Visual Disorders Health Article

Definition

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.

Description

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 have good vision in the center but poor vision around the edges (peripheral visual field). Patients with very poor vision may be unable to view any letters on the eye chart; they then will be asked to count fingers at a given distance from their eyes. This distance becomes the measure of their 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°) 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

The leading causes of blindness include:

• macular degeneration
• glaucoma
• cataracts
• diabetes mellitus

Other possible etiologies include infections, injury, or poor nutrition.

Infections

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.