A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z

The ability of the eye muscles to improve the eye’s focus by contracting or relaxing in order to change the shape of the lens; in other words, accommodation is the extent to which the eye can accommodate for imperfect refraction.

Age-Related Macular Degeneration (AMD)
A disease that destroys the macula, a cluster of light-sensitive cells in the central part of the retina.

See age-related macular degeneration (AMD).

A type of strabismus in which the eyes are unable to coordinate their alignment or to maintain parallel focus, causing the brain to accept images from the stronger eye and ignore images from the weaker eye.

Amsler Grid
An Amsler grid is a chart composed of horizontal and vertical black lines on a white background with a black dot at the center. This chart is a simple screening tool for age-related macular degeneration (AMD). If you have AMD, staring at the center dot will make the grid lines around it appear gray, blurry, or distorted.

Angle-Closure Glaucoma
A type of glaucoma in which fluid pressure mounts rapidly when a sudden protrusion of the iris blocks drainage channels.

Antiretroviral Agents
Drugs that keep the human immunodeficiency virus (HIV) from replicating. See also HAART.

Aqueous Humor
A clear fluid that circulates in a chamber behind the cornea and in front of the iris.

See computer eye strain.

Basal Cell Carcinoma
A carcinoma that can appear on the eyelid or elsewhere near the eye, such as beneath the brow; it arises from cells in the basal (meaning "base") layer of the epithelium, where new skin cells are produced.

Eyelid inflammation, redness, and tenderness caused by bacterial overgrowth and consequent inflammation of the oil-secreting meibomian glands near the eyelashes.

Chalazion Cyst
A chronic accumulation of sebum, white blood cells, and other waste products, encased in a fibrous capsule within an oil gland (meibomian gland) in the eyelid; the cyst appears as a painless, elevated patch that tends to be larger than a stye.

A clouding of the lens of the eye that obscures vision by making objects look hazy.

A rich network of blood vessels beneath the sclera that nourishes and supplies oxygen to the retina and other internal structures of the eye.

Computer Eye Strain
Also known as "digital eye fatigue" and "computer vision syndrome," computer eye strain is a collection of problems related to prolonged computer use. Symptoms include eye fatigue, headaches, and dry, itchy, watery, or burning eyes. The medical term for eye strain is asthenopia.

Congenital Glaucoma
A type of glaucoma that occurs in infants born with a narrow angle of drainage in the eye.

Contact Tonometry
Measurement of eye pressure using a pressure gauge that makes contact with the eye after anesthetic (numbing) drops are instilled. It is more accurate than non-contact tonometry.

The transparent, dome-like window at the front of the eye that covers the iris and pupil. The cornea admits light but screens out nearly all of the UVB radiation in sunlight, protecting the lens and retina.

Cytomegalovirus (CMV) Retinitis
Inflammation of the retina caused by infection with a specific virus from the herpes family.

A problem in which the eye appears to turn in or out; in other words, the gaze deviates from a normal path. Deviation is associated with amblyopia, strabismus, and other disorders.

Diabetic Retinopathy
A disease associated with diabetes in which chronic high blood glucose causes fragile blood vessels in the eye to grow where they’re not needed (a process called neovascularization). These blood vessels tend to swell and burst, leaking blood and other fluids beneath the retina, which produces inflammation and eventual scarring. Diabetic retinopathy is the leading cause of preventable low vision and blindness in the United States.

A unit of measurement used to express refraction. Myopia is indicated by a negative sign before the number of diopters, whereas hyperopia is designated by a positive number.

Dry Age-Related Macular Degeneration (AMD)
A mild form of age-related macular degeneration (AMD) in which the light-sensitive cells and supportive tissues of the macula break down over time and central vision gradually deteriorates. In dry AMD, no bleeding or fluid leakage occurs; however, this stage of the disease often precedes a more dangerous form of AMD called wet age-related macular degeneration (AMD).

Deposits on the retina associated with a higher risk of developing age-related macular degeneration (AMD).

Dry Eye
A condition in which the mucous membranes of the eye become inflamed and the eye’s corneal surface dries out, causing discomfort and sometimes permanently damaging the cornea. The condition occurs when too little tear film is produced or when the tear film has an abnormal composition, is inadequately distributed, or evaporates too quickly.

Thin, delicate tissues that limit the amount of light entering the eye, help keep the eye moist, and block pollen and dust particles and other foreign bodies.

See hyperopia.

Fluorescein Angiography
Use of a dye called fluorescein to screen for age-related macular degeneration (AMD). The dye is administered intravenously (injected into a vein) and taken up by vessels of the eye, allowing the vessels of the choroid and retina to be visualized.

