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Glossary of Eye Conditions

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ACCOMMODATION: The ability of the eye to maintain a clear focus as objects are moved closer to it by changing the shape of the lens.

ACHROMATOPSIA (color blindness): Inability to distinguish colors. A congenital cone disorder. Tinted lenses can improve visual function. This is an autosomal recessive condition with no associated abnormalities. Functional visual problems include:

ALBINISM: Total or partial lack of pigment in eyes, hair, and skin. A congenital condition. Corrective lens for refractive errors, contact lens, or tinted lenses for light sensitivity may be prescribed. Functional problems include:

AMBLYOPIA (lazy eye): Reduced visual functioning in one eye without discernable ocular abnormality.

ANIRIDIA: A rare congenital condition in which the iris does not develop fully (partial to almost complete absence of iris) that is often associated with glaucoma and/or cataracts. Corrective lenses may be prescribed. Functional vision problems include:

ANOPHTHALMIA: Absence of one or both eyeballs.

ANOXIA/HYPOXIA: Condition in which there is an absence of oxygen supply to an organ’s tissues although there is adequate blood flow through the tissue.

APHAKIA: Absence of the lens. May be congenital condition or following cataract removal. Results in an inability to accommodate, which can lead to depth perception problems or peripheral field distortions. Corrective lenses may be prescribed.

AQUEOUS HUMOR: Clear, watery fluid bathing the lens that fills the space between the back surface of the cornea and the front surface of the vitreous; nourishes the cornea, iris, and lens and maintains intraocular pressure.

ASTIGMATISM: A refractive error caused by a sphero-cylindrical curvature of the cornea; corrected with a cylindrical lens. Blurred vision is a functional vision problem associated with astigmatism.

AXENFELD-REIGER SYNDROME: Syndrome associated with high intraocular pressures accompanied by hazy corneas (congenital glaucoma).

AUGMENTATIVE VISUAL CUE: An object, such as a light, that highlights another object to draw attention to it.


BINOCULAR VISION: Vision that uses both eyes to form a fused image in the brain and results in three-dimensional vision.


CATARACT: Cloudiness of the lens that may prevent passage of light through the lens. Surgical removal is usually recommended. Functional visual problems include:

CENTRAL VISION: Vision that results when an eye looks directly at an object so that its image is focused on the fovea.

CHARGE ASSOCIATION: Rare disorder in which at least four of six characteristics are necessary for diagnosis. A sensorineural or mixed hearing loss can occur, which can be mild to profound. Each letter of the word “CHARGE” stands for one of the characteristics.

CHORIORETINITIS: Inflammation of the choroid and retina of the eye. This condition sometimes leads to a scotoma. Treatment includes medications such as penicillin. Functional visual problems include:

CHOROID: Vascular layer of the eye, between the sclera and retina, which nourishes the retina; part of the uveal tract.

CILIARY BODY: Tissue inside the eye, composed of the ciliary processes and ciliary muscle; the former secretes aqueous, the latter controls the shape of the lens.

CLINICAL VISION ASSESSMENT: Eye examination that includes tests conducted in an eye health care worker’s office. The exam may include dilating the pupil to view blood vessels and nerves, using a hand-held slit lamp exam to view the front and some internal parts of the eye, using an indirect ophthalmoscope to determine the health of internal structures including the retina, macula, optic nerve, and sometimes the lens. A three-dimensional view of the eye as well as the outer edges of the periphery of the retina can be visualized. Muscle balance, acuity, field, motility, glaucoma, and color deficiency exams may also be included in a clinical vision assessment depending upon the specific questions asked of the doctor.

COLOBOMA: Cleft or defect in the pupil, iris, ciliary body, lens, retina, choroids, or optic nerve caused by improper fusion of fetal tissue during gestation. Functional visual problems include:

CONES: Visual cells in the retina. They are concentrated in the macula area, and are responsible for both color perception and detail vision.

CONE DYSTROPHY: Degeneration of the retinal receptors that results in a progressive decrease in vision and loss of color discrimination. There is no known treatment. Functional visual problems include:

CONGENITAL: Present at birth or shortly thereafter.

CONGENITAL STATIONARY NIGHT BLINDNESS: Heredity retinal disorder characterized by poor night vision. This condition is non-progressive.

