTOIMMUNE DISORDERS, cicatricial pemphigoid tends to be chronic and recurrent. The persistent irritation can result in damage to the cornea that causes VISUAL IMPAIRMENT and, when severe, results in blindness.

See also ANTIBODY; CONJUNCTIVITIS; CORNEAL TRANSPLANTATION; ECTROPION; HUMAN LEUKOCYTE ANTI- GEN (HLA).

color deficiency A VISION IMPAIRMENT in which the ability to distinguish certain, and rarely all, colors is impaired. Color deficiency represents a shortage of normal cones, the specialized cells on the RETINA that detect color. Cones contain photo- sensitive chemicals that react to red, green, or blue. The most common presentation of color deficiency, accounting for about 98 percent of color deficiency, is red/green deficiency, in which the person cannot distinguish red and green. A small percentage of people cannot distinguish blue and yellow. Rarely, a person sees only in shades of gray.

Color perceptions occur when lightwaves of certain frequencies (lengths) activate the photo- chemicals in cones that are sensitive to the fre- quency. The BRAIN interprets the varying intensities and blends of the photochemical responses. Color deficiency occurs when the cones that perceive one of the three primary colors (red, green, blue) do not function properly.

The most common test for color vision and color deficiency is a series of disks that contain dots of color in random patterns with a structured pattern of differing color within the field. The structured pattern may be a number (most commonly) or an object. There is no treatment to compensate for color deficiency. People who are color-deficient learn to accommodate the deficiency through mechanisms such as memorizing the locations of colored objects (such as the sequence of lights in a traffic signal) and by making adaptations in their personal environments. A person may have friends or family members sort clothing by color, for example, and label the color groups. Some people who have mild color deficiency experience benefit from devices such as colored glasses and colored contact lenses that filter the lightwaves that enter the EYE. A yellow tint may improve blue-deficient color vision, for example.

Most color deficiency is an X-linked genetic MUTATION, affecting about 8 percent of men and 1⁄2 percent of women. Color deficiency may also develop with AGE-RELATED MACULAR DEGENERATION (ARMD), RETINOPATHY, neurologic disorders such as MULTIPLE SCLEROSIS, and HEAVY-METAL POISONING such as lead or mercury. Antimalarial drugs can cause permanent changes in the RETINA that affect color vision; the ERECTILE DYSFUNCTION medication sildenafil (Viagra) can temporarily intensify the perception of blue.

See also VISION HEALTH; VISUAL ACUITY.

conjunctivitis An INFLAMMATION of the conjunctiva, or mucous tissue that lines the inside of the eyelids. There are many causes for conjunctivitis, commonly called pink EYE, including INFECTION (bacterial, viral, or fungal) and contact contamination such as due to pollen or substances in the air or on the fingers that irritate the tissues. Infectious conjunctivitis is highly contagious and very common, especially in children. Symptoms include

• red, swollen conjunctiva and sclera (inner eye- lids and the “white” of the eye)

• itchy or scratchy sensation

• thick, yellowish discharge that crusts

• PHOTOPHOBIA (sensitivity to light)

The doctor can usually diagnose conjunctivitis from its appearance. Typical treatment is application of an antibiotic medication in ophthalmic preparation (drops or ointment). Most conjunctivitis dramatically improves with 48 hours of initiating treatment, though symptoms may resolve gradually over 10 to 14 days, and does not require further medical attention. The doctor may culture the discharge when there is reason to suspect CHLAMYDIA or GONNORHEA is the cause, or when symptoms do not improve with treatment. Warm, moist compresses help relieve discomfort and clear away the discharge. Frequent HAND WASHING helps prevent spreading the infection. Untreated conjunctivitis, particularly when chlamydia or gonorrhea is the infectious agent, can cause permanent damage to the CORNEA, which results in VISUAL IMPAIRMENT or blindness.

See also ALLERGIC CONJUNCTIVITIS; ANTIBIOTIC MED- ICATIONS; BACTERIA; FUNGUS; SEXUALLY TRANSMITTED DISEASE (STD) PREVENTION; VIRUS.

cornea The transparent portion of the sclera, the EYE’s outer layer. The cornea functions as a window to allow light to enter the eye and is the first point of refraction (bending lightwaves to focus them on the RETINA). Irregularities in the surface of the cornea can distort refraction, resulting in ASTIGMATISM. Though the cornea has no BLOOD vessels it has numerous NERVE endings that make it highly sensitive. Because it is the outermost portion of the eye, the cornea is also highly vulnerable to injury.

For further discussion of the cornea within the context of ophthalmologic structure and function please see the overview section “The Eyes.”

See also CORNEAL INJURY; CORNEAL TRANSPLANTA- TION; LENS; REFRACTIVE ERRORS.

corneal injury Lacerations, punctures, and blunt trauma to the CORNEA. Because of its position, somewhat protruding at the front of the EYE, the cornea is at risk for damage that can jeopardize vision.

Corneal injuries require immediate medical attention. Any puncture or penetrating wound to the eye is a medical emergency. Loosely patch both eyes to minimize eye movement

Dust, dirt, pollen, and other particulates in the air can scratch the surface of the cornea. Particles that adhere to the inside of the upper eyelid or objects that slash across the cornea before the eye- lid reflexively closes may cause lacerations (cuts) to the cornea. Though the cornea has no blood vessels and thus cannot bleed, it has numerous nerve endings that unmistakably sound the alert when injury occurs. Injury to the cornea also can diminish VISUAL ACUITY. Puncture or penetrating injuries can destroy the cornea and expose the inner eye to traumatic damage as well as INFECTION. Even minor ABRASIONS and lacerations can cause temporary vision impairment as well as present the risk for infection. Loss of the eye is possible when a significant penetrating wound allows the inner contents of the eye to escape.

Symptoms of corneal injury include

• discomfort ranging from a scratchy sensation to frank PAIN

• PHOTOPHOBIA (sensitivity to light)

• excessive tearing

• inability to keep the eye open

• blurred or distorted vision

The ophthalmologist can identify a corneal injury with FLUORESCEIN STAINING, a simple and pain- less procedure. Any areas of injury on the cornea absorb the fluorescein dye, causing them to glow green under blue light. Serious injuries to the cornea, or embedded foreign objects, may require immediate surgery to minimize loss of vision. Treatment for injuries that affect only the surface of the cornea may include ophthalmic ANTIBIOTIC MED- ICATIONS (drops or ointment) and patching of the affected eye. Protective eyewear, worn whenever there is the potential for particles or objects to strike the eye, helps prevent corneal injuries.

See also CORNEAL TRANSPLANTATION; TRAUMA TO THE EYE.

corneal transplantation The replacement of a Corneal injuries require immediate medical attention. Any puncture or penetrating wound to the eye is a medical emergency. Loosely patch both eyes to minimize eye movement.

diseased CORNEA with a healthy donor cornea. In the United States, ophthalmologists perform more than 45,000 corneal transplantations each year; up to 90 percent of people who receive trans- planted corneas experience restored vision; success depends on the reason for the transplant. Ophthalmologists may recommend corneal trans- plantation to treat:

• BULLOUS KERATOPATHY

• KERATOCONUS

• KERATITIS

• significant CORNEAL INJURY

Donor corneas are harvested within a few hours of death and can be preserved for up to 14 days. Current practice does not employ blood type or tissue type matching for corneal transplantation, though some studies suggest matching the blood type of donor and recipient reduces the risk for rejection.

