Blindness disables millions of people throughout the world. In developing countries, it is mostly caused by cataract, malnutrition, or infection. Then there are vision problems and blindness due to chronic systemic diseases, such as tuberculosis, pneumonia, and sexually transmitted diseases. Among preventable eye diseases, the three most common are vitamin A deficiency; trachoma, an inflammatory disease endemic in Asia and Africa that is caused by a micro-organism tra! nsmitted by flies; and onchocerciasis, also called river blindness, a parasitic infestation transmitted by the blackfly.

In developed countries, ready access to medical and surgical care mostly eliminates these problems. Instead, almost all blindness and low vision (functionally disabling poor sight that is uncorrectable by lenses) is associated with diseases that destroy tissue in the retina, optic nerve, or visual cortex. Further, because people live much longer, age-related degeneration of the macula (AMD), a key portion of the retina, is the leading cause of blindness.

The visual system can fall prey to other attacks. In glaucoma, pressure in the eye or weakness of support tissue damages the fibers of the optic nerve.

The group of diseases known as retinal dystrophies affect the photoreceptors and may be inherited. One of the best-known is retinitis pigmentosa, commonly first made manifest by night blindness and loss of the mid-periphery of the visual field; this area of loss then expands to form a ring hole in the field of view that spreads outward and inward until only a narrow island of central vision is left.

Other conditions affecting the retina derive from systemