Commonly known as Lazy eye. A loss of vision in a young child due to the eye not being used. The eye is normal but the brain tends to suppress or ignore the image received by the amblyopic eye. The most common causes include a muscle imbalance, a focusing problem, or a problem such as a cataract or corneal scar. Sometimes both eyes can be affected.


A hereditary eye problem in which the iris, the colored part of the eye, is absent. There is poor vision, sensitivity to sunlight, nystagmus, and a tendency to develop glaucoma.


A difference in the size of the two pupils. It is present in about 5% of normal children. The most serious cause of an acquired Anisocoria follows a head injury with some brain or nerve damage; a disease such as a tumor also causes it.


A difference in the focusing power of the two eyes. One of the major causes of amblyopia; the brain is not able to clearly focus both eyes simultaneously. This is a "hidden" cause of amblyopia and very difficult to detect without an eye exam.


An absence of the lens in the eye. The lens is removed during a cataract operation. The natural lens may be replace with an artificial lens during the cataract operation.


An irregular curvature of either the cornea (front of the eye) or the lens. If either structure is shaped more like a football rather than a basketball, light is not sharply focused on the retina. This results in blurry vision for both distance and near.


Legal blindness is defined as: 1) visual acuity of 20/200 (only being able to see the big E on the eye chart) or less in the best eye even with the eyes corrected by glasses or contact lenses; or, 2) The peripheral visual field is reduced to 20 degrees of visual angle or less. Twenty degrees of visual angle is about the size of a one foot ruler held at arms length.


An opacity or haziness of the lens of the eye. A cataract is noticed particularly at night when oncoming headlights produce glare disability or/and discomfort. It may or may not reduce the vision depending on size, density and location. If a cataract reduces visual acuity significantly, an Ophthalmologist can replace the defective lens with an artificial lens.


Photoreceptors, cells in the back of the eye that gather light and convert light into electrical impulses that travel to the brain, used for seeing in bright lights and used for color vision. Cone photoreceptors are located mostly in the fovea and in the macula. Cone photoreceptors are used for fine detailed vision, color vision, central vision and for seeing in bright daylight. Under a microscope, the cone photoreceptors are cone shaped (like traffic cones or ice cream cones), thus the name.


The front part of the eye that acts as a window for the entrance of light rays. It is attached to the other outer coat of the eye, the sclera; the white part of the eye. The cornea provides a significant amount of focusing power for the eye (the rest is provided by the lens). Because it has many nerve fibers, an injury or foreign body causes significant pain and discomfort.

Cover Test

A test for a muscle imbalance. While the person is looking at a distant object, one eye is covered and then uncovered (cover-uncover). This is repeated on the other eye. Finally it is performed on both eyes, covering one then the other (alternate-cover). If one or both eyes shift during this test, there is a problem with alignment of the eyes. The misalignment with the eyes often cannot be seen with both eyes opened.


A paralysis of the ciliary muscles following the instillation of eye drops. This produces a loss of accommodation or focusing ability. With the lens relaxed, a better estimate of the refractive error is possible in most cases. Most cycloplegic eye drops also dilate the pupil. Cycloplegia may last from a few hours to several days, depending on certain factors such as skin color – the lighter the longer.


The unit used to measure the amount of refractive or focusing power of the eye. It also refers to the strength of lens required to provide clear vision. In general, the higher the refractive error, as measured in diopters, the worse the eye.


Commonly known as double vision. In children, diplopia is often associated with a muscle imbalance such as esotropia. A refractive error may also cause enough blurring that a person sees two objects.


A learning problem in which a person has difficulty with letter or word recognition. Children often are of normal or above normal intelligence; however, they have difficulty reading and sometimes naming pictures of objects. More recent evidence suggests that dyslexia is a decoding problem based on phonemes – the basic language components. This is a higher cortical processing problem and NOT an eye problem, per se.


A tendency for an eye to turn inward a little bit. It occurs under certain conditions such as fatigue. An esophoria is sometimes uncovered by the cover test.


Commonly known as "crossed eyes". One eye is constantly turned inward toward the nose. In children, esotropia may lead to suppression of the visual signals from the eye to the brain and lead to amblyopia and decreased depth perception. In adults with previously straight eyes (for example after head trauma), esotropia causes constant double vision.


A tendency for an eye to turn outward a little bit. Occurs sometimes under certain conditions such as fatigue, bright sun light or prolonged use of the eyes.


Sometimes called "Wall Eyes". One eye is constantly turned outward. A child may have an exophoria which progresses to a stage where the eye is straight one minute and turned-out the next (as when daydreaming or in bright sunlight). This is called intermittent exotropia. Exotropia may require surgical correction, prisms placed in the glasses or orthoptic (eye muscle) training.


Also called hyperopia. A refractive error in which the light rays entering the eye are focused behind the retina. With moderate degrees of farsightedness, accommodation can compensate so that glasses are not required. In later life everyone loses the ability to accommodate (presbyopia) and reading glasses become necessary. Children, up to about the age of 8 years, are often farsighted.


