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Vision Diseases, Disorders & Definitions

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C Defintions


Opacity or cloudiness of the crystalline lens, which may prevent a clear image from forming on the retina. Surgical removal of the lens is necessary if visual loss becomes significant, with lost optical power replaced with an intraocular lens.

A cataract is a clouding of the lens in the eye that affects vision. Most cataracts are related to aging. Cataracts are very common in older people. By age 80, more than half of all Americans either have a cataract or have had cataract surgery.

A cataract can occur in either or both eyes. It cannot spread from one eye to the other.

What is the lens? The lens is a clear part of the eye that helps to focus light, or an image, on the retina. The retina is the light-sensitive tissue at the back of the eye.

anatomy image of an eye
Image Courtesy: National Eye Institute, National Institutes of Health

In a normal eye, light passes through the transparent lens to the retina. Once it reaches the retina, light is changed into nerve signals that are sent to the brain.

The lens must be clear for the retina to receive a sharp image. If the lens is cloudy from a cataract, the image you see will be blurred.

Are there other types of cataract? Yes. Although most cataracts are related to aging, there are other types of cataract:

  1. 1. Secondary cataract. Cataracts can form after surgery for other eye problems, such as glaucoma. Cataracts also can develop in people who have other health problems, such as diabetes. Cataracts are sometimes linked to steroid use.
  2. 2. Traumatic cataract. Cataracts can develop after an eye injury, sometimes years later.
  3. 3. Congenital cataract. Some babies are born with cataracts or develop them in childhood, often in both eyes. These cataracts may be so small that they do not affect vision. If they do, the lenses may need to be removed.
  4. 4. Radiation cataract. Cataracts can develop after exposure to some types of radiation.

What causes cataracts? The lens lies behind the iris and the pupil (see diagram). It works much like a camera lens. It focuses light onto the retina at the back of the eye, where an image is recorded. The lens also adjusts the eye's focus, letting us see things clearly both up close and far away. The lens is made of mostly water and protein. The protein is arranged in a precise way that keeps the lens clear and lets light pass through it.

But as we age, some of the protein may clump together and start to cloud a small area of the lens. This is a cataract. Over time, the cataract may grow larger and cloud more of the lens, making it harder to see.

Researchers suspect that there are several causes of cataract, such as smoking and diabetes. Or, it may be that the protein in the lens just changes from the wear and tear it takes over the years.

How can cataracts affect my vision? Age-related cataracts can affect your vision in two ways:

  1. 1. Clumps of protein reduce the sharpness of the image reaching the retina.
    The lens consists mostly of water and protein. When the protein clumps up, it clouds the lens and reduces the light that reaches the retina. The clouding may become severe enough to cause blurred vision. Most age-related cataracts develop from protein clumpings.
    When a cataract is small, the cloudiness affects only a small part of the lens. You may not notice any changes in your vision. Cataracts tend to "grow" slowly, so vision gets worse gradually. Over time, the cloudy area in the lens may get larger, and the cataract may increase in size. Seeing may become more difficult. Your vision may get duller or blurrier.The clear lens slowly changes to a yellowish/brownish color, adding a brownish tint to vision

  2. 2. As the clear lens slowly colors with age, your vision gradually may acquire a brownish shade. At first, the amount of tinting may be small and may not cause a vision problem. Over time, increased tinting may make it more difficult to read and perform other routine activities. This gradual change in the amount of tinting does not affect the sharpness of the image transmitted to the retina.
    If you have advanced lens discoloration, you may not be able to identify blues and purples. You may be wearing what you believe to be a pair of black socks, only to find out from friends that you are wearing purple socks.

Learn more about Cataracts

Cataract extraction: Removal of a cloudy lens from the eye. Extracapsular cataract extraction leaves the rear lens capsule intact; with an intracapsular extraction (usually by cryoextraction) there is complete removal of the lens with its capsule.

Color blindness: Reduced ability to discriminate among colors, especially shades of red and green. Usually hereditary; much more common in men.

Computer Vision Syndrome: Computer Vision Syndrome is caused by prolonged computer use; it refers to a wide range of eyestrain, pain, and discomfort experienced by computer users, with the level of discomfort typically increasing with continued computer use.

Computer Vision Syndrome is caused by our eyes and brain reacting differently to text and characters on a computer screen than they do to printed characters. Printed material has dense black text with well-defined edges. But characters on a computer screen are made up of pixels and do not have the same density or degree of contrast and definition. In addition, the presence of glare and reflections on computer screens can make viewing difficult.

Computer Vision Syndrome symptoms include:

  • Blurred vision/loss of focus
  • Burning eyes
  • Double vision
  • Dry eyes
  • Eye irritation
  • Eye strain
  • Eye twitching
  • Headaches
  • Neck and shoulder pain
  • Red eyes
  • Tired eyes

A person's visual abilities can also impact the degree to which one experiences symptoms. Near or farsightedness, astigmatisms, changes due to aging of the eyes, eye coordination, and other eye and vision problems can all be contributing factors.

