Visual disorders are an impairment in vision, the ability to see. Total blindness is the inability to tell light from dark, or the total lack of vision. Visual impairment or low vision is a severe reduction in vision that cannot be corrected with standard glasses or contact lenses, and reduces a patient's ability to function at certain tasks. Legal blindness, defined as a severe visual impairment, refers to a best-corrected central vision of 20/200 or worse in the better eye, best corrected, or a visual acuity of better than 20/200 but with a visual field no greater than 20°—for example, side vision that is so reduced that it appears as if the person is looking through a tunnel.
Vision is measured, as a rule, using a Snellen chart. A Snellen chart has letters of different sizes that are read, one eye at a time, from a distance of 20 ft. People with normal vision are able to read the 20 ft line at 20 ft—20/20 vision—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 as the distance from the chart over the size of the smallest letter that can be read.
Eye care professionals measure vision in many ways. Vision clarity indicates the strength of an individual's central visual status. The diopter is the unit of measure for refractive errors such as nearsightedness, farsightedness, and astigmatism, and indicates the strength of corrective lenses needed. Patients do not just see straight ahead; the entire vision area is called the visual field. Some patients see clearly but have areas of reduced vision or blind spots in parts of their visual field. Others
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°) enters category 3; less than 5° into category 4, even if the tiny spot of central vision is perfect.
Color blindness represents the reduced ability to perceive certain colors, usually red and green. It is a hereditary defect and affects few tasks. Contrast sensitivity describes the ability to distinguish one object from another. Patients with reduced contrast sensitivity may have problems seeing things in the fog, for instance, due to decreased contrast between the object and the fog.
According to the WHO over 40 million people worldwide have vision that is category 3 or worse, 80% of whom live in developing countries. Half of the blind population in the United States is older than 65.
Causes and symptoms
Other possible etiologies include infections, injury, or poor nutrition.
Most infectious eye diseases have been eliminated in the industrialized nations through 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. A viral infection of the cornea. Repeated occurrences may lead to corneal scarring.
- Trachoma. Trachoma is caused by an incomplete bacterium, Chlamydia trachomatis, that is easily treated with standard antibiotics. It is transmitted directly from eye to eye, mostly by flies. The chlamydia gradually destroy the cornea. This disease accounts worldwide for six to nine million of the third of a billion documented cases of blindness.
- Leprosy (Hansen's disease). This is bacterial disease that has a high affinity for the eyes. It can be effectively treated with medicines.
- River blindness. Much of the tropics of the Eastern Hemisphere are infested with Onchocerca volvulus, a worm that causes "river blindness." This worm is transmitted by fly bites and can be treated with a drug called ivermectin. Twenty-eight million people suffer from the disease, and 40% of those have incurred blindness as a result.
When a pregnant woman is exposed to certain diseases, such as, rubella or toxoplasmosis, congenital eye problems can occur in her child. Also, eye injuries can result in blindness. Brain disease, or disease in the optic nerves accounts for a minimal amount of blindenss. Multiple sclerosis and similar nervous system diseases, brain tumors, eye socket diseases, and head injuries are also 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 become blind.
A low vision examination differs from a general examination. In many cases, the patient already has had a complete eye exam and is referred to a low vision specialist. These specialists can be either optometrists (O.D.s) or ophthalmologists (M.D.s). Case history, visual status, and eye health evaluation are common to both examinations, but other elements vary.
Because the low vision examination often results from a general examination, the specialists focus more intensely on the specific complaints detailing a patient's daily visual demands. Examiners must determine the exact source and outcome of the patient's visual challenges. Because many of these patients are elderly and may not want to complain about poor vision, it is crucial that the physician or ophthalmic assistant document the
Tests may include depth perception, color vision, and contrast sensitivity. Eye charts, with a larger range of letters than a Snellen eye chart, will be used. Testing distance will vary depending on the patient's ability to see. Refraction is facilitated with the use of a trial frame. Patients with poor vision may not be able to distinguish between lenses in the phoropter. The take-home test is more "real-life" in that it ennables the patient to utilize his or her side vision as well.
There are many options for patients with visual impairment. There are optical and nonoptical aids. Optical aids include:
- Telescopes. May be used to read street signs, watch television, and attend plays and sporting events.
- Hand magnifiers. May be used to read labels on items at the store, and menus.
- Stand magnifiers. May be used to read books, magazines, and other material.
- Prisms. Are utilized to move the image onto a healthy part of the retina, providing a helpful technique for vision only in eye diseases in which the healthy part of the retina exists.
- Closed circuit television (CCTV). For large magnification (for example, for reading fine print).
Nonoptical aids include special illumination, large print books and magazines, check-writing guides, large print dials on the telephone, and more. Special computer software is also used to provide low vision patients usable access—access which ennable the individual to read what is being accessed—to computer programs.