A central portion of the retina and macula that contains only cones. The fovea is the only part of the eye that is capable of 20/20 or better vision.


The back part of the eye that can be seen with an instrument called an ophthalmoscope. Visible features include the retina with its blood vessels, the optic nerve and choroid. The fundus surrounds the fovea, that part of the eye used for reading.


Commonly known as farsightedness. Most children are hyperopic and see things in the distance better than very close things.


A tendency for one eye to drift upward. A vertical type of muscle imbalance between the eyes.


A muscle imbalance in which one eye is straight and the other is turned upward.


The colored part of the eye with a hole (pupil) in the center. It regulates the amount of light entering the eye – the dimmer the lighting the more light the iris lets into the eye by widening the pupil.


A refractive surgery that requires the cutting of a large "flap" of cornea and the removal of the inner layer of cells, usually by use of a laser, in order to change the refractive state of the eye. Risks include problems with glare, particularly at night, and surgical complications. If serious infection occurs, blindness could result. Long term risks are not known.

Lazy Eye

A term often used instead of amblyopia. A loss of visual function, usually measured by visual acuity, in one or both eyes that cannot be explained by identifiable causes(s) such as a cataract or retinal disease. An eye that turns in (esotropia) or out (exotropia) may have a certain degree of central visual loss (amblyopia). A lazy eye is often treated by placing a patch over the stronger eye and forcing use of the lazy eye. The earlier the detection of the lazy eye the better for recovery of central vision with patching. If left untreated, the child will have a permanent loss of vision and loss of binocular vision and depth perception.


The lens of the eye is like an adjustable lens of a camera and focuses light rays on to the retina for sharp images. A condition called presbyopia occurs when the lens is no longer able to adjust for objects at different distances.

Lens Capsule

The lens capsule is a membrane that surrounds the lens of the eye. In cataract surgery, the lens is usually replaced with an intraocular lens but the lens capsule remains in the eye.


A specialized part of the retina containing mostly cones. The macula is used for all detailed visual tasks. The center of the macula is called the fovea. If a disease process harms or destroys the macula, vision is usually reduced to 20/200 (legal blindness). When a person looks directly at something they are using the center part of the macula called the fovea. The fovea contains only photoreceptors for daylight vision, called cones.

Maintenance Therapy

Amblyopia therapy continued following the return of normal or near normal vision in the amblyopic eye. Sometimes, the eye doctor will continue to treat the lazy eye, for example with occlusion therapy, even when vision in the amblyopic eye is normal or near normal. Maintenance therapy is used to decrease the chance of the amblyopia returning following the termination of therapy. For example, the eye doctor may recommend occlusion for one hour per day until the patient is 9 years or older to reduce the chance that the amblyopia will return following the termination of therapy.

Mixed Amblyopia

Amblyopia caused by a combination of misaligned eyes (strabismus) and a difference in refractive error between the eyes (anisometropia). Both of these conditions, alone, can cause amblyopia and the combination can cause very deep amblyopia which is more difficult to cure. Also, sometimes amblyopia can return after the termination of therapy, called regression, and regression is more likely with this form of amblyopia.


Commonly known as nearsightedness. A refractive error in which the light rays focus in front of the retina producing blurry distance vision. External optical correction (glasses or contact lenses) are required for clear distance vision. It is now believed that myopia is partly hereditary; you’re more likely to become myopic if your parents are myopic. Also, near work can lead to a further worsening of the myopia. If the myopia is greater than 6 diopters, a condition known as high myopia, the possibility of retinal detachment is increased.


See myopia.

(OD) Ocular Dexter

Right eye

(OS) Oculus Sinister

Left eye

(OU) Oculus Uterque

Both eyes

Ophthalmologist (M.D., D.O.)

A physician (MD or DO) who specializes in the diagnosis and treatment of eye problems and diseases. The ophthalmologist works with the use of glasses, contact lenses, eye medication and surgery.

Optic Disc

The visible part of the optic nerve inside the eye. The axons of the ganglion cells of the inner retina make-up the optic nerve.


A technician who fits a person for glasses. He/she does not test for glasses. Some opticians also fit contact lenses.

Optometrist (O.D.)

Doctors of Optometry are independent primary health care providers who examine, diagnose, treat and manage diseases and disorders of the visual system, the eye and associated structures as well as diagnose related systemic conditions.

Peripheral Vision

Also called "side vision". That part of vision that detects objects outside of where we are directly focusing our eyes. When we look directly at something we are using the fovea – that part of our retina where there is a high density of cone photoreceptors and thus allows for detailed vision. The fovea is part of the macula – that part of our retina with mostly cone photoreceptors and used for day time vision. Outside of the macula is what is typically referred to as peripheral vision, and peripheral vision is dominated by the rod photoreceptors. Peripheral vision is used mainly for detecting objects and in directing where we should fixate our fovea or central vision. Peripheral vision is used mostly during the night. Without peripheral vision, we would have "tunnel vision". If a person has a significant loss of peripheral vision the person would be legally blind. See legal blindness.