A tiny pit at the center of the macula that contains a high concentration of photoreceptor cells; the fovea is responsible for producing color and sharp-detail vision.

Geographic Atrophy
A round, well-defined, nearly transparent spot in the center of the macula of a person with dry age-related macular degeneration (AMD). The atrophied tissue thins and loses pigment, causing the person to see a blur or blank spot.

A group of related diseases in which the optic nerve becomes damaged by excessive fluid pressure within the eyeball.

A painless test that measures the angle at which fluid drains from the eye.

An acronym for highly active antiretroviral therapy, a combination of drugs used to keep HIV from replicating in the bloodstream of an infected person.

A stye at the base of the eyelid. Derived from the Latin word for "barley," in reference to the stye’s grain-like size and appearance.

Hypercholesterolemia, Familial
An inherited disorder that causes high blood cholesterol

The medical term for farsightedness, a condition in which a person has trouble seeing objects close to him or her. Hyperopia is measured in diopters. The farther away from zero the number of diopters is, the more hyperopia the person has.

A muscle made up of specialized fibers that are able to constrict the pupil to about 1 mm or dilate it to about 10 mm.

Keratoconjunctivitis Sicca
The medical term for dry eye.

To drain; A healthcare professional may use this technique to relieve pressure within a lesion, such as a stye.

A transparent elastic disc that lies behind the iris suspended by muscles that can pull taut or slacken to change the shape of the lens in order to focus images on the retina.

A term used to describe any tissue damage (for example, a chemical burn to the cornea), pathology (for example, basal cell carcinoma, cataracts), or loss of function (for example, a macula that is no longer functioning properly because of age-related macular degeneration [AMD]).

Light Reflex
The narrowing and widening of the pupil in response to changing light conditions.

Low-Tension Glaucoma
A type of open-angle glaucoma in which the optic nerve is damaged despite low-to-normal eye pressure (tension).

Low Vision
World Health Organization definition: "A person with low vision is one who has impairment of visual functioning even after treatment and/or standard refractive correction, and has a visual acuity of less than 6/18 to light perception, or a visual field of less than 10 degrees from the point of fixation, but who uses, or is potentially able to use,
vision for planning and/or execution of a task."

A cluster of light-sensitive cells in the central part of the retina that is responsible for crisp central vision and perception of fine detail.

Macular Edema
Fluid accumulation that causes the macula to swell, thickening it and elevating it from its normal position. Macular edema can occur during any stage of diabetic retinopathy.

Of or related to a physical obstruction or dysfunction (as opposed to a pathological disease process).

Meibomian Glands
Tiny glands at the base of the eyelid that secrete sebum, which helps keep tear film from evaporating. Each eyelid has about 100 such glands.

Multifactorial Disease
A condition caused not by a single factor, but by several factors in combination

The medical term for nearsightedness, a condition characterized by an elongated eyeball, in which light is focused in front of the retina instead of on it, causing images to look blurry. Myopia is the most common eye disorder in the world and is a leading cause of visual impairment. It is measured in diopters. The farther away from zero the number of diopters is, the more myopia you have.

See myopia.

Abnormal growth of new blood vessels in the retina. Neovascularization occurs in wet age-related macular degeneration (AMD), proliferative diabetic retinopathy, and neovascular glaucoma.

Non-Contact Tonometry
Measurement of eye pressure using an instrument that estimates eye pressure by gauging the cornea’s resistance to a quick blast of air. It is less accurate than contact tonometry.

Nonproliferative Diabetic Retinopathy (also called simple or background retinopathy)
An early stage of diabetic retinopathy during which no new blood vessel growth (proliferation) has occurred. Nonproliferative diabetic retinopathy usually does not diminish vision and may cause no symptoms.

A treatment for amblyopia in which the stronger eye is covered (occluded) with a patch to obscure vision for several weeks or months, forcing the brain to accept images from the weaker eye.

Ocular Melanoma
Melanoma is a dangerous cancer that arises from cells that produce a pigment called melanin. This pigment is found in both your skin and your eyes, so a melanoma may be either a skin cancer or an eye (ocular) cancer. Research has found a clear association between UVA radiation and the development of melanoma of the eye (see ultraviolet [UV] radiation).

Ocular Tuberculosis
Infection with the bacillus Mycobacterium tuberculosis causes uveitis, an inflammatory condition associated with retinal detachment, glaucoma, cataracts, and other causes of low vision. Infection with this pathogen also affects the bones and joints, skin, gastrointestinal system, genitourinary system, and other body organs and systems.

Open-Angle Glaucoma
A type of glaucoma in which drainage fails to keep pace with the output of new fluid.