CORNEA DONATION

Nearly anyone can donate his or her corneas after death. There is no cost to the donor. An eye bank coordinates the harvesting, testing, storage, and dispensing of donated corneas. Many states incorporate organ donor authorization with driver’s licenses. People should tell family members that they wish to donate their corneas.

Corneal transplantation surgery takes place with a local anesthetic to numb the eye and an intravenous sedative medication for relaxation and comfort. The operation takes 45 to 60 minutes. From the donor cornea, the ophthalmologist uses a trephine, a device that cleanly punches out a buttonlike segment of the cornea’s center. Using a surgical microscope, the ophthalmologist then removes a similarly shaped segment from the dis- eased cornea and places the donor button in its place. Very fine suture, sometimes thinner than a human hair, secures the donor corneal button in position and remains in the eye for three months to one year. The ophthalmologist often removes the sutures a few at a time as HEALING progresses, using an ophthalmic anesthetic to numb the affected eye, depending on the rate of vision improvement. Some sutures may remain in place indefinitely.

Full recovery typically takes about a year. Some people will have ASTIGMATISM and require CORREC- TIVE LENSES following corneal transplantation, resulting from irregularities in the shape of the cornea that develop during healing. Corneal trans- plantation may correct another VISION IMPAIRMENT such as HYPEROPIA (farsightedness) or MYOPIA (near- sightedness) because the OPERATION changes the shape of the cornea.

The most common complication of corneal transplantation is rejection of the transplanted cornea, which occurs overall in about 15 percent of corneal transplantations. Rejection is most likely to take place in the first two years after the operation. Early detection and prompt treatment with ophthalmic CORTICOSTEROID MEDICATIONS can reverse the rejection process. Signs of rejection include

• diminished VISUAL ACUITY

• PAIN

• redness of the eye

• PHOTOPHOBIA (sensitivity to light)

Other complications that can occur include INFECTION and bleeding within the eye.

See also ORGAN TRANSPLANTATION; PHOTOTHERAPEU- TIC KERATECTOMY (PTK).

corrective lenses Eyeglasses or contact lenses that alter the focal point of the lightwaves entering the EYE to correct REFRACTIVE ERRORS of vision, including HYPEROPIA (farsightedness), MYOPIA (near- sightedness), ASTIGMATISM (blurred or distorted vision), and PRESBYOPIA (age-related hyperopia). The eye’s natural focusing structures, the CORNEA and the LENS, gather lightwaves and refract (bend) them toward their centers. The cornea refracts the lightwaves first. The lens, which can thicken or flatten to refine its focal efforts, refracts the some- what focused lightwaves that come to it from the cornea. In normal vision, this sequence results in the focal point of the lightwaves striking the RETINA.

When refractive errors exist the focal point falls in front of or behind the retina, resulting in blurred images. Corrective lenses add a third level of refraction to compensate for the error, bending the lightwaves before they enter the cornea to realign their focal point. The direction of refraction depends on the refractive error:

• In myopia, the focal point falls short of the retina. A lens that corrects for myopia bends the lightwaves inward, narrowing the span of light as it enters the cornea to lengthen the focal point. Such a lens is thicker at the edges than in the middle (concave); it is a minus spherical correction.

• In hyperopia, the focal point extends beyond the retina. A lens that corrects for hyperopia bends the lightwaves outward, broadening the span of light as it enters the cornea to shorten the focal point. Such a lens is thicker in the center than at the edges (convex); it is a plus spherical correction.

• In astigmatism, irregularities in the surface of the lens cause a second focal point. A lens that corrects for astigmatism refracts along a specific axis, realigning the lightwaves. This is a cylinder correction.

Corrective lenses can, and often do, combine spherical and cylindrical corrections. A multifocal lens further incorporates a correction for presbyopia in the form of a bifocal, trifocal, or progressive lens. The bottom of the lens is a plus section,

added to the corrective prescription, that accommodates the limited ability of the lens to focus on near objects (such as when reading).

Eyeglasses

Eyeglasses are plastic resin or polycarbon, and less commonly glass, lenses ground to the thicknesses and shapes necessary to achieve the desired refractive specifications. Because eyeglasses are external to the eye, they can correct for a broad range of refractive errors and are the most common means of refractive correction. Eyeglasses also can contain tints and dyes that change their color; some have additives that provide protection from ultraviolet light. About 85 percent of people who have refractive errors of vision wear eye- glasses to correct them.

Bifocal and trifocal eyeglasses have a clear shift (sometimes visible as a line on the lens) to the presbyopic correction; a progressive lens transitions to the presbyopic correction. Reading glasses such as those available without an eye care practitioner’s prescription, are magnifying lenses that enlarge close objects, requiring the lens to make less of an adjustment to bring them into focus. How well reading glasses work depends on whether there are refractive errors that remain uncorrected. With aging, most people develop at least a small degree of astigmatism, which can result in blurred or distorted images not related to presbyopia.

The primary risk of wearing eyeglasses is trau-matic injury due to a blow that strikes the glasses. The energy of such a blow concentrates initially at the contact points on the NOSE. The frame may break, causing lacerations to the face. Of more significant consequence is a blow that breaks the lens, which can result in vision-threatening injury to the eye. Polycarbonate lenses have the highest inherent shatter resistance; plastic resin and glass lenses should have shatter-resistant coatings or additives. People who engage in physical activities such as ball sports should wear polycarbonate eye- glasses or custom protective eyewear.

Contact Lenses

Contact lenses fit directly onto the eye, covering the cornea. There are two basic kinds of contact lenses in use today: gas permeable (hard) and hydrophilic (soft). Gas-permeable contact lenses float on a layer of tears over the center of the cornea and often are the contact lens of choice to correct for moderate to significant astigmatism as well as KERATOCONUS, a condition in which the cornea’s center bulges outward. Gas-permeable lenses also can correct for mild to moderate myopic and hyperopic refractive errors. Made of rigid polymers of fluorocarbon and polymethyl methacrylate, gas-permeable lenses allow oxygen molecules to pass through but do not absorb moisture from the eye. Hydrophilic contact lenses cover the entire cornea and can correct for mild to moderate myopia and hyperopia. Soft and flexible, hydrophilic lenses contain a high percentage of water and draw additional moisture from the tears to remain hydrated. A special kind of hydrophilic lens, the toric lens, is necessary to correct for astigmatism. A toric lens has varying thicknesses that compensate for corneal irregularities to correct refraction.

Contact lenses can incorporate correction for moderate presbyopia, though this tends to be a less satisfactory approach than eyeglasses. There are two methods for accommodating presbyopia with contact lenses: progressive or bifocal lenses and monovision. Progressive or bifocal contacts function much the same as progressive or bifocal eyeglasses, with the lower portion of the lens containing the presbyopic correction. Because contact lenses shift position on the eye with blinking and when the wearer alters the angle of the head (such as when lying down), the presbyopic correc- tion may not remain in an effective position. Monovision takes the approach of modifying the BRAIN’s interpretation of visual signals. One eye, usually the dominant eye, wears a contact lens with the refractive correction. The other eye wears a contact lens with the presbyopic correction. The brain learns to distinguish which signals to inter- pret, accepting those from the dominant eye during normal visual activities and those from the other eye when reading or doing close-focus work.

