Encyclopedia - Low Vision

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Encyclopedia - Low Vision

What causes low vision?
What help is available for persons with low vision?

Low vision is a general inability to see clearly, ranging from a slight visual loss to blindness. The term describes the visual ability of people who, even after correction with eyeglasses, contact lenses or surgery, have less than 20/40 vision. The American Academy of Ophthalmology defines low vision as what happens "if ordinary eyeglasses, contact lenses or intraocular lens implants don't get you clear vision."

A person who has visual acuity of no more than 20/70 in the better eye with the best corrective lens in place is classified as partly sighted or visually impaired. A person who has visual acuity of no more than 20/200 in the best corrected lens and/or a field of view no greater than 20 degrees is legally blind. An estimated 20 million Americans age 40 and older have severe visual impairments, which can include decreased side vision, some degree of color blindness, or inability to adjust to contrast and glare.

What causes low vision?

There are many causes of low vision which include disease, injury, genetic factors, and the effects of aging. Some leading causes of low vision, listed alphabetically, are:

age-related macular degeneration -- progressive deterioration of the macula and the central region of the retina. It causes loss of central vision and is the leading cause of severe loss of vision in older persons.
anaridia -- the failure of the iris to develop normally. The resulting partial or full absence of the iris can be accompanied by other vision-decreasing conditions, such as nystagmus, glaucoma and extreme sensitivity to light.
cataract -- the clouding of the lens of the eye that generally occurs with aging but can be present at birth or be caused by injury to the eye
corneal lesions -- damage to the cornea caused by injury, ulcerations, or dystrophy, such as deterioration, of the basement membrane of the cornea
diabetic retinopathy -- in which adverse changes in the blood vessels supplying the retina result in deterioration of function. Diabetes is the leading cause of blindness in the United States.
glaucoma -- eye damage often caused by a buildup of fluid pressure within the eye. Often age-related, glaucoma is the third most common cause of blindness in the United States.
keratoconus -- deformation of the cornea that usually becomes evident during puberty. It usually affects both eyes and occurs more often in women than men.
retinal detachment -- in which part or all of the retina becomes separated from its supporting tissues, causing a blind spot to develop. It can be due to disease or injury.
retinitis pigmentosa -- genetically caused deterioration of the retina that can be accompanied by cataracts or glaucoma.
retinopathy of prematurity -- damage to the blood vessels of the retina that can occur when premature babies or other newborns are given oxygen therapy.

A number of aids for persons with low vision are available. Before selecting a device, an individual needs a complete vision examination. The exam allows the physician to determine the nature and extent of the problem causing loss of vision, so that any condition can be treated, e.g., surgery for cataracts. At the eye examination, the effects of low vision on the person's daily personal and business activities are determined, one or more devices can be prescribed, and the person can be trained in its use. In addition, proper illumination gives the best contrast, and it keeps glare and other distracting elements at a minimum.

What help is available for persons with low vision?

One class of low-vision aids comprises magnifiers. They can be as simple as handheld magnifiers used to read dials, tags, labels and mail. Stand magnifiers, which can be equipped with their own light sources, enable hands-free reading and close work. Loupes are magnifying devices that can be mounted on eyeglasses.

Special eyeglasses can help, although contact lenses may provide better optics, less distortion, and wider fields of view than eyeglasses. Nevertheless, many persons are more comfortable with high-intensity reading glasses than with contact lenses.

Projection magnifiers give an enlarged image on a screen, as do closed-circuit television systems. TV systems can be adjusted in several ways to provide images best suited for a specific kind of low vision.

Some persons with low vision can be helped by using reading slits or typoscopes. These are black masking devices that allow the viewer to see one or several lines of type through a rectangular opening, often with improved contrast.

For persons who have deficits in specific parts of the visual field, prisms or mirrors can be prescribed to let them view objects in that region with minimum effort.

Large print editions of many books, magazines, newspapers and other reading materials are available.

Audio devices, ranging from reading machines to talking clocks to books on tape, aid persons who cannot take advantage even of large print material.

Blind persons can use tactile methods, such as classic Braille texts, for reading, and can use orientation methods, from simple canes to guide dogs to advanced electronic aids.

Help is available at the low-vision centers that have been established throughout the United States. Most centers have multidisciplinary staffs that can include an ophthalmologist, optometrist, a nonmedical low-vision specialist and rehabilitation experts who can help adjust the home environment to make the most of a person's remaining visual abilities.

Persons with low vision can ask a physician for a referral to such organizations as the American Federation for the Blind, the National Association for the Visually Handicapped and the Lighthouse, all in New York City, and the National Federation of the Blind in Baltimore. The Library of Congress in Washington, D.C., maintains a National Library for the Blind and Physically Handicapped.