Dilation of the pupil to view the interior structures of the eye, including the optic nerve, with the aid of an ophthalmoscope.

Opportunistic Infection
An infection that exploits the body’s weakened state during a period of illness, particularly when the immune system is impaired by HIV/AIDS, cancer, or some other serious condition.

Optical Coherence Tomography (OCT)
A safe, painless test that produces high-resolution images of three-dimensional cross-sections of the retina, allowing its thickness to be measured. Areas of thinning may indicate age-related macular degeneration (AMD).

Optic Nerve
A bundle of more than a million long, slender retinal fibers that carry visual images from the retina to the visual cortex of the brain.

Orbital Cavity
The "eye socket," a cone-shaped pocket within the skull that protects the eyeball from trauma, such as injuries that occur during car accidents, falls, and assaults.

A procedure in which a series of specially fitted rigid contact lenses, worn while during sleep, is used to reshape the corneas. Retainer lenses must be worn at night every few days, or the eyes will gradually return to their previous curvature. Orthokeratology is an alternative to wearing corrective lenses or having refractive surgery.

Measurement of corneal thickness using a pachymeter. This measurement helps your vision care provider interpret your eye pressure readings when screening for glaucoma.

A small, benign, wart-like growth arising from squamous skin cells and sometimes attached by a stalk of connective tissue called a peduncle. A papilloma can have a smooth or coarse surface and may be either pigmented or similar in color to the surrounding skin.

An infectious organism, such as a virus, bacterium, or fungus.

Of or related to a disease caused by an infectious organism, a genetic mutation, or some other non-mechanical process.

A stalk or stem of connective tissue that anchors a pathological lesion such as a papilloma.

Penetrating Eye Trauma
A type of trauma (injury) that occurs when a sharp object penetrates the eye. Objects that can cause penetrating trauma include knives, metal fragments, BB gun pellets, glass, plastic, and even animal bites or scratches.

To detach, as in a perforated cornea or retina.

A specialized instrument used to measure refraction; it resembles a pair of oversized goggles hanging from a boom.

A painful corneal burn caused by short-term exposure to UVB rays from any highly reflective surface, such as concrete, water, sand, or snow. See ultraviolet (UV) radiation.

Photoreceptor Cells
The light-sensitive rod and cone cells that capture electrical impulses and transmit images along the optic nerve to the visual cortex of the brain.

A yellowish patch or lump on the conjunctiva. More nodular in shape than a pterygium, a pinguecula is a noncancerous accumulation of protein and fat deposits. Like a pterygium, it grows slowly; however, it does not invade the cornea.

Hyperopia (farsightedness) that develops as the lens of the eye and its surrounding muscles lose elasticity and can no longer change shape. This condition keeps the lens from focusing light on the retina in order to produce crisp visual images. Presbyopia is different from normal farsightedness, which is caused by a distortion in the shape of the eyeball.

Primary Biliary Cirrhosis
Inflammation of the bile ducts of the liver; this condition is a risk factor for styes and chalazion cysts.

Proliferative Diabetic Retinopathy
An advanced stage of diabetic retinopathy during which lack of blood flow (because vessels are bleeding out into the vitreous rather than reaching the retina) starves the retina of oxygen and nutrients. As more and more vessels fail to deliver nourishment, abnormal new blood vessel growth (proliferation) begins. Hemorrhaging of these fragile new vessels causes inflammation and scarring.

A thin, wedge-shaped growth of fibrous tissue extending from the corner of the eye to the cornea. A pterygium is not cancerous and grows slowly, but may spread over the cornea enough to impede vision.

When applied to the eyelids, the term ptosis refers to a loss of elasticity that causes the lids to droop, giving them a draped or hooded appearance that may be pronounced enough to obscure vision.

A round opening in the center of the iris. The pupil appears to be black because most of the light that passes through it is absorbed by pigmented cells in the tissue at the back of the eye.

The power of a lens (such as the lens of the eye) to bend light.

Refraction Testing
The process of measuring how the eye focuses light. The test is performed with the aid of a phoropter. Your vision care provider adjusts a series of lenses inside the phoropter as you read the letters on a projection screen. Your provider selects stronger and stronger lenses until the letters look crisp to you.

Refractive Error
A mechanical problem in which the light fails to bend properly as it moves through the eye. Myopia (nearsightedness) and hyperopia (farsightedness) are examples of refractive errors.

Refractive Surgery
Surgery to reshape the cornea as an alternative to wearing contact lenses or glasses. Methods of refractive surgery include various laser procedures, such as LASIK surgery and photorefractive keratectomy (PRK).