The primary risks of wearing contact lenses are damage to the cornea and INFECTION. Even hydrophilic lenses can irritate the cornea and cause corneal ABRASIONS, particularly in dusty, windy, or dry environmental conditions. Contact lenses tend to accumulate protein deposits that cause irritation. Most hydrophilic lenses are disposable, so frequent replacement helps minimize this as a problem. The optician may need to clean or gently grind the surface of gas-permeable lenses to clear away deposits. Contact lens hygiene, including diligent HAND WASHING before handling lenses and storing lenses in the appropriate disinfectant solution, is essential.

Reading a Corrective Lens Prescription Optometrists and ophthalmologists measure refractive errors in diopters, a representational scale of the distance in front of or behind the eye’s lens the focal point of lightwaves entering the eye must shift to allow the light waves to clearly focus on the retina. The larger the diopter number, the more the lens refracts, or bends, the light. A corrective lens prescription represents the diopter as

minus or plus, according to the direction the correction shifts the focal point. For example, the following prescription corrects for myopia and astigmatism:

This prescription denotes different refractive corrections for the right eye (OD) and left eye (OS). The minus diopter is the spherical correction for the myopia; the plus diopter is the cylindrical correction for the astigmatism, and the last number is the axis position for the cylindrical correction. A lens with a strong correction may also include an adjustment that tilts the lens to alter its optical center, the prism, allowing a thinner lens to deliver the same corrective power or to accommodate a significant difference in the refractive correction for each eye (anisometropia).

See also REFRACTION TEST; REFRACTIVE SURGERY; VISION IMPAIRMENT.

cataract Cloudiness and discoloration of the LENS. Cataracts become increasingly common with advancing age, affecting half of all people age 80 and older. Cataracts were once a leading cause of age-related blindness. Today ophthalmologists surgically remove cataracts and replace the lens with a prosthetic intraocular lens (IOL) that restores vision.

Cataracts result from protein deposits that accumulate within the lens. These deposits disperse light in much the same way cracks in a window might splinter sunlight shining through. The fragmented light creates areas of accentuated bright- ness, causing the halos and sensitivity to lights at night. The opacity of the cataract interferes with the refractive function of the lens, causing blurry or hazy vision. The yellow or gray discoloration of the lens common with mature or “ripe” cataracts filters the lightwaves that enter the EYE, particularly affecting those in the spectrum of blue. The location of the cataract on the lens determines the nature and extent of VISION IMPAIRMENT.

Age-related cataracts Most cataracts develop as a function of aging. Protein structures within the body, including the lens of the eye, begin to change. The lens becomes less resilient. Such changes make it easier for proteins to clump together, forming areas of opacity that eventually form cataracts. Nuclear cataracts form in the nucleus (gelatinous center) of the lens and are the most common type of age-related cataract. Corti- cal cataracts form in the cortex, or outer layer, of the lens and often do not affect vision.

Congenital cataracts Infants may be born with cataracts. A congenital cataract affecting only one eye typically is idiopathic (without identifiable cause); congenital cataracts affecting both eyes

often suggest genetic disorders such as DOWN SYN- DROME. A congenital cataract that is in the line of vision (on the visual axis) can cause significant vision impairment or blindness because the path- ways for vision develop in the infant’s first few months of life. Ophthalmologists usually remove such cataracts as soon as possible. Other congeni- tal cataracts may be small and located so they are inconsequential to vision; ophthalmologists generally take an approach of watchful waiting with these.

Cataracts of diabetes GLUCOSE, which can be present in high blood levels with DIABETES, inter- acts with the protein structure of the lens, causing protein clumping. People who have type 1 (INSULIN-dependent) diabetes are at greatest risk for cataracts of diabetes, which often develop at a young age. People who have type 2 diabetes or insulin resistance also are at increased risk. Developing cataracts account in part for the vision disturbances that are among the symptoms of diabetes. Treatment for cataracts of diabetes is the same as for age-related cataracts.

Symptoms and Diagnostic Path

Because cataracts develop slowly, symptoms become gradually noticeable. Symptoms usually affect only one eye (though cataracts may develop concurrently in both eyes) and may include

• blurry or hazy vision

• double vision

• halos around lights at night

• difficulty seeing at night

• colors appearing faded or dull, or difficulty perceiving shades of blue and purple

Gradual loss of vision at middle age and beyond may be a symptom of AGE- RELATED MACULAR DEGENERATION (ARMD) or GLAUCOMA. Untreated, these conditions result in significant and permanent vision impairments. Any decrease in vision requires an ophthalmologist’s or optometrist’s prompt evaluation.

The ophthalmologist can see cataracts during OPHTHALMOSCOPY, a painless procedure for examining the interior of the eye.

Treatment Options and Outlook

CATARACT EXTRACTION AND LENS REPLACEMENT is the treatment of choice for nearly all cataracts. There is no element of time-sensitivity for the surgery. Though VISUAL ACUITY will progressively deteriorate as the cataract enlarges, there is no permanent harm to vision by waiting to extract the cataract. Following cataract surgery, more than 90 percent of people experience vastly improved vision. Some people who are unable to receive an IOL because of other eye conditions will need to wear a special contact lens or eyeglasses to carry out the refractive functions of the extracted lens. Nearly every- one will still need reading glasses to accommodate PRESBYOPIA.

Risk Factors and Preventive Measures Cataracts are primarily a consequence of aging. Cataracts also can develop as a SIDE EFFECT of long- term STEROID use (therapeutic or performance enhancing). Cigarette smoking, excessive ALCOHOL consumption, and extended exposure to sunlight (ultraviolet rays) are among the lifestyle factors associated with early or accelerated cataract development. There are no known methods for pre- venting cataracts.  See also AGING, EYE AND VISION CHANGES THAT OCCUR WITH; ANABOLIC STEROIDS AND STEROID PRECUR- SORS; CORTICOSTEROID MEDICATIONS; SMOKING AND HEALTH.

stage of its development. The vast majority of people who undergo cataract extraction fully recover without complications and experience VISUAL ACU- ITY correctable to 20/40 or better.

Surgical Procedure

Cataract extraction is nearly always an outpatient surgery performed under local anesthetic and a mild general sedative for comfort. There are three surgical procedures for cataract extraction. Each takes 20 to 30 minutes for the ophthalmologist to complete. Many variables influence the ophthalmologist’s choice for which to use.

Phacoemulsification The most commonly per- formed cataract extraction procedure is phacoemulsification, which requires a tiny incision into the capsule containing the lens. The ophthalmologist first uses ULTRASOUND to liquefy the central nucleus (inner, gelatinous portion of the lens) and then uses aspiration to remove it. Last the ophthalmologist removes the cortex (outer layer of the lens) from the capsule in multiple segments.

Extracapsular cataract extraction The extra- capsular cataract extraction procedure requires a slightly larger incision in the capsule, through which the ophthalmologist removes the central nucleus of the lens intact, then removes the cortex in multiple segments.

Lens replacement After extracting the cataract, the ophthalmologist inserts either a monofocal or multifocal IOL to give the eye the ability to focus. Contemporary lens designs allow the ophthalmologist to fold the lens, insert it into the lens capsule through the tiny incision used to extract the cataract, and unfold the IOL to place it in position.