Most Computer Vision Syndrome-related symptoms are temporary and improve or stop after discontinuing computer work. However, some people may continue to feel symptoms even after ending their computer session.

Symptoms of Computer Vision Syndrome can be prevented or reduced by using appropriate ergonomics, including proper lighting conditions, chair comfort and height, the position of the computer and monitor, the location of reference materials, and taking breaks when using the computer for long periods of time.

Taking regular breaks is an enormous benefit to your eye and vision health, and it is easy to remember to do by following the 20-20-20 rule. For every 20 minutes spent working on a computer, look at an object 20 feet away for 20 seconds. This brief activity will help rest your eyes and avoid or lessen many Computer Vision Syndrome symptoms.

See you eye care provider if you experience any Computer Vision Syndrome symptoms or any other significant vision problems for a prolonged period of time. Your eye care provider may recommend computer glasses, which aid in the reduction of computer-related eye strain by eliminating the constant refocusing effort your eyes go through when viewing the screen. Take serious steps to enhance all facets of your ergonomics because if nothing is done to address the problem, the symptoms will likely continue and perhaps worsen with additional computer use.

Conjunctiva: Transparent mucous membrane covering the outer surface of the eyeball except the cornea, and lining the inner surfaces of the eyelids.

Cornea: Transparent, dome-shaped front part of the eye that covers the iris, pupil, and anterior chamber and provides most of an eye's optical power.

Corneal disease: Group of infections, dystrophies, and injuries affecting the cornea.

What is the cornea? The cornea is the eye's outermost layer. It is the clear, dome-shaped surface that covers the front of the eye.

Image of the cornea
Image Courtesy: National Eye Institute, National Institutes of Health

Refractive Errors About 120 million people in the United States wear eyeglasses or contact lenses to correct nearsightedness, farsightedness, or astigmatism. These vision disorders–called refractive errors–affect the cornea and are the most common of all vision problems in this country.

Refractive errors occur when the curve of the cornea is irregularly shaped (too steep or too flat). When the cornea is of normal shape and curvature, it bends, or refracts, light on the retina with precision. However, when the curve of the cornea is irregularly shaped, the cornea bends light imperfectly on the retina. This affects good vision. The refractive process is similar to the way a camera takes a picture. The cornea and lens in your eye act as the camera lens. The retina is similar to the film. If the image is not focused properly, the film (or retina) receives a blurry image. The image that your retina "sees" then goes to your brain, which tells you what the image is.

When the cornea is curved too much, or if the eye is too long, faraway objects will appear blurry because they are focused in front of the retina. This is called myopia, or nearsightedness. Myopia affects over 25 percent of all adult Americans.

Hyperopia, or farsightedness, is the opposite of myopia.

Distant objects are clear, and close-up objects appear blurry. With hyperopia, images focus on a point beyond the retina. Hyperopia results from an eye that is too short.

Astigmatism is a condition in which the uneven curvature of the cornea blurs and distorts both distant and near objects. A normal cornea is round, with even curves from side to side and top to bottom. With astigmatism, the cornea is shaped more like the back of a spoon, curved more in one direction than in another. This causes light rays to have more than one focal point and focus on two separate areas of the retina, distorting the visual image. Two-thirds of Americans with myopia also have astigmatism.

Refractive errors are usually corrected by eyeglasses or contact lenses. Although these are safe and effective methods for treating refractive errors, refractive surgeries are becoming an increasingly popular option.

What are some diseases and disorders affecting the cornea? Some diseases and disorders of the cornea are:

  • Allergies
  • Conjunctivitis (Pink Eye)
  • Corneal Infections
  • Dry Eye
  • Fuchs' Dystrophy
  • Herpes Zoster (Shingles)
  • Iridocorneal Endothelial Syndrome
  • Keratoconus
  • Lattice Dystrophy
  • Map-Dot-Fingerprint Dystrophy
  • Ocular Herpes
  • Pterygium
  • Stevens-Johnson Syndrome

Learn more about Corneal Disease

Cross-eyes: Eye misalignment in which one eye turns inward (toward nose) while the other fixates normally.

Crystalline lens: The eye's natural lens. Transparent, biconvex-shaped tissue that helps bring rays of light to a focus on the retina.


Portions of this page are excerpted from Dictionary of Eye Terminology, copyright 1990-2006 by Barbara Cassin and Triad Communications. Reprinted with permission.

Some of the information above is reprinted from the National Eye Institute, National Institutes of Health, and is for educational purposes only. Superior Vision strongly recommends eye exams as the best way to diagnose eye conditions and learn more about potential vision problems.