CONTRAST: Luminance contrast is the difference in brightness between a foreground (what you are looking at) and its background (e.g., a high contrast situation occurs when a black object is on a yellow background). Light-dark contrast is the amount of light reflected off of different areas of similar surfaces (e.g., a wall corner may stand out because light will strike one side differently from the other, making one side brighter).

CONTRAST SENSITIVITY: Ability to detect differences in grayness and background. Because more of the retina is used in discriminating contrast, it may be a better test of visual function, since it utilizes more than central visual acuity.

CONVERGENCE: Movement, as an object approaches, of both eyes toward each other in an effort to maintain fusion of separate images.

CORNEA: The transparent tissue at the front of the eye that is curved to provide most of the eye’s refractive power.

CORNEAL DEFECTS/DISEASE: Clouding of the cornea; certain metabolic diseases, trauma, infectious diseases, nutritional deficiencies, and environmental situations may cause the clouding.

CORTICAL/CEREBRAL VISUAL IMPAIRMENT: Temporary or permanent visual impairment caused by the disturbance of the posterior visual pathways and/or the occipital lobes of the brain. The degree of vision impairment can range from severe visual impairment to total blindness. Nystagmus is not usually observed in children with CVI. The degree of neurological damage and visual impairment depends upon the time of onset, as well as the location and intensity of the insult. It is a condition that indicates that the visual systems of the brain do not consistently understand or interpret what the eyes see. Functional vision problems include:

CRYSTALINE LENS: The eye’s natural lens. A transparent biconvex intraocular tissue that helps bring rays of light to a focus on the retina.

CYTOMEGALOVIRUS (CMV): A virus that may occur prior to birth or at any age. CMV is the most common of the infections that occur prior to delivery. It can range in severity from a silent infection with no consequences, to a disease manifested by fever, hepatitis, and in newborns, severe brain damage or stillbirth. Effects on vision include chorioretinitis (inflammation of the choroid and retina) and microphthalmia (small eyes).


DELAYED VISUAL MATURATION: Condition in which a child does not use vision during the first few months of life. A magnetic resonance imaging study will not reveal abnormalities. Nystagmus may be present and the delay in visual function may be related to anterior visual pathway abnormalities. Visual function generally develops at 6-12 months with continued improvement by two to three years of age.

DIOPTER: Unit to designate the refractive power of a lens.


ECCENTRIC VIEWING: Visual abnormality in which a retinal area other than the fovea is used for visual fixation. Vision within the peripheral or patchy surrounding regions is used when fixating eccentrically.

ENUCLEATION: Removal of the eyeball, leaving eye muscles and remaining orbital contents intact.

ESOTROPIA: Misalignment of eye in which one or both eyes deviate inward.

EXOTROPIA: Misalignment of eye in which one or both eyes deviates outward (away from nose).


FARSIGHTEDNESS (HYPEROPIA): A refractive error caused by an eyeball that is too short; corrected with a plus (convex) lens.

FIELD LOSS: An inability to see part of an area of view when looking straight ahead, measured in degrees from the fixation point.

FIGURE-GROUND: Ability to sort out single figures (e.g., picture, person, event, and object) from a background, and elements of a scene that are at different distances from the eye.

FIXATING: Coordinating eye movements to enable an image to focus on the fovea.

FORCED CHOICE PREFERENTIAL LOOKING TEST: A means of testing the vision of nonverbal or preverbal children in which patterned stimuli are presented and the movement of the individual’s eyes is noted to determine resolution acuity.

FOVEA: Center of the macula that has the highest concentration of cones in the eye, resulting in the sharpest vision.

FUNCTIONAL VISION ASSESSMENT: An assessment of an individual’s use of vision in a variety of tasks and settings that include measures of near and distance vision; eye movement; and responses to specific environmental characteristics such as light and color. The assessment report includes recommendations for instructional procedures, modifications-adaptations, and additional tests.


GLAUCOMA: Group of diseases characterized by an increase in intraocular pressure associated with a buildup of aqueous fluid that may cause damage to the retina and the optic nerve and eventual visual field defects if left untreated. Pressure can be reduced with medication, surgery, or both. Functional vision problems include:


HEMIANOPSIA (Hemianopia): Non-seeing area in the right or left half of the visual field.

HYPEROPIA (farsightedness): Refractive error caused by an eyeball that is too short; corrected with a plus (convex) lens.