Risks and Complications

Most ophthalmologists prescribe antibiotic and anti-inflammatory eye drops applied to the eye for four to six weeks following surgery, and recommend wearing dark glasses in bright light to help protect the eye from light sensitivity. Swelling and irritation of the tissues around the operated eye is normal in the first few weeks following surgery. Clear vision may take four to six weeks, though many people experience dramatic improvement immediately. Though the short-term risks of cataract extraction and lens replacement are minor, RETINAL DETACHMENT can occur months to years following surgery.

Cataract extraction is a permanent solution for cataracts. Once removed, cataracts cannot grow back. Some people do develop a complication called posterior capsule opacity, in which the membrane behind the IOL becomes cloudy (opaque). This is a complication that results when residual cells that remain after removal of the lens begin to grow across the membrane, causing the membrane to thicken. A follow-up procedure, either yttrium-aluminum-garnet (YAG) laser cap- sulotomy or conventional surgery, is necessary to remove the membrane.

Outlook and Lifestyle Modifications

About 90 percent of people experience vastly improved vision after cataract extraction. How- ever, other eye problems or underlying conditions (such as RETINOPATHY of diabetes) can affect the quality of vision. Many people do need eyeglasses after cataract extraction, as the IOL does not adjust for focus as does a natural lens. It is important to see the ophthalmologist for follow-up and routine eye care as recommended.

See also AGE-RELATED MACULAR DEGENERATION (ARMD); BULLOUS KERATOPATHY; HYPEROPIA; MYOPIA; PRESBYOPIA; SMOKING AND HEALTH; SURGERY BENEFIT AND RISK ASSESSMENT.

chalazion A painless, hard nodule that arises from a gland (meibomian or sebaceous) along the edge of the eyelid, the result of glandular secretions that granulate. A chalazion may extend deep into the structure of the eyelid. A chalazion some- times forms at the site of a recurrent HORDEOLUM (an infected eyelid SEBACEOUS GLAND, also called a stye). Often a small chalazion will go away on its own, without treatment. Moist heat applied to the eyelid helps dissolve the granulated material and draw it from the gland. Because of the risk of scar-

ring and pain, the ophthalmologist may recommend excising (surgically removing) a chalazion that does not go away or that recurs. The procedure, with local anesthetic to numb the eyelid, takes only a few minutes in the doctor’s office. The wound typically heals within two weeks and leaves no scarring. Inflammatory skin conditions such as DERMATITIS or ROSACEA can block the eye- lid’s glands, causing a chalazion to develop. Care- ful eyelid hygiene helps keep secretions from accumulating.

See also BLEPHARITIS; CONJUNCTIVITIS; OPERATION.

cicatricial pemphigoid An autoimmune disorder in which painful blisters form on the inner surfaces of the eyelids (and may form on other mucus membranes, such as in the MOUTH and NOSE). SCAR tissue that forms after the blisters heal continues to irritate the inner eyelids as well as the outer surface of the EYE (sclera and CORNEA). The blisters commonly involve the lacrimal (tear) glands and ducts, reducing tear production and causing DRY EYE SYNDROME. Cicatricial pemphigoid occurs when the body’s IMMUNE SYSTEM produces antibodies that attack the cells that form the mucus membranes. Trauma appears to activate the eruptions of blisters and may be as inconsequential as rubbing the eye or the irritation such as occurs with exposure to environmental particulates such as pollen and dust. Some people first experience outbreaks of cicatricial pemphigoid following eye operations such as CATARACT EXTRACTION AND LENS REPLACEMENT or BLEPHAROPLASTY.

The diagnostic path includes laboratory tests to assess the levels of antibodies in the blood, particularly IMMUNOGLOBULIN G (IGG) and IMMUNOGLOBIN A

(IGA), the antibodies most closely associated with cicatricial pemphigoid. Treatment focuses on reducing BLISTER formation and minimizing scar- ring, typically by taking oral CORTICOSTEROID MED- ICATIONS or IMMUNOSUPPRESSIVE MEDICATIONS. As with

other AUTOIMMUNE DISORDERS, cicatricial pemphigoid tends to be chronic and recurrent. The persistent irritation can result in damage to the cornea that causes VISUAL IMPAIRMENT and, when severe, results in blindness.

See also ANTIBODY; CONJUNCTIVITIS; CORNEAL TRANSPLANTATION; ECTROPION; HUMAN LEUKOCYTE ANTI- GEN (HLA).

color deficiency A VISION IMPAIRMENT in which the ability to distinguish certain, and rarely all, colors is impaired. Color deficiency represents a shortage of normal cones, the specialized cells on the RETINA that detect color. Cones contain photo- sensitive chemicals that react to red, green, or blue. The most common presentation of color deficiency, accounting for about 98 percent of color deficiency, is red/green deficiency, in which the person cannot distinguish red and green. A small percentage of people cannot distinguish blue and yellow. Rarely, a person sees only in shades of gray.

Color perceptions occur when lightwaves of certain frequencies (lengths) activate the photo- chemicals in cones that are sensitive to the fre- quency. The BRAIN interprets the varying intensities and blends of the photochemical responses. Color deficiency occurs when the cones that perceive one of the three primary colors (red, green, blue) do not function properly.

The most common test for color vision and color deficiency is a series of disks that contain dots of color in random patterns with a structured pattern of differing color within the field. The structured pattern may be a number (most commonly) or an object. There is no treatment to compensate for color deficiency. People who are color-deficient learn to accommodate the deficiency through mechanisms such as memorizing the locations of colored objects (such as the sequence of lights in a traffic signal) and by making adaptations in their personal environments. A person may have friends or family members sort clothing by color, for example, and label the color groups. Some people who have mild color deficiency experience benefit from devices such as colored glasses and colored contact lenses that filter the lightwaves that enter the EYE. A yellow tint may improve blue-deficient color vision, for example.

Most color deficiency is an X-linked genetic MUTATION, affecting about 8 percent of men and 1⁄2 percent of women. Color deficiency may also develop with AGE-RELATED MACULAR DEGENERATION (ARMD), RETINOPATHY, neurologic disorders such as MULTIPLE SCLEROSIS, and HEAVY-METAL POISONING such as lead or mercury. Antimalarial drugs can cause permanent changes in the RETINA that affect color vision; the ERECTILE DYSFUNCTION medication sildenafil (Viagra) can temporarily intensify the perception of blue.

See also VISION HEALTH; VISUAL ACUITY.

conjunctivitis An INFLAMMATION of the conjunctiva, or mucous tissue that lines the inside of the eyelids. There are many causes for conjunctivitis, commonly called pink EYE, including INFECTION (bacterial, viral, or fungal) and contact contamination such as due to pollen or substances in the air or on the fingers that irritate the tissues. Infectious conjunctivitis is highly contagious and very common, especially in children. Symptoms include

• red, swollen conjunctiva and sclera (inner eye- lids and the “white” of the eye)

• itchy or scratchy sensation

• thick, yellowish discharge that crusts

• PHOTOPHOBIA (sensitivity to light)

The doctor can usually diagnose conjunctivitis from its appearance. Typical treatment is application of an antibiotic medication in ophthalmic preparation (drops or ointment). Most conjunctivitis dramatically improves with 48 hours of initiating treatment, though symptoms may resolve gradually over 10 to 14 days, and does not require further medical attention. The doctor may culture the discharge when there is reason to suspect CHLAMYDIA or GONNORHEA is the cause, or when symptoms do not improve with treatment. Warm, moist compresses help relieve discomfort and clear away the discharge. Frequent HAND WASHING helps prevent spreading the infection. Untreated conjunctivitis, particularly when chlamydia or gonorrhea is the infectious agent, can cause permanent damage to the CORNEA, which results in VISUAL IMPAIRMENT or blindness.