A structure that lies beneath the choroid and contains light-sensitive rod cells and cone cells, which convert light into electrical impulses and transmit them to the brain via the optic nerve. The retina makes up the innermost layer, or lining, of the eye.

Retinal Pigment Epithelium (RPE)
A thin layer of cells just beneath the rod cells of the retina. The RPE digests waste shed from the rod cells.

Schirmer Test
A painless test in which specially treated blotting paper is used to determine how much tear film the eyes are producing.

Sometimes called the "white of the eye," the sclera is an envelope of tissue that extends from the cornea, at the front of the eye, to the optic nerve, at the back of the eye, providing a sheath of protection for the eye’s internal structures.

Scoring Tool for Assessing Risk (STAR)
A tool designed to aid medical decision making by estimating your risk of developing glaucoma. STAR is used if you have elevated eye pressure, making you a glaucoma suspect. The tool is a kind of slide rule that takes into account age, corneal thickness (see pachymetry), and several other technical measurements to estimate the likelihood that elevated eye pressure will convert to glaucoma during the next 5 years. Using STAR allows providers to identify at-risk patients before any vision loss has occurred.

Secondary Glaucoma
Glaucoma is said to be secondary if it is caused by another disease. For instance, glaucoma might be associated with (secondary to) diabetes, which would then be considered the primary disease.

The aging process.

An itchy rash or scaly patches on the skin or scalp (called "dandruff" when it affects the scalp).

Oil secretions from meibomian glands in the eyelid.

Describes a condition that improves or disappears on its own, without medical attention, within a brief, predictable period of time.

Snellen Eye Chart
A visual acuity chart with a large block letter (usually an E) at the top and progressively smaller rows of letters beneath it. The chart, named for the ophthalmologist who invented it in 1863, helps your provider determine how well you see at various distances.

Squamous Cell Carcinoma
A relatively aggressive cancer that arises in the squamous epithelium of the skin. It is associated with chronic sun exposure and may not appear for decades.

A disorder in which the eyes are improperly aligned because of poor muscle coordination, causing substantial deviation of one or both eyes. See also amblyopia.

Stye (Sty)
An actively infected oil gland (meibomian gland) that appears as a tender lump or bump near the eyelashes or on the eyelid.

See contact tonometry.

Applied to the skin (for example, a topical antibiotic ointment or cream).

Toxoplasmosis Retinitis
Inflammation of the choroid and retina that causes eye pain, sensitivity to light, and blurred vision, blind spots, or vision loss. Toxoplasmosis retinitis is a manifestation of toxoplasmosis infection and often occurs in patients who have advanced AIDS.

A network of channels for draining aqueous humor from the eye.

Laser surgery to improve passage of fluid out of the eye.

Traumatic Glaucoma
A type of glaucoma associated with blunt eye injury, such as being punched in a fight or being struck with a hockey puck, tennis racket, or rock.

Ultraviolet (UV) Radiation
Energy of a particular wavelength that spreads out (radiates) from its source, such as the sun. The sun’s electromagnetic energy consists of UVA, UVB, and UVC rays.

Swelling of the middle layer of the eye.

Viral Load
The amount of a virus, such as human immunodeficiency virus (HIV), in the bloodstream of an infected person.

Vision Rehabilitation
Programs that help people adapt to visual impairment, make decisions about their care, and teach friends and loved ones in caregiver roles how to help effectively. Teaching hospitals, community centers, senior centers, and government agencies often offer vision rehabilitation services.

Visual Acuity
Visual ability at various distances.

Visual Acuity Testing
Evaluation of visual acuity using a Snellen eye chart. From a specified distance, usually 20 feet, your vision care provider will ask you to read aloud progressively smaller rows of capital letters on the chart.

Visual Cortex
A specialized area of the brain in which electrical impulses transmitted by the optic nerve are reconstituted into fully formed images.

Visual Field
The radius of your vision while your eyes are fixed on a central point.

Visual Field Test
Testing one eye at a time, a visual field test maps your response to a series of lights flashed in the central and peripheral vision (perimeter, or side vision). The map indicates which areas you can and can’t see (the visual field), based on which lights you responded to.

Vitreous Humor
A clear, gelatin-like substance that makes up 80% of the mass of the eyeball and allows it to maintain its spherical shape.

Wet Age-Related Macular Degeneration (AMD)
The advanced form of age-related macular degeneration (AMD), in which the blood vessels of the choroid migrate beneath and into the retina, leaking blood and other fluid that inflames and eventually scars the macula. Ninety percent of people with AMD–related low vision have this form of the disease.

Raised, bumpy discolorations, caused by an accumulation of cholesterol, that often resemble raw chicken skin or have a cobblestone appearance.



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