Ophthalmic occupational therapists or rehabilitation specialists usually work in tandem with low vision specialists to help patients use these devices properly. Many times these professionals will make visits to the patient's home to ensure correct use of these aids and answer any
questions about low vision. Patients sometimes will be able to use the device correctly in the physician's office, but may be unable to do so at home. This inability can be due to forgetfulness; more often it is due to a low vision plan that has not been correctly adapted to the person's home environment. Home visits are crucial for effective treatment.
For those who are blind, extensive resources are available to improve the quality of life. For the legally blind, financial assistance for help may be possible from state and private organizations. Braille and audio books are increasingly available. Books-on-tape are provided free of charge from the Library of Congress to those who qualify as legally blind; and the service is usually arranged through the local public library. Guide dogs provide well-trained eyes and independence. Occupational therapists and rehabilitation specialists can provide orientation and mobility training. Special schools for blind children exist throughout the United States, as well as access to disability support through Social Security and private institutions.
The prognosis is often determined by the severity of the impairment and the ability of the aids to correct it. It also depends on the patient's ability and willingness to learn how to utilize the devices. The benefits of a throrough low vision examination include presentation of the most current low vision aids.
Health care team roles
Skilled ophthalmic nurses, technicians, and assistants help the O.D.s and M.D.s diagnose low vision by assisting with testing. These professionals log the patient history and perform many of the preliminary tests. Highly skilled technicians perform visual field tests and refractions.
Occupational therapists and rehabilitation specialists play an important role in treating low vision patients. They answer questions about low vision aids and instruct them on the devices' proper uses. They also help provide a sense of independence to these patients who may have previously been restricted in their activities by a total lack of, or limited vision.
These therapists and specialists also help totally blind patients adjust to daily life by providing orientation and mobility training. They evaluate home and job environments and make recommendations for adaptation. These professionals also consult with family members to ensure effective care methods. Especially with older adults, a total care plan that includes family and caretakers is essential to the success of offsetting the negative effects or trauma of decreased vision.
Low vision specialists, the referring O.D.s and M.D.s, and ophthalmic staff need to make sure their patients fully understand their conditions. Many elderly patients are confused by the diagnosis and need to be carefully told what their condition means, what treatment options they can utilize. Some practitioners use a video explaining macular degeneration, for example, to further emphasize the disease's impact. Large-print brochures also are helpful. Occupational therapists and rehabilitation specialists need to make sure they emphasize the correct use of visual aids so patients can receive the maximum benefit from them.
Cataracts should be removed when they interfere with a person's quality of life.
Primary prevention addresses the causes before they begin to cause eye damage. In those climates and environments where it is an issue for eye diseases, fly control can be accomplished by simple sanitation methods. Public health measures can reduce the incidence of many infectious diseases. Vitamin A supplementation, when appropriate, will eliminate xerophthalmia completely. Isome studies show that protecting the eyes against ultraviolet (UV) light will reduce the incidence of cataracts, macular degeneration, and some other eye diseases. UV coatings can be placed on regular glasses, sunglasses, and ski goggles. Protective goggles should also be worn during certain activities for protection.
Secondary prevention addresses treating established diseases before they cause irreversible eye damage. Regular general physical examinations can also detect systemic diseases such as diabetes or high blood pressure. Diabetes control is a crucial factor in preserving sight in people affected by the disease.
"Neuro-ophthalmology." Cecil Textbook of Medicine, edited by J. Claude Bennett, and Fred Plum. Philadelphia, PA:W. B. Saunders, 1996.
American Academy of Ophthalmology. P.O. Box 7242, San Francisco, CA 94120-7242. (415) 561-8500. <http://www.eyenet.org>.
American Foundation for the Blind. 11 Penn Plaza, Suite 300, New York, NY 10001. (800) 232-5463. <http://www.afb.org>.
Guide Dogs for the Blind. P.O. Box 1200, San Rafael, CA 94915. (415) 499-4000. <http://www.guidedogs.org>.
International Eye Foundation. 7801 Norfolk Avenue, Bethesda, MD 20814. (301) 986-1830.
The Lighthouse National Center for Education. 111 E. 59th Street. New York, NY 10022. (800) 334-5497. <http://www.lighthouse.org>.
National Association for the Visually Handicapped. 22 West 21st Street, New York, NY 10010. (212) 889-3141.
National Center For Sight. (800) 221-3004.
National Children's Eye Care Foundation. One Clinic Center, A3-108, Cleveland, OH 44195. (216) 444-0488.
National Eye Institute of the NIH. 9000 Rockville Pike, Bethesda, MD 20892. (301) 496-5248. <http://www.nei.nih.gov>.