See also ALLERGIC CONJUNCTIVITIS; ANTIBIOTIC MED- ICATIONS; BACTERIA; FUNGUS; SEXUALLY TRANSMITTED DISEASE (STD) PREVENTION; VIRUS.

cornea The transparent portion of the sclera, the EYE’s outer layer. The cornea functions as a window to allow light to enter the eye and is the first point of refraction (bending lightwaves to focus them on the RETINA). Irregularities in the surface of the cornea can distort refraction, resulting in ASTIGMATISM. Though the cornea has no BLOOD vessels it has numerous NERVE endings that make it highly sensitive. Because it is the outermost portion of the eye, the cornea is also highly vulnerable to injury.

For further discussion of the cornea within the context of ophthalmologic structure and function please see the overview section “The Eyes.”

See also CORNEAL INJURY; CORNEAL TRANSPLANTA- TION; LENS; REFRACTIVE ERRORS.

corneal injury Lacerations, punctures, and blunt trauma to the CORNEA. Because of its position, somewhat protruding at the front of the EYE, the cornea is at risk for damage that can jeopardize vision.

Corneal injuries require immediate medical attention. Any puncture or penetrating wound to the eye is a medical emergency. Loosely patch both eyes to minimize eye movement

Dust, dirt, pollen, and other particulates in the air can scratch the surface of the cornea. Particles that adhere to the inside of the upper eyelid or objects that slash across the cornea before the eye- lid reflexively closes may cause lacerations (cuts) to the cornea. Though the cornea has no blood vessels and thus cannot bleed, it has numerous nerve endings that unmistakably sound the alert when injury occurs. Injury to the cornea also can diminish VISUAL ACUITY. Puncture or penetrating injuries can destroy the cornea and expose the inner eye to traumatic damage as well as INFECTION. Even minor ABRASIONS and lacerations can cause temporary vision impairment as well as present the risk for infection. Loss of the eye is possible when a significant penetrating wound allows the inner contents of the eye to escape.

Symptoms of corneal injury include

• discomfort ranging from a scratchy sensation to frank PAIN

• PHOTOPHOBIA (sensitivity to light)

• excessive tearing

• inability to keep the eye open

• blurred or distorted vision

The ophthalmologist can identify a corneal injury with FLUORESCEIN STAINING, a simple and pain- less procedure. Any areas of injury on the cornea absorb the fluorescein dye, causing them to glow green under blue light. Serious injuries to the cornea, or embedded foreign objects, may require immediate surgery to minimize loss of vision. Treatment for injuries that affect only the surface of the cornea may include ophthalmic ANTIBIOTIC MED- ICATIONS (drops or ointment) and patching of the affected eye. Protective eyewear, worn whenever there is the potential for particles or objects to strike the eye, helps prevent corneal injuries.

See also CORNEAL TRANSPLANTATION; TRAUMA TO THE EYE.

corneal transplantation The replacement of a Corneal injuries require immediate medical attention. Any puncture or penetrating wound to the eye is a medical emergency. Loosely patch both eyes to minimize eye movement.

diseased CORNEA with a healthy donor cornea. In the United States, ophthalmologists perform more than 45,000 corneal transplantations each year; up to 90 percent of people who receive trans- planted corneas experience restored vision; success depends on the reason for the transplant. Ophthalmologists may recommend corneal trans- plantation to treat:

• BULLOUS KERATOPATHY

• KERATOCONUS

• KERATITIS

• significant CORNEAL INJURY

Donor corneas are harvested within a few hours of death and can be preserved for up to 14 days. Current practice does not employ blood type or tissue type matching for corneal transplantation, though some studies suggest matching the blood type of donor and recipient reduces the risk for rejection.

CORNEA DONATION

Nearly anyone can donate his or her corneas after death. There is no cost to the donor. An eye bank coordinates the harvesting, testing, storage, and dispensing of donated corneas. Many states incorporate organ donor authorization with driver’s licenses. People should tell family members that they wish to donate their corneas.

Corneal transplantation surgery takes place with a local anesthetic to numb the eye and an intravenous sedative medication for relaxation and comfort. The operation takes 45 to 60 minutes. From the donor cornea, the ophthalmologist uses a trephine, a device that cleanly punches out a buttonlike segment of the cornea’s center. Using a surgical microscope, the ophthalmologist then removes a similarly shaped segment from the dis- eased cornea and places the donor button in its place. Very fine suture, sometimes thinner than a human hair, secures the donor corneal button in position and remains in the eye for three months to one year. The ophthalmologist often removes the sutures a few at a time as HEALING progresses, using an ophthalmic anesthetic to numb the affected eye, depending on the rate of vision improvement. Some sutures may remain in place indefinitely.

Full recovery typically takes about a year. Some people will have ASTIGMATISM and require CORREC- TIVE LENSES following corneal transplantation, resulting from irregularities in the shape of the cornea that develop during healing. Corneal trans- plantation may correct another VISION IMPAIRMENT such as HYPEROPIA (farsightedness) or MYOPIA (near- sightedness) because the OPERATION changes the shape of the cornea.

The most common complication of corneal transplantation is rejection of the transplanted cornea, which occurs overall in about 15 percent of corneal transplantations. Rejection is most likely to take place in the first two years after the operation. Early detection and prompt treatment with ophthalmic CORTICOSTEROID MEDICATIONS can reverse the rejection process. Signs of rejection include

• diminished VISUAL ACUITY

• PAIN

• redness of the eye

• PHOTOPHOBIA (sensitivity to light)

Other complications that can occur include INFECTION and bleeding within the eye.

See also ORGAN TRANSPLANTATION; PHOTOTHERAPEU- TIC KERATECTOMY (PTK).

corrective lenses Eyeglasses or contact lenses that alter the focal point of the lightwaves entering the EYE to correct REFRACTIVE ERRORS of vision, including HYPEROPIA (farsightedness), MYOPIA (near- sightedness), ASTIGMATISM (blurred or distorted vision), and PRESBYOPIA (age-related hyperopia). The eye’s natural focusing structures, the CORNEA and the LENS, gather lightwaves and refract (bend) them toward their centers. The cornea refracts the lightwaves first. The lens, which can thicken or flatten to refine its focal efforts, refracts the some- what focused lightwaves that come to it from the cornea. In normal vision, this sequence results in the focal point of the lightwaves striking the RETINA.

When refractive errors exist the focal point falls in front of or behind the retina, resulting in blurred images. Corrective lenses add a third level of refraction to compensate for the error, bending the lightwaves before they enter the cornea to realign their focal point. The direction of refraction depends on the refractive error:

• In myopia, the focal point falls short of the retina. A lens that corrects for myopia bends the lightwaves inward, narrowing the span of light as it enters the cornea to lengthen the focal point. Such a lens is thicker at the edges than in the middle (concave); it is a minus spherical correction.

• In hyperopia, the focal point extends beyond the retina. A lens that corrects for hyperopia bends the lightwaves outward, broadening the span of light as it enters the cornea to shorten the focal point. Such a lens is thicker in the center than at the edges (convex); it is a plus spherical correction.

