Blindness

Excerpt

Total blindness, low vision, and visual impairment encompass a diverse range of conditions, each with its unique characteristics and implications for daily life.

Total blindness describes those who have a complete lack of light perception, documented as no light perception (NLP). Only about 15% of people with eye disorders have total blindness, and the majority of those with visual impairment have some level of vision.

Low vision describes those whose vision cannot be fully corrected by conventional methods such as glasses, contact lenses, medicine, surgery, magnification aids, or assistive technology.

Visual impairment describes those whose decreased visual function interferes with the ability of one to perform their activities of daily living, such as reading, driving, and watching TV. Visual impairment is defined based on function instead of using visual acuity or visual field cutoff values, and includes those who have low vision or who are blind.

Symptoms of visual impairment include:

1. Blurred vision

2. Hazy/cloudiness

3. Wavy or spots in central vision

4. Restricted peripheral vision

5. Poor night vision

6. Difficulty seeing colors

Legal blindness is defined by the United States Social Security Administration (SSA) to determine those who are eligible to receive disability benefits, tax exemption programs, and rehabilitation training. SSA uses visual acuity or visual field results to determine this eligibility. A person is considered legally blind if he/she has central visual acuity of 20/200 or worse in the better-seeing eye with the best correction (using glasses or contact lenses) at a distance or if he/she has visual field restriction where the widest diameter is 20 degrees or less in the better-seeing eye.

There are several tests used to measure visual acuity or visual fields. Visual acuity testing for distance is carried out using the Snellen visual acuity chart or another test that is comparable to the Snellen methodology. In 2007, SSA updated the criteria for measuring visual acuity by allowing newer low-vision test charts to be used instead of Snellen acuity charts alone. Under this update, if a person cannot read at least one letter on the 20/100 line, he or she will be classified as legally blind.

Acceptable tests for visual field testing include automated static perimetry such as Humphrey Field Analyzer (HFA) 30-2, HFA 24-2, and Octopus 32, kinetic perimetry such as Goldmann perimetry or HFA "SSA Test Kinetic." Screening tests, including confrontation tests, tangent screen tests, and static screening tests, are not accepted forms of testing to determine legal blindness.

The World Health Organization (WHO) classifies visual impairment into the following categories based on visual acuity or a visual field of the better-seeing eye.

1. Normal: 20/10-20/25

2. Near normal visual impairment: 20/30-20/60

3. Moderate visual impairment: 20/70-20/160

4. Severe visual impairment: 20/200-20/400 or 11-20 degrees on visual field

5. Profound visual impairment: 20/500-20/1000 visual acuity or 6- 10 degrees on visual field

6. Near total visual impairment: Counting fingers, hand motion, light perception, or 5 degrees or less on visual field

7. Total visual impairment: No light perception

WHO also defines blindness as visual acuity of 3/60 (Snellen’s chart) or less or its equivalent (see Table. WHO and the National Programme on Control of Blindness [NPCB] classification of blindness). In the absence of visual acuity charts, when paramedical staff takes the visual acuity, it is defined as the inability to count fingers at a distance of 3 meters in daylight to indicate 3/60 or its equivalent.