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Target Population
Many terms have been used to refer to visual impairment. For consistency, the committee will rely primarily on "visual disability," "low vision," ''blindness," and "mild visual impairment" as defined below and in the glossary.1
It has been estimated by the National Advisory Eye Council (1993) that there are more than three million people in the United States who are visually disabled. "Visually disabled" refers to those who are blind or have low vision. There are approximately 200,000 people who are blind; they have no useful pattern vision, but some may retain light perception. The remaining people in the visually disabled category have low vision. Low vision is often defined as best-corrected letter acuity less than 20/60 in the better eye (World Health Organization, 1966) or the inability to read regular newsprint with optimal reading glasses.2 Some people with severe peripheral-field loss are classified as having low vision even though their letter acuity may be higher than the 20/60 criterion. The estimated number of people with low vision and blindness in the United States divided by age group is given in Table 2-1 and is also shown in Figure 2-1.3 The data in this table were generated by applying the data analysis technique explained in Genensky (1978) to the 1990 U.S. Census data (Genensky, 1994). According to these data, there are approximately 3.7 visually disabled Americans.
The leading causes of low vision and blindness are diseases that are common in old age: age-related maculopathy, cataract, glaucoma, diabetic retinopathy, and optic nerve atrophy. According to data from the Health Interview Survey, more than two-thirds of all people with low vision are 65 years of age or older, coming to an estimated total of 2.9 million people in 1990 (Nelson and Dimitrova, 1993). It is estimated that more than 25 percent of all people over 85 years of age are visually disabled (Genensky, 1994). As the geriatric population grows, the number of people with low vision and other age-related disabilities will increase.
The two traditional clinical measures of low vision are acuity and visual field. A recent detailed discussion of the measurement of visual acuity and visual field has been provided by the National Research Council Committee on Vision (NRC, 1994).
1
"Legal blindness" is commonly defined as a visual acuity in the better eye (with best refractive correction) of no more than 20/200 or a visual field of no more than 20 degrees. Approximately 600,000-900,000 persons in the United States fall into this category, which is primarily used for legal and official purposes. Visual disability and visual impairment encompass a broader population than legal blindness; blindness encompasses a narrower population.
2
The level of best—corrected visual acuity at which a person is said to have "low vision" has several definitions. Measured levels of 20/60 or 20/70 are commonly used and correspond roughly to the more qualitative definition of inability to read regular newsprint. The definition used by each source of the data cited in the chapter is given with that data.
3
Because the braille print version of this report will not contain the graphics, each figure is described in detail in the text.
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