• In astigmatism, irregularities in the surface of the lens cause a second focal point. A lens that corrects for astigmatism refracts along a specific axis, realigning the lightwaves. This is a cylinder correction.

Corrective lenses can, and often do, combine spherical and cylindrical corrections. A multifocal lens further incorporates a correction for presbyopia in the form of a bifocal, trifocal, or progressive lens. The bottom of the lens is a plus section,

added to the corrective prescription, that accommodates the limited ability of the lens to focus on near objects (such as when reading).

Eyeglasses

Eyeglasses are plastic resin or polycarbon, and less commonly glass, lenses ground to the thicknesses and shapes necessary to achieve the desired refractive specifications. Because eyeglasses are external to the eye, they can correct for a broad range of refractive errors and are the most common means of refractive correction. Eyeglasses also can contain tints and dyes that change their color; some have additives that provide protection from ultraviolet light. About 85 percent of people who have refractive errors of vision wear eye- glasses to correct them.

Bifocal and trifocal eyeglasses have a clear shift (sometimes visible as a line on the lens) to the presbyopic correction; a progressive lens transitions to the presbyopic correction. Reading glasses such as those available without an eye care practitioner’s prescription, are magnifying lenses that enlarge close objects, requiring the lens to make less of an adjustment to bring them into focus. How well reading glasses work depends on whether there are refractive errors that remain uncorrected. With aging, most people develop at least a small degree of astigmatism, which can result in blurred or distorted images not related to presbyopia.

The primary risk of wearing eyeglasses is trau-matic injury due to a blow that strikes the glasses. The energy of such a blow concentrates initially at the contact points on the NOSE. The frame may break, causing lacerations to the face. Of more significant consequence is a blow that breaks the lens, which can result in vision-threatening injury to the eye. Polycarbonate lenses have the highest inherent shatter resistance; plastic resin and glass lenses should have shatter-resistant coatings or additives. People who engage in physical activities such as ball sports should wear polycarbonate eye- glasses or custom protective eyewear.

Contact Lenses

Contact lenses fit directly onto the eye, covering the cornea. There are two basic kinds of contact lenses in use today: gas permeable (hard) and hydrophilic (soft). Gas-permeable contact lenses float on a layer of tears over the center of the cornea and often are the contact lens of choice to correct for moderate to significant astigmatism as well as KERATOCONUS, a condition in which the cornea’s center bulges outward. Gas-permeable lenses also can correct for mild to moderate myopic and hyperopic refractive errors. Made of rigid polymers of fluorocarbon and polymethyl methacrylate, gas-permeable lenses allow oxygen molecules to pass through but do not absorb moisture from the eye. Hydrophilic contact lenses cover the entire cornea and can correct for mild to moderate myopia and hyperopia. Soft and flexible, hydrophilic lenses contain a high percentage of water and draw additional moisture from the tears to remain hydrated. A special kind of hydrophilic lens, the toric lens, is necessary to correct for astigmatism. A toric lens has varying thicknesses that compensate for corneal irregularities to correct refraction.

Contact lenses can incorporate correction for moderate presbyopia, though this tends to be a less satisfactory approach than eyeglasses. There are two methods for accommodating presbyopia with contact lenses: progressive or bifocal lenses and monovision. Progressive or bifocal contacts function much the same as progressive or bifocal eyeglasses, with the lower portion of the lens containing the presbyopic correction. Because contact lenses shift position on the eye with blinking and when the wearer alters the angle of the head (such as when lying down), the presbyopic correc- tion may not remain in an effective position. Monovision takes the approach of modifying the BRAIN’s interpretation of visual signals. One eye, usually the dominant eye, wears a contact lens with the refractive correction. The other eye wears a contact lens with the presbyopic correction. The brain learns to distinguish which signals to inter- pret, accepting those from the dominant eye during normal visual activities and those from the other eye when reading or doing close-focus work.

The primary risks of wearing contact lenses are damage to the cornea and INFECTION. Even hydrophilic lenses can irritate the cornea and cause corneal ABRASIONS, particularly in dusty, windy, or dry environmental conditions. Contact lenses tend to accumulate protein deposits that cause irritation. Most hydrophilic lenses are disposable, so frequent replacement helps minimize this as a problem. The optician may need to clean or gently grind the surface of gas-permeable lenses to clear away deposits. Contact lens hygiene, including diligent HAND WASHING before handling lenses and storing lenses in the appropriate disinfectant solution, is essential.

Reading a Corrective Lens Prescription Optometrists and ophthalmologists measure refractive errors in diopters, a representational scale of the distance in front of or behind the eye’s lens the focal point of lightwaves entering the eye must shift to allow the light waves to clearly focus on the retina. The larger the diopter number, the more the lens refracts, or bends, the light. A corrective lens prescription represents the diopter as

minus or plus, according to the direction the correction shifts the focal point. For example, the following prescription corrects for myopia and astigmatism:

This prescription denotes different refractive corrections for the right eye (OD) and left eye (OS). The minus diopter is the spherical correction for the myopia; the plus diopter is the cylindrical correction for the astigmatism, and the last number is the axis position for the cylindrical correction. A lens with a strong correction may also include an adjustment that tilts the lens to alter its optical center, the prism, allowing a thinner lens to deliver the same corrective power or to accommodate a significant difference in the refractive correction for each eye (anisometropia).

See also REFRACTION TEST; REFRACTIVE SURGERY; VISION IMPAIRMENT.

black eye Bleeding into the tissues around the EYE resulting from trauma to the area, such as a blow or surgical OPERATION. As with any other bruise, the bleeding causes swelling and discoloration. The most significant concerns with a black eye are damage to the eye or fractures of the orbital bones, which require immediate medical attention.

Seek immediate medical attention if these symptoms accompany a black eye:

• seeing dark spots (FLOATERS) or flashes of light

• any cuts on the insides of the eyelids or on the eye

• blurry, distorted, or double vision

• numbness on same side of the face

• trouble moving the eye to look up, down, or to either side

Treatment for a simple black eye is cold to the area as quickly as possible after the injury occurs and at frequent intervals during the first 24 hours or until the PAIN subsides and the swelling stabilizes. A black eye may take two weeks to fully heal, and undergoes a number of color changes as HEALING progresses. People who participate in sports such as softball, baseball, basketball, tennis, racquetball, soccer, and similar events should wear appropriate eye protection.

See also BLEPHAROPLASTY; ORBITAL CELLULITIS; RETI- NAL DETACHMENT; RHINOPLASTY; TRAUMA TO THE EYE; VITREOUS DETACHMENT.

blepharitis INFLAMMATION of the eyelids. The most common causes of blepharitis are INFECTION and irritation. Anterior blepharitis is INFLAMMATION of the outer surface of the eyelid, typically along the rim at the base of the eyelashes. Posterior blepharitis affects the inner surface of the eyelid, typically resulting from blocked oil glands (meibomian glands) along the eyelid. Symptoms of either form of blepharitis may include

• itching or burning sensation

• crusting along the eyelids, especially upon awakening

• swelling and redness

• PHOTOPHOBIA (excessive light sensitivity)

• excessive tearing

• blurry vision

Blepharitis may develop as a result of other conditions such as DERMATITIS or ROSACEA (disorders that cause SKIN inflammation). When this is the case, treatment targets the underlying condition. When the cause of the inflammation is bacterial, treatment is topical and sometimes oral ANTIBIOTIC MEDICATIONS. Occasionally the viruses HERPES SIM- PLEX I (which causes cold sores) and HERPES ZOSTER (which causes shingles) can infect the eyes. Viral infections such as these cause symptoms until they run their course, typically in 7 to 10 days. Whatever the cause of the inflammation, moist, warm compresses help loosen crusted secretions and keep the eyelids clean. EYE care professionals recommend gently washing the eyelids with a mixture of water and baby (tear-free) shampoo. Blepharitis tends to be chronic so good eyelid hygiene helps minimize recurrences as well as discomfort during episodes.

