Visual impairment and blindness
Total blindness is the inability to tell light from dark, or the total inability to see. Visual impairment or low vision is a severe reduction in vision that can't be corrected with standard glasses or contact lenses and reduces aperson's ability to function at certain or all tasks. Legal blindness (whichis actually a severe visual impairment) refers to a best-corrected central vision of 20/200 or worse in the better eye or a visual acuity of better than 20/200 but with a visual field no greater than 20° (e.g., side vision thatis so reduced that it appears as if the person is looking through a tunnel).
Vision is normally measured using a Snellen chart. A Snellen chart has letters of different sizes that are read, one eye at a time, from a distance of 20ft. People with normal vision are able to read the 20 ft line at 20 ft--20/20vision--or the 40 ft line at 40 ft, the 100 ft line at 100 ft, and so forth.If at 20 ft the smallest readable letter is larger, vision is designated asthe distance from the chart over the size of the smallest letter that can beread.
Eye care professionals measure vision in many ways. Clarity (sharpness) of vision indicates how well a person's central visual status is. The diopter is the unit of measure for refractive errors such as nearsightedness, farsightedness, and astigmatism and indicates the strength of corrective lenses needed.People do not just see straight ahead; the entire area of vision is called the visual field. Some people have good vision (e.g., see clearly) but have areas of reduced or no vision (blind spots) in parts of their visual field. Others have good vision in the center but poor vision around the edges (peripheral visual field). People with very poor vision may be able only to count fingers at a given distance from their eyes. This distance becomes the measure oftheir ability to see.
The World Health Organization (WHO) defines impaired vision in five categories:
- Low vision 1 is a best corrected visual acuity of 20/70.
- Low vision 2 starts at 20/200.
- Blindness 3 is below 20/400.
- Blindness 4 is worse than 5/300
- Blindness 5 is no light perception at all
- A visual field between 5° and 10° (compared with a normal visual field of about 120°) goes into category 3; less than 5° into category 4, even if the tiny spot of central vision is perfect.
Color blindness is the reduced ability to perceive certain colors, usually red and green. It is a hereditary defect and affects very few tasks. Contrast sensitivity describes the ability to distinguish one object from another. A person with reduced contrast sensitivity may have problems seeing things in thefog because of the decrease in contrast between the object and the fog.
According to the WHO there are over forty million people worldwide whose vision is category 3 or worse, 80% of whom live in developing countries. Half ofthe blind population in the United States is over 65 years of age.
The leading causes of blindness include:
- Macular degeneration
- Diabetes mellitus.
Other possible causes include infections, injury, or nutrition.
Most infectious eye diseases have been eliminated in the industrialized nations by sanitation, medication, and public health measures. Viral infections are the main exception to this statement. Some infections that may lead to visual impairment include:
- Herpes simplex keratitis.
- Leprosy (Hansen's disease).
- River blindness.
Exposure of a pregnant woman to certain diseases (e.g., rubella or toxoplasmosis) can cause congenital eye problems. Injuries to the eyes can result in blindness. Very little blindness is due to disease in the brain or the optic nerves. Multiple sclerosis and similar nervous system diseases, brain tumors, diseases of the eye sockets, and head injuries are rare causes of blindness.
Vitamin A deficiency is a widespread cause of corneal degeneration in children in developing nations. As many as five million children develop xerophthalmia from this deficiency each year. Five percent end up blind.
A low vision exam is slightly different from a general exam. While a case history, visual status, and eye health evaluation are common to both exams, somethings do differ. Eye charts other than a Snellen eye chart will be used. Testing distance will vary. A trial frame worn by the patient is usually used instead of the instrument containing the lenses the patient sits behind (phoropter). Because the low vision exam is slightly more goal oriented than a general exam, for example, what specifically is the patient having trouble with (reading, seeing street signs, etc.), different optical and nonoptical aids will generally be tried. Eye health is the last thing to be checked so that thelights necessary to examine the eyes won't interfere with the rest of the testing.
There are many options for patients with visual impairment. There are opticaland nonoptical aids. Optical aids include:
- Hand magnifiers
- Stand magnifiers
- Closed circuit television (CCTV)
Nonoptical aids can include large print books and magazines, check-writing guides, large print dials on the telephone, and more.
For those who are blind, there are enormous resources available to improve the quality of life. For the legally blind, financial assistance for help may be possible. Braille and audio books are increasingly available. Guide dogs provide well-trained eyes and independence. Orientation and mobility training is available. There are special schools for blind children and access to disability support through Social Security and private institutions.
The prognosis generally relates to the severity of the impairment and the ability of the aids to correct it. A good low vision exam is important to be aware of the latest low vision aids.