Blindness and Vision Loss
Visual impairment: a functional limitation in seeing, including both those with:
- "non-severe limitation" ("difficulty seeing words and letters") and
- those with "severe limitation" ("unable to see words and letters").
Legal blindness: a level of visual impairment that has been defined by law to determine eligibility for benefits.
Legal blindness refers to central visual acuity of 20/200 or less in the better eye with the best possible correction, as measured on a Snellen vision chart, or a visual field of 20 degrees or less.
Vision correctable to 20/20 with at least 180-degree field is considered 'normal vision'. A simplified example of visual acuity is that a person who is legally blind with 20/200 vision sees at approximately 20 feet what a person with 20/20 vision sees at 200 feet. A person with a visual field of 20 degrees or less is able to see no more than a 20-degree field without scanning.
Types of vision loss
General types of vision loss
- Reduced acuity
- Restricted fields (central or peripheral)
- Combination of reduced acuity and restricted fields
- Total blindness or light perception only
Reduced acuity can refer to a large range of functional vision from vision tested as 20/20 to totally blind. Lighting and contrast affect functional vision and are not reflected in the clinical measurements.
The general category of restricted fields can be further divided into central field loss and peripheral field loss.
The picture below represents a street crossing as it might be seen by a person with general reduced visual acuity. An overall loss of acuity, sensitivity to glare, and loss of contrast sensitivity is common in the elderly population.
Central field loss
Individuals with a central field loss usually will have difficulty seeing pedestrian signals, some signs, and details directly in front of them. Central field loss is typical of macular degeneration, the leading cause of blindness in those over 60.
Peripheral field loss
Individuals with peripheral field loss, sometimes referred to as tunnel vision, may see details directly in front of them clearly, but have difficulty with objects and signs off to the side. In addition, depth perception is often impaired.
Glaucoma and retinitis pigmentosa are the main causes of peripheral field loss.
Decrease in attentional field
Research by Brabyn, HaegerstrÃ¶m-Portnoy, Schneck, and Lott (Brabyn et al., 2000), demonstrated that over age 60-65 the prevalence of problems detecting objects in the peripheral visual field increases dramatically. This is known as a decrease in attentional field, and it may be present with or without other types of visual impairment. By age 90, 40% of people have an attentional field of less than 10 degrees left and right. Thus, if they are looking at a pedestrian signal head, they are unlikely to be visually aware of vehicles that may be turning across their path of travel, until it is too late to take appropriate action.
Total blindness or light perception
Individuals who are considered totally blind usually cannot see any difference in light and dark. Individuals who have light perception may be able to tell if it is dark or light and the direction of a bright light source, but do not have vision that is useable for discerning objects or the travel path.
Prevalence of blindness
Different sources provide different estimates of the prevalence of blindness in the U.S. There is no registry of individuals with vision loss in the U.S. and different methodologies are used to derive these estimates. Adams, Hendershot, & Marano (1999) estimate that some degree of vision impairment affects 8.3 million (3.1%) Americans of all ages.
Centers for Disease Control and Prevention (CDC) states that more than one million Americans are legally blind and 12 million are visually impaired. It is projected that the number of blind and visually impaired people will double by 2030 (CDC, 2003).
The Lighthouse National Survey reports that 8.7 million (9%) Americans age 45 and older report a severe vision impairment, defined as an inability to recognize a friend at arm's length even when wearing corrective lenses; an inability to read ordinary newspaper print with corrective lenses; poor or very poor vision even when wearing corrective lenses; or blindness in both eyes. (The Lighthouse Inc., 1995).
A Health and Activity Limitations post-censal survey (HALS) by Statistics Canada reported that 635,000 Canadians identified themselves as having a "seeing disability" (HALS, 1995).
Of the 635,000 Canadians with a seeing disability:
- 511,000 were adults living in households
- 94,000 were adults living in institutions
- 30,000 of these were children aged 14 years and under
Area of residence
Of people with vision impairments, 33% live in cities, 37% live in suburbs, 28% live in non-metropolitan areas (e.g., small towns) and 1% live in farm areas (Schmeidler & Halfmann, 1998). In comparison to the general population, persons who are visually impaired are somewhat under-represented in the suburbs (i.e., 48% of general population live in suburbs vs. 37% of visually impaired) and over-represented in cities.
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