See also BACTERIA; CHALAZION; CONJUNCTIVITIS; DANDRUFF; DRY EYE SYNDROME; HORDEOLUM; ORBITAL CELLULITIS.

blepharoplasty A surgical OPERATION to remove excess tissue from the eyelids to correct drooping upper eyelids and “baggy” lower eyelids. Such conditions most commonly develop as a consequence of aging or extensive weight loss or when there is damage to the nerves that control the eye- lid muscles (such as with PARKINSON’S DISEASE). An ophthalmologist or a plastic surgeon performs blepharoplasty, usually as an AMBULATORY SURGERY (also called outpatient or same-day surgery). Recovery takes two to four weeks; there can be significant swelling, bruising, and discoloration especially during the first two weeks after the operation. Cold compresses help reduce these symptoms. The risks of blepharoplasty include excessive bleeding and INFECTION.

See also BLACK EYE; BLEPHAROSPASM; PLASTIC SUR- GERY; PTOSIS; RHINOPLASTY; RHITIDOPLASTY; SURGERY BENEFIT AND RISK ASSESSMENT.

blepharospasm Involuntary closure of the eyelid that results from dysfunction of or damage to the nerves that control the muscles of the eyelids. Episodes of closure may range from brief (a minute) to extended (several hours). Extended closure interferes with vision. Doctors do not know what causes blepharospasm, though it is a symptom of numerous neurologic and neuromus- cular disorders that affect MUSCLE control such as PARKINSON’S DISEASE and DYSTONIA. Blepharospasm that develops without an apparent underlying dis- order is benign essential blepharospasm. Ble- pharospasm often begins with minor twitches and tics or squinting, progressing over time to forceful and prolonged contraction of the eyelid muscles. PHOTOPHOBIA (sensitivity to light) is common. Fatigue and CAFFEINE may initiate episodes of spasms.

The diagnostic path may include a NEUROLOGIC EXAMINATION to determine whether underlying neurologic disorders exist. Blepharospasm requires treatment when it begins to interfere with the activities of everyday life. Moderate blepharospasm often responds to MUSCLE RELAXANT MEDICATIONS such as clonazepam (Klonopin) or Lioresal (Baclofen), or to medications used to treat Parkinson’s disease such as levodopa. Many people obtain long-term relief from injections of botulinum toxin, which temporarily paralyzes the eyelid muscles. Surgery to remove muscle tissue (myectomy) or to cut the nerves supplying the eyelid muscles (neurectomy) may provide relief. Though therapeutic measures can control symptoms, as yet there is no cure for blepharospasm.

See also BOTULINUM THERAPY; PTOSIS; SURGERY BEN- EFIT AND RISK MANAGEMENT; TIC.

blindness See VISION IMPAIRMENT.

braille A tactile (touch-based) system of written language that features patterns of raised dots to represent letters of the alphabet, common words and contractions, mathematical symbols, and punctuation. Named after its developer, Louis Braille (1809–1852), braille allows people who are blind to read and, with adaptive typewriters and computer technology, to write. Six dots, in two columns of three dots each, form the foundation for braille; the presence or absence of dots in specific patterns identifies the letter, number, symbol, or concept. There are a number of braille variations, or codes, in common use in the United States. The major ones are these:

• American literary braille code uses about 250 patterns to create book-length materials using short-form words, contractions, single-cell words, and symbols; patterns may have multiple meanings interpreted by context.

• Grade 2 braille code is an abbreviated variation of American literary braille code used primarily for recreational reading materials such as novels and nonacademic nonfiction.

• Grade 1 braille code is the basic alphabet and numerals 0 through 9.

• Nemeth braille code contains about 600 unique, specialized patterns that are distinct from American literary braille code for use in mathematics and science.

• Computer braille code provides a mix of American literary braille code, Nemeth braille code, and unique symbols for computer programming and instruction documentation.

• Music braille code is specialized for transcribing musical scores.

Learning each variation of braille code is like learning a different language. Most people learn the one or two variations they are most likely to use. People whose vision is intact also can learn braille, and should if they have regular interactions with people who are blind. Many communities have schools and consultants who teach braille as well as libraries that provide braille publications. Most public signage in the United States includes braille translations.

See also VISION IMPAIRMENT.

bullous keratopathy Swelling (edema) and blistering of the CORNEA. Bullous keratopathy most commonly develops as a complication following CATARACT EXTRACTION AND LENS REPLACEMENT or other surgery on the EYE, though it also may develop as a consequence of chronic irritation such as might occur with DRY EYE SYNDROME.

The healthy cornea is about 75 percent water. One function of the cells that surround the cornea is to maintain this fluid balance. Irritation and trauma that damage these cells diminishes their ability to function, and the cornea retains more water. The swelling stretches the surface of the cornea, pushing the cornea into closer contact with the eyelid and resulting in further irritation. Bullae, or blisters, develop as the cornea’s attempt to relieve the discomfort, much as blisters develop on the feet or hands in reaction to friction.

Early symptoms of bullous keratopathy are a sensation of grittiness in the eye, blurred vision, excessive tearing, and PHOTOPHOBIA (sensitivity to light). When bullae form, and especially when they rupture, the PAIN often is severe. The ophthalmologist can diagnose bullous keratopathy using SLIT LAMP EXAMINATION of the cornea, a pain- less procedure that combines an intense light focused in a slit with magnification through a ophthalmologic microscope. Eye drops or ointment with a higher saline concentration than tears helps draw fluid out of the cornea, reducing the swelling. Soft contact lenses, which absorb fluid from the eye and shield the cornea from con- tact with the eyelid, relieve discomfort for many people. Bullous keratopathy tends to be chronic, and over time may result in damage to the cornea that requires the cornea’s surgical removal (kera- totomy) or CORNEAL TRANSPLANTATION.

See also BLISTER; KERATITIS; UVEITIS.

black eye Bleeding into the tissues around the EYE resulting from trauma to the area, such as a blow or surgical OPERATION. As with any other bruise, the bleeding causes swelling and discoloration. The most significant concerns with a black eye are damage to the eye or fractures of the orbital bones, which require immediate medical attention.

Seek immediate medical attention if these symptoms accompany a black eye:

• seeing dark spots (FLOATERS) or flashes of light

• any cuts on the insides of the eyelids or on the eye

• blurry, distorted, or double vision

• numbness on same side of the face

• trouble moving the eye to look up, down, or to either side

Treatment for a simple black eye is cold to the area as quickly as possible after the injury occurs and at frequent intervals during the first 24 hours or until the PAIN subsides and the swelling stabilizes. A black eye may take two weeks to fully heal, and undergoes a number of color changes as HEALING progresses. People who participate in sports such as softball, baseball, basketball, tennis, racquetball, soccer, and similar events should wear appropriate eye protection.

