Low vision is a subspecialty within the professions of optometry and ophthalmology and Opticianry dealing with individuals who have less than normal vision even with the most accurate conventional prescription available. It can be a result of either congenital or acquired factors. An example of the former is Leber's congenital amaurosis and of the latter Age-Related Macular Degeneration.
Classifying Low Vision
Anyone with noncorrectable reduced vision is considered to be visually impaired, and can have a wide range of causes. The World Health Organization uses the following classifications of visual impairment. When the vision in the better eye with best possible glasses correction is:
- 20/30 to 20/60 : is considered mild vision loss, or near-normal vision
- 20/70 to 20/160 : is considered moderate visual impairment, or moderate low vision
- 20/200 to 20/400 : is considered severe visual impairment, or severe low vision
- 20/500 to 20/1,000 : is considered profound visual impairment, or profound low vision
- less than 20/1,000 : is considered near-total visual impairment, or near total blindness
- No Light Perception : is considered total visual impairment, or total blindness
There are also levels of visual impairment based on visual field loss (loss of peripheral vision).
In the United States, any person with vision that cannot be corrected to better than 20/200 in the best eye, or who has 20 degrees (diameter) or less of visual field remaining, is considered to be "legally blind" or eligible for disability classification and possible inclusion in certain government sponsored programs.
Low Vision and its Lifestyle Implications
Visual impairments may take many forms and be of varying degrees. Visual acuity alone is not always a good predictor of the degree of problems a person may have. Someone with relatively good acuity (e.g., 20/40) can have difficulty with daily functioning, while someone with worse acuity (e.g., 20/200) may function reasonably well if their visual demands are not great.
Some people who fall into this category can use their considerable residual vision - their remaining sight - to complete daily tasks without relying on alternative methods. The role of a low vision specialist (optometrist or ophthalmologist) is to maximize the functional level of a patient's vision by optical or non-optical means. Primarily, this is by use of magnification in the form of telescopic systems for distance vision and optical or electronic magnification for near tasks.
The Low Vision Examination
It is critical that all patients be examined by an optometrist or ophthalmologist specializing in Low Vision Care prior to other rehabilitation training to rule out potential medical or surgical correction for the problem and to establish a careful baseline refraction and prescription of both normal and low vision glasses and optical aids. Only a doctor is qualified to evaluate visual functioning of a compromised visual syetem effectively. American Optometric Association web site
Low Vision Aids
Some people with significantly reduced acuity may benefit from training conducted by individuals trained in the provision of technical aids. Rehabilitation professionals, some of whom are connected to an agency for the blind, can provide advice on lighting and contrast to maximize remaining vision. These professionals also have access to non-visual aids, and can instruct patients in their uses.
Visually impaired patients may benefit from high-tech aids such as OCR scanners that can, in conjunction with text-to-speech software, read the contents of books and documents aloud via computer. Vendors also build closed-circuit televisions that electronically magnify paper, and even change its contrast and color, for visually impaired users.
The vast majority of patients with low vision can be helped to function at a higher level with the use of low vision devices. Low vision specialists recommend appropriate low vision devices and counsel patients on how better to deal with their reduced vision in general. Many government and private organizations exist to aid the visually impaired.
- American National Eye Institute Health Education Program
- Envision Choices and resources for people who are blind or low vision
- Journal of Visual Impairment & Blindness Special Issue on Low Vision
- National Eye Institute
- Lighthouse International Helping people with vision loss
- Low Vision Tips
- VisionAWARE - Self Help for Vision Loss Independence for adults with low vision
- Checkered Eye Project