Eyes A-Z - Volunteer Eyecare

Eyes A-Z

The American Optometric Association provides doctor-reviewed, doctor-approved information about the most common eye conditions. Find out more below. If you are having vision or eye problems, see an AOA-member doctor of optometry today.


Acanthamoeba is one of the most common organisms in the environment. Although it rarely causes infection, when it does occur, it can threaten your vision. Recently, there have been increased reports of Acanthamoeba keratitis. The best defense against Acanthamoeba keratitis infection is proper contact lens hygiene.

Anterior Uveitis

Anterior uveitis is an inflammation of the middle layer of the eye.Anterior uveitis is an inflammation of the middle layer of the eye. This middle layer includes the iris (colored part of the eye) and adjacent tissue (known as the ciliary body). If untreated, glaucoma, cataract or retinal edema can develop and cause permanent loss of vision. It usually responds well to treatment, but the inflammation tends to recur.


Astigmatism is a vision condition that causes blurred vision due either to the irregular shape of the cornea (the clear front cover of the eye) or sometimes the curvature of the lens inside the eye.


Blepharitis is an inflammation of the eyelids and eyelashes. It causes red, irritated, itchy eyelids and dandruff-like scales on eyelashes.


A cataract is a cloudy area in the normally clear lens of the eye. Depending upon its size and location, it can interfere with normal vision. Most cataracts develop in people over age 55, but they occasionally occur in infants and young children. Usually cataracts develop in both eyes, but one may be worse than the other.


A chalazion is a slowly developing lump that forms due to blockage and swelling of an oil gland in the eyelid. It is more common in adults between the ages of 30 to 50 than children.

Color Deficiency

Color vision deficiency is the inability to distinguish certain shades of color. In severe cases, people can’t see colors at all. The term “color blindness” is also used to describe this visual condition, but very few people are completely color blind.

Conjunctivitis (Red Eyes)

Conjunctivitis is an inflammation of the conjunctiva, the thin, transparent layer that lines the inner eyelid and covers the white part of the eye. The three main types of conjunctivitis are infectious, allergic and chemical. The infectious type, commonly called “pink eye,” is caused by a contagious virus or bacteria.

Diabetic Retinopathy

Diabetes is a disease that interferes with the body’s ability to use and store sugar and can cause many health problems. Diabetic retinopathy occurs when small blood vessels in your eyes become damaged. These blood vessels nourish your eye’s retina, the delicate, light-sensitive lining of the back of the eye.

Dry Eye

The tears your eyes produce are necessary for overall eye health and clear vision. Dry eyes occur when the body can’t produce enough tears or the specific components that make up tears.

Eye Coordination

Eye coordination is the ability of both eyes to work together as a team. Each of your eyes sees a slightly different image. Your brain, by a process called fusion, blends these two images into one three-dimensional picture. Good eye coordination keeps the eyes in proper alignment. Poor eye coordination results from a lack of adequate vision development or improperly developed eye muscle control.


Glaucoma is an eye disease in which the internal pressure in your eyes increases enough to damage your optic nerve and cause vision loss. The increased pressure occurs when the passages that allow fluid in your eyes to drain become clogged. The reasons the passages become blocked are not known.

Hyperopia (Farsightedness)

Hyperopia is a condition where light comes to a focus behind the retina.Farsightedness, or hyperopia, is a vision condition in which distant objects are usually seen clearly, but close ones do not come into proper focus. Farsightedness occurs if your eyeball is too short or the cornea has too little curvature, so light entering your eye is not focused correctly.


Keratoconus is a vision disorder that occurs when the normally round cornea (the front part of the eye) becomes thin and cone-shaped. This abnormal shape prevents the light entering the eye from being focused correctly on the retina and causes distorted vision.

Lazy Eye (Amblyopia)

Lazy Eye (Amblyopia) results in a loss of central vision.Lazy eye, or amblyopia, is the loss or lack of development of normal vision in one eye that is unrelated to any eye health problem and cannot be corrected with lenses. It can result from a failure to use both eyes together. Lazy eye is often associated with crossed eyes or a large difference in the degree of nearsightedness or farsightedness between the two eyes. It usually develops before age 6, and it does not affect side vision.

Macular Degeneration

Macular Degeneration is often accompanied by blurred vision.Macular degeneration is the leading cause of blindness in America. It results from changes to the macula, a portion of the retina that is responsible for clear, sharp vision and is located at the back of the eye.

Myopia (Nearsightedness)

Nearsightedness, or myopia, is a vision condition in which near objects are seen clearly, but distant objects do not come into proper focus. Nearsightedness occurs if your eyeball is too long or the cornea has too much curvature, so the light entering your eye is not focused correctly.


Nystagmus is a vision condition in which the eyes make repetitive, uncontrolled movements. It often results in reduced vision. These involuntary eye movements can be side to side, up and down, or in a circular pattern. As a result, both eyes are unable to steadily view objects. People with nystagmus may hold their head in unusual positions or nod their head in an attempt to compensate for the condition.

Ocular Hypertension

Ocular hypertension is an above-normal increase in the pressure in your eyes with no detectable changes in vision or damage to the eye structure. The term is used to distinguish people with elevated pressure from those with glaucoma, a serious eye disease that causes damage to the optic nerve and vision loss.


Presbyopia is a vision condition in which the lens of your eye loses its flexibility, making it difficult to focus on close objects.

Retinitis Pigmentosa

Retinitis pigmentosa is a group of inherited diseases that damage the light-sensitive rods and cones located in the retina (the back part of the eyes). Rods, which provide side (peripheral) and night vision, are affected more than the cones, which provide color and clear central vision.


Retinoblastoma is the most common cancer involving the eye. In the United States, this fast-growing cancer occurs in 1 in every 20,000 children, making it the 10th most common pediatric cancer.

Spots and Floaters

Spots (often called floaters) are small, semi-transparent or cloudy specks or particles in the fluid that fills the inside of your eyes. They appear as various-sized specks, threadlike strands or cobwebs. Because they are within your eyes, they move as your eyes move and seem to dart away when you try to look at them directly.

Strabismus (Crossed Eyes)

Strabismus occurs when one or both of your eyes turns in, out, up or down. Poor eye muscle control usually causes strabismus. This misalignment often first appears before a child reaches 21 months but may develop as late as age 6.

Visual Acuity

20/20 vision is a term used to express normal visual acuity (the clarity or sharpness of vision) measured at a distance of 20 feet. If you have 20/20 vision, you can see clearly at 20 feet what should normally be seen at that distance. If you have 20/100 vision, you must be as close as 20 feet to see what a person with normal vision can see at 100 feet.