A child’s eyes appear to be out-of-alignment, and usually one eye appears to turn in. In infants this appearance is especially noticeable when there is excessive skin on either side of the nose that covers the inner corner of each eye. As the child looks to one side, part of the eye disappears under this skin and looks crossed. This condition is common in Asian – Americans.


A drooping of the upper eyelid. In children it is usually a congenital problem. It rarely causes amblyopia. Most children simply hold their heads back if the droop is severe. Surgery, the only treatment, is usually suggested prior to starting school when the appearance is cosmetically unacceptable.


A circular opening in the center of the iris. The size of the pupil changes according to the amount of light present. It is small in sunlight and large in a dark room.


In order for an eye to see clearly, the light rays must come to a perfect focus when they reach the fovea. The bending of the light rays is called refraction. Each eye has its own characteristic refractive error. An instrument (retinoscope) is used to determine this error. The examination is called refracting the eye. From the refraction, the examiner learns the strength of lens necessary to provide the clearest vision for each eye.

Refractive Error

If rays of light that enter an eye do not focus perfectly on the back of the eye, retina, the patient is said to have a refractive error. The main types of refractive error include myopia, when the rays of light focus in front of the retina, and hyperopia, when the rays of light focus behind the retina. When the rays of light focus perfectly on the retina, it is referred to as emmetropia. Patients who have myopia are often called "nearsighted" because they can see things only when they are close to the face. Patients that have hyperopia are often called "farsighted" because they can see things better if the objects are farther away. Glasses, also called lenses including contact lenses, are used to correct the refractive error. Infants often have hyperopia and as they get older they may have emmetropia (perfect eyes) and later may develop myopia. Usually the eyes will have similar refractive errors. When the eyes have different refractive errors; for example, one eye may have emmetropia (i.e., no need for glasses) and the other eye may have hyperopia, the condition is called anisometropia and may lead to amblyopia if left untreated.


Amblyopia may sometimes return after the termination of therapy and this is referred to as regression.


The inner lining of the back of the eye that contains the visual cells (rods and cones). The function of the retina and visual cells is to convert light rays into electrical impulses that are transmitted to the brain by way of the optic nerve.


The rods are photoreceptors are used for gathering light and for converting light into electrical impulses that travel to the brain. The rod photoreceptors are located in the retina and are used for side "peripheral" vision, particularly at night. The rod photoreceptors sends only black-and-white vision to the brain. The rod photoreceptors are used for seeing in very dim light such as during the night. There are no rod photoreceptors in the very center part of the eye, called the fovea. Under a microscope, rod photoreceptors look like little straight sticks or "rods" thus the name.


An area of reduced vision or a "blind spot" in vision in one eye. A scotoma may be present in the central part of the amblyopic eye when both eyes are open in an amblyopic patient. A scotoma will prevent an amblyopic patient from having "binocular" vision; that is, the combined use of both eyes. A special instrument or test called a "Worth 4 dot test" is sometimes used to determine if an amblyopic patient has a scotoma in one eye while both eyes are open. Some other types of patients may have a scotoma in one or both eyes, such as patients with macular degeneration.

Snellen Chart

The familiar eye chart with larger letters at the top and smaller ones at the bottom. It is used for measuring central vision.


Also known as depth perception. The separation between the eyes provides for slightly different views of an object by each eye. The brain for the purpose of telling the location of an object in 3D space uses this difference in views between the eyes or disparity.


Misaligned eyes. See exotropia, esotropia and hyperopia. See article on strabismus surgery.

Vision Therapy

Vision therapy refers to a large group of "exercises" or "activities" designed to improve certain aspects of vision, visual performance, learning or eye muscle balance. This is a complex field and there appears to be fundamental differences about the value of vision therapy and, specifically, "vision behavioral therapy" between Ophthalmologists and Optometrists. For the Ophthalmologist’s general view of vision therapy see: http://www.aao.org/eyecare/treatment/alternative-therapies/vision-therapies-learning-disabilities.cfmVisual. For the Optometrist’s general view of vision therapy see: http://www.aoa.org/x5411.xml. The Ohio Amblyope Registry does not have a position regarding vision therapy and leaves it up to the individual eye doctors on how best to treat amblyopia.

Visual Evoked Response (VER)

The VER is a test of the function of the visual pathways from the retina, along the optic nerve and optic tract to the early parts of the visual centers of the brain. Usually, EEG electrodes are placed on the head and the patient is required to view a flashing light and an alternating pattern (e.g., stripes or checks) on a TV. The VER is a diagnostic test for such things as Multiple Sclerosis, optic neuritis, optic neuropathies, cortical visual impairment and certain types of brain tumors. The pattern VER can also provide an objective estimate of a patient’s visual acuity, even if the patient is nonverbal (e.g., too young, comatose or mentally impaired).

Visual Suppression

This occurs when the brain ignores the visual image being transmitted from one eye. It is not voluntary. In the younger child it is associated with strabismus and amblyopia. An eye that is misaligned or is out of focus is likely to be suppressed by the child.