IRIS: The colored portion of the eye that expands or contracts to control the amount of light entering the eye.


LEA GRATING PADDLES: Created by Dr. Lea Hyvarinen, these resolution acuity paddles can be used to screen the acuity of infants and toddlers birth to 18 months of age. Infants detect the presence of parallel lines of decreasing width. When a striped pattern is presented in front of an infant at 2 feet simultaneously with a gray surface of the same size and luminance, the infant is likely to look at the striped pattern because there is more to see than on a gray surface. The gratings on the paddles are held about 2 feet from the infant’s face.

LEGAL BLINDNESS: In the U.S., visual acuity of 20/200 or worse in the better eye with corrective lenses (20/200 means that a person must be at 20 feet from an eye chart to see what a person with normal vision can see at 200 feet) or visual field restricted to 20 degrees diameter or less (tunnel vision) in the better eye with corrective lenses.

LEBER’S CONGENITAL AMAUROSIS: Reduced retinal function as documented by an Electroretinogram. Visual function can vary widely; however, profound or total visual loss is common.

LENS: Transparent lentil-shaped body behind the iris controlled by the ciliary muscle. To focus on close-up objects, the ciliary muscle squeezes the lens to make it thicker. To focus on far-away objects, the ciliary muscle flattens the lens to make it thinner.

LIGHT PERCEPTION: The ability to discern the presence or absence of light but not its source or direction.

LIGHT PROJECTION: The ability to discern the source or direction of light, but not enough vision to identify objects, people, shapes, or movements.

LOCALIZING: Identifying the position of an object in space.

LOW VISION: Visual impairment after correction but with the potential for use of available vision with or without optical or nonoptical compensatory visual strategies, devices, and environmental modification to plan and perform daily tasks.


MACULA: A thin layer of nerve cells near the center of the retina with high concentrations of cones for detailed vision. The macula provides sharp, clear central vision that allows a person to see form, color, and detail.

MICROPHTHALMIA (microphthalmos): Abnormally small eyeball. Functional visual problems include:

MYOPIA (nearsightedness): A refractive error resulting from an eyeball that is too long; corrected with a concave (minus) lens.


NEAR VISION: Use of vision to accomplish tasks that occur within 16” distance from individual. Common near vision tasks include eating, playing with a toy, or reading.

NIGHT BLINDNESS: Diminished rod function resulting in deficient visual acuity at night and in dim light.

NYSTAGMUS: Rhythmic, involuntary movement of one or both eyes in any direction. This condition is often secondary to other ocular problems. There is no known treatment. Functional visual problems include:


OCULAR MOTILITY: Eye movement that allows a child to scan or track a broad array of objects, people, or events.

OCULAR PURSUIT: Act of tracking a moving object in all cardinal directions (e.g., vertical, horizontal, oblique, and circular).

OCCIPITAL LOBE: Section of the brain that is responsible for vision and visual perception.

OCULOMOTOR APRAXIA: Difficulty controlling eye movements.

OPHTHALMOLOGIST: A physician who specializes in the medial and surgical care of the eyes and is qualified to prescribe ocular mediations and to perform surgery on the eyes. May also perform refractive and low vision work, including eye examination and other vision services.

OPTIC NERVE GLIOMA: Slow-growing, non-malignant tumor of the optic nerve or optic chiasm. Reduced visual acuity is a functional visual problem that results from optic nerve glioma.

OPTIC NERVE ATROPHY: Degeneration of the optic nerve fibers causing loss of vision. May or may not be progressive. Any visual loss is permanent. Functional visual problems include:

OPTIC NERVE: Largest sensory nerve of the eye. This cranial nerve carries impulses for sight from the retina to the brain.

OPTIC NERVE HYPOPLASIA (ONH): Results from underdevelopment of the optic nerve during fetal development resulting in a small gray or pale nerve head surrounded by a light halo; often associated with central nervous system or endocrine disorders; visual function varies widely and can include total blindness. Other problems may include:

OPTIC NEURITIS: Inflammation of the optic nerve. This condition is associated with diseases such as multiple sclerosis. Characterized by rapid onset of decreased vision and, usually, discomfort with eye movements. Functional visual problems include:

OPTOMETRIST: A health care provider who specializes in refractive errors, prescribes eyeglasses or contact lenses, and diagnoses and manages certain conditions of the eye as regulated by state law.