See also BLEPHAROPLASTY; ORBITAL CELLULITIS; RETI- NAL DETACHMENT; RHINOPLASTY; TRAUMA TO THE EYE; VITREOUS DETACHMENT.

blepharitis INFLAMMATION of the eyelids. The most common causes of blepharitis are INFECTION and irritation. Anterior blepharitis is INFLAMMATION of the outer surface of the eyelid, typically along the rim at the base of the eyelashes. Posterior blepharitis affects the inner surface of the eyelid, typically resulting from blocked oil glands (meibomian glands) along the eyelid. Symptoms of either form of blepharitis may include

• itching or burning sensation

• crusting along the eyelids, especially upon awakening

• swelling and redness

• PHOTOPHOBIA (excessive light sensitivity)

• excessive tearing

• blurry vision

Blepharitis may develop as a result of other conditions such as DERMATITIS or ROSACEA (disorders that cause SKIN inflammation). When this is the case, treatment targets the underlying condition. When the cause of the inflammation is bacterial, treatment is topical and sometimes oral ANTIBIOTIC MEDICATIONS. Occasionally the viruses HERPES SIM- PLEX I (which causes cold sores) and HERPES ZOSTER (which causes shingles) can infect the eyes. Viral infections such as these cause symptoms until they run their course, typically in 7 to 10 days. Whatever the cause of the inflammation, moist, warm compresses help loosen crusted secretions and keep the eyelids clean. EYE care professionals recommend gently washing the eyelids with a mixture of water and baby (tear-free) shampoo. Blepharitis tends to be chronic so good eyelid hygiene helps minimize recurrences as well as discomfort during episodes.

See also BACTERIA; CHALAZION; CONJUNCTIVITIS; DANDRUFF; DRY EYE SYNDROME; HORDEOLUM; ORBITAL CELLULITIS.

blepharoplasty A surgical OPERATION to remove excess tissue from the eyelids to correct drooping upper eyelids and “baggy” lower eyelids. Such conditions most commonly develop as a consequence of aging or extensive weight loss or when there is damage to the nerves that control the eye- lid muscles (such as with PARKINSON’S DISEASE). An ophthalmologist or a plastic surgeon performs blepharoplasty, usually as an AMBULATORY SURGERY (also called outpatient or same-day surgery). Recovery takes two to four weeks; there can be significant swelling, bruising, and discoloration especially during the first two weeks after the operation. Cold compresses help reduce these symptoms. The risks of blepharoplasty include excessive bleeding and INFECTION.

See also BLACK EYE; BLEPHAROSPASM; PLASTIC SUR- GERY; PTOSIS; RHINOPLASTY; RHITIDOPLASTY; SURGERY BENEFIT AND RISK ASSESSMENT.

blepharospasm Involuntary closure of the eyelid that results from dysfunction of or damage to the nerves that control the muscles of the eyelids. Episodes of closure may range from brief (a minute) to extended (several hours). Extended closure interferes with vision. Doctors do not know what causes blepharospasm, though it is a symptom of numerous neurologic and neuromus- cular disorders that affect MUSCLE control such as PARKINSON’S DISEASE and DYSTONIA. Blepharospasm that develops without an apparent underlying dis- order is benign essential blepharospasm. Ble- pharospasm often begins with minor twitches and tics or squinting, progressing over time to forceful and prolonged contraction of the eyelid muscles. PHOTOPHOBIA (sensitivity to light) is common. Fatigue and CAFFEINE may initiate episodes of spasms.

The diagnostic path may include a NEUROLOGIC EXAMINATION to determine whether underlying neurologic disorders exist. Blepharospasm requires treatment when it begins to interfere with the activities of everyday life. Moderate blepharospasm often responds to MUSCLE RELAXANT MEDICATIONS such as clonazepam (Klonopin) or Lioresal (Baclofen), or to medications used to treat Parkinson’s disease such as levodopa. Many people obtain long-term relief from injections of botulinum toxin, which temporarily paralyzes the eyelid muscles. Surgery to remove muscle tissue (myectomy) or to cut the nerves supplying the eyelid muscles (neurectomy) may provide relief. Though therapeutic measures can control symptoms, as yet there is no cure for blepharospasm.

See also BOTULINUM THERAPY; PTOSIS; SURGERY BEN- EFIT AND RISK MANAGEMENT; TIC.

blindness See VISION IMPAIRMENT.

braille A tactile (touch-based) system of written language that features patterns of raised dots to represent letters of the alphabet, common words and contractions, mathematical symbols, and punctuation. Named after its developer, Louis Braille (1809–1852), braille allows people who are blind to read and, with adaptive typewriters and computer technology, to write. Six dots, in two columns of three dots each, form the foundation for braille; the presence or absence of dots in specific patterns identifies the letter, number, symbol, or concept. There are a number of braille variations, or codes, in common use in the United States. The major ones are these:

• American literary braille code uses about 250 patterns to create book-length materials using short-form words, contractions, single-cell words, and symbols; patterns may have multiple meanings interpreted by context.

• Grade 2 braille code is an abbreviated variation of American literary braille code used primarily for recreational reading materials such as novels and nonacademic nonfiction.

• Grade 1 braille code is the basic alphabet and numerals 0 through 9.

• Nemeth braille code contains about 600 unique, specialized patterns that are distinct from American literary braille code for use in mathematics and science.

• Computer braille code provides a mix of American literary braille code, Nemeth braille code, and unique symbols for computer programming and instruction documentation.

• Music braille code is specialized for transcribing musical scores.

Learning each variation of braille code is like learning a different language. Most people learn the one or two variations they are most likely to use. People whose vision is intact also can learn braille, and should if they have regular interactions with people who are blind. Many communities have schools and consultants who teach braille as well as libraries that provide braille publications. Most public signage in the United States includes braille translations.

See also VISION IMPAIRMENT.

bullous keratopathy Swelling (edema) and blistering of the CORNEA. Bullous keratopathy most commonly develops as a complication following CATARACT EXTRACTION AND LENS REPLACEMENT or other surgery on the EYE, though it also may develop as a consequence of chronic irritation such as might occur with DRY EYE SYNDROME.

The healthy cornea is about 75 percent water. One function of the cells that surround the cornea is to maintain this fluid balance. Irritation and trauma that damage these cells diminishes their ability to function, and the cornea retains more water. The swelling stretches the surface of the cornea, pushing the cornea into closer contact with the eyelid and resulting in further irritation. Bullae, or blisters, develop as the cornea’s attempt to relieve the discomfort, much as blisters develop on the feet or hands in reaction to friction.

Early symptoms of bullous keratopathy are a sensation of grittiness in the eye, blurred vision, excessive tearing, and PHOTOPHOBIA (sensitivity to light). When bullae form, and especially when they rupture, the PAIN often is severe. The ophthalmologist can diagnose bullous keratopathy using SLIT LAMP EXAMINATION of the cornea, a pain- less procedure that combines an intense light focused in a slit with magnification through a ophthalmologic microscope. Eye drops or ointment with a higher saline concentration than tears helps draw fluid out of the cornea, reducing the swelling. Soft contact lenses, which absorb fluid from the eye and shield the cornea from con- tact with the eyelid, relieve discomfort for many people. Bullous keratopathy tends to be chronic, and over time may result in damage to the cornea that requires the cornea’s surgical removal (kera- totomy) or CORNEAL TRANSPLANTATION.

See also BLISTER; KERATITIS; UVEITIS.