Information on Common Eye Disorders
 
Optics And Refraction

Low Vision
 
Over three million people in the United States do not have normal vision even with corrective lenses. If ordinary eyeglasses do not provide clear vision, one is said to have low vision. This should not be confused with blindness. People with low vision still have useful vision that can often be improved with low-vision devices.

Low vision can result from birth defects, inherited diseases, injuries, diabetes, glaucoma or macular degeneration. Although reduced central or reading vision is most common, a person can have low vision in their side (peripheral) vision, or a loss of color vision or contrast sensitivity.

Low vision devices or aides are available in optical and non-optical types. Optical devices use lenses or combinations of lenses to provide magnification. They should not be confused with standard eyeglasses. There are five main kinds of optical devices: magnifying spectacles, hand magnifiers, stand magnifiers, telescopes and closed-circuit television. Different devices may be needed for different purposes. If possible, try the optical device before purchasing it and be sure you understand how to use it.

The simplest non-optical technique is to bring the object of interest closer. Non-optical low vision devices include large print books, check writing guides, enlarged phone dials, talking appliances (timers, clocks, computers), and machines that scan print and read out loud.

Government and private agencies have social services available for people with low vision. For more information, contact the following resources:

American Academy of Ophthalmology Web Site   www.eyenet.org
American Foundation for the Blind   (800) 232-5463
National Association for Visually Handicapped   (212) 889-3141
National Library Service for the Blind and Physically Handicapped   (800) 424-8567
Lighthouse International   (800) 334-5497
National Eye Institute   (301) 496-5248
Prevent Blindness America   (800) 331-2020
Resources for Rehabilitation   (617) 862-6455
VISION Foundation, Inc.   (800) 852-3029
Visions/Services for the Blind and Visually Impaired   (212) 425-2255

Veterans may contact the Visual Impairment Services coordinator at their local VA facility.
Myopic Degeneration
Myopic degeneration is an uncommon condition characterized by progressive stretching of the eye that damages the retina, the layer of light-sensitive cells that lines the back of the eye. People with severe nearsightedness (high myopia) are at greater risk for myopic degeneration.

Myopic degeneration commonly occurs during young adulthood with a gradual decrease in central vision. Vision can decrease more abruptly, but typically vision loss is gradual. Although central vision may be lost, side (peripheral) vision usually remains unaffected. Remaining sight can still be very useful and with the help of low-vision optical devices, people can continue many of their normal activities.

The causes of myopic degeneration are not clearly understood but may include biomechanical abnormalities or hereditary factors. The biomechanical theory assumes that the retina, in a myopic eye, is stretched over a larger than normal area because the eye is longer than usual. Over time, the outer coat of the eye, known as the sclera, also stretches in response to forces like internal eye pressure. This stretching of the sclera is thought to lead to retinal degeneration. In the hereditary theory, the retinal changes are thought to be an unavoidable, inherited process.

The only treatment for myopic degeneration is surgery to reinforce the scleral wall. This has been performed with varying degrees of success. 
Refractive Errors

To see clearly, light rays must be bent or refracted to focus on the retina, the light- sensitive nerve layer that lines the back of the eye. The cornea and lens of the eye work together to bend or refract light rays and bring them together on the retina. If a refractive error is present, the light is not focused directly on the retina, so images appear blurry.

Myopia (nearsightedness): Distance vision is impaired when the eye is too long in relation to the curvature of the cornea. This causes light to focus before it reaches the retina. Close objects look clear but distant objects appear blurry.

Hyperopia (farsightedness): Close vision is impaired, with some impairment of distance vision, as well. The eye is too short in relation to the curvature of the cornea. Light rays are not yet in focus when they reach the retina, so images appear blurry.

Astigmatism (the cornea is oval shaped instead of round): The irregular curvature of the cornea causes light to focus on more than one point on the retina. Uncorrected astigmatism impairs both distance and near vision.

Presbyopia (aging eyes): When young, the lens of the eye is soft and flexible, allowing people to see objects both close and far away. After the age of 40, the lens of the eye becomes more rigid, making it more difficult for the lens to change its shape, or accommodate, to do close work such as reading. This condition is known as presbyopia and is the reason reading glasses or bifocals are necessary at some point after age forty.