PERIPHERAL VISION: Perception of objects or motion from the parts of the retina that are beyond the macula.

PHOTOPHOBIA: Abnormal sensitivity to, and discomfort from, light. May be associated with excessive tearing. Usually symptomatic of other ocular disorders or diseases.

PTOSIS: A drooping of the eyelid caused by paralysis or weak eyelid muscles; it may be congenital. Ptosis requires surgical correction if it interferes with vision.

PUPIL: Variable-sized black circular opening in the center of the iris that regulates the amount of light that enters the eye.


REFRACTIVE ERROR: Conditions such as myopia, hyperopia, and astigmatism caused by irregularities in the shape of the eye or defractive media of the eye. Parallel rays of light are not brought to a focus on the retina.

RETINA: Inner sensory nerve layer next to the choroid that lines the posterior two-thirds of the eyeball. The retina converts images to electrical pulses that are sent to the brain that interpret these impulses are interpreted as images.

RETINAL DETACHMENT (non-ROP): Separation of retina from its normal position in the back of the eye. Functional visual problems include:

RETINITIS PIGMENTOSA (RP; rod/cone dystrophy): Degeneration primarily of the light sensitive cells in the periphery of the retina. Functional visual problems include:

RETINOBLASTOMA: Malignant tumor within the eye. Most common childhood malignancy, which may require enucleation. The first sign of this disorder may be strabismus. If one eye is removed, the person will not have depth perception.

RETINOPATHY OF PREMATURITY (ROP; Previously known as retrolental fibroplasia): A disease of the retina that primarily affects infants born prematurely resulting in a discontinuation of normal retinal vessel growth and abnormal growth of new vessels that can result in retinal detachment. Visual function can range from near normal to total blindness. Other problems may include:

RODS: Cylindrical, rod-shaped photoreceptors that do not sense color but enhance peripheral vision and vision in dim light.


SCLERA: The tough white opaque outer covering of the eye that protects the inner contents from most injuries.

SHAKEN BABY SYNDROME (SBS): Impairments and/or disabilities that result from child abuse in which a child is forcefully shaken as a result of either direct traumatic injury or as a secondary complication of intraocular bleeding. Visual impairment may be due to retinal detachment, optic atrophy, and/or damage to visual pathways in the brain.

SHIFTING GAZE: Alternating eye fixation from one object, person or event to another at near intermediate or far distances.

SNELLEN CHART (Test chart): Chart that is routinely used to assess recognition visual acuity at specified distances. Rows of Es of standard sizes are used to determine distance acuity.

STARGARDT’S DISEASE: Deterioration of the cone cells of the macula. This hereditary condition is progressive. Central vision loss is a functional vision problem that results from Stargardt’s disease.

STRABISMUS: Extrinsic muscle imbalance that causes misalignment of the eyes; includes deviations upward, downward, toward the nose and away from the nose. This condition is often secondary to other visual impairments. Surgery and eye patching may be used to correct this condition. Functional vision problems include:


TELLER ACUITY CARDS: A method for measuring resolution (grating) acuity in infants, young children, and individuals who cannot respond verbally to standard acuity measurements. Each card contains a black-and-white grating located to the left or to the right of a central peephole. An individual is tested using cards with different gratings.

TRACKING: Visually following a moving object.


VISION: Process of deriving meaning from what is seen. It involves fixation and eye movement abilities, accommodation (eye focusing), convergence (eye aiming), visual perception, and visual-motor integration.

VISUAL ACUITY: Assessment of the eye’s ability to distinguish object details and shape, using the smallest identifiable object that can be seen at a specified distance (usually 20 ft. for distance acuity or 16 in. for near acuity).

VISUAL CLUTTER: A combination of images and background that provides distracting details for some individuals who are unable to select a single object form its background (figure-ground difficulties).

VISUAL IMPAIRMENT: The terms low vision, legally blind, and totally blind are used in the educational context to describe students with visual impairments. They are defined as follows:

VISUAL FIELD: Area that can be seen when looking straight ahead, measured in degrees form the fixation point; also called field of vision.

VITREOUS HUMOR: Transparent, gelatinous mass that fills the rear of the eyeball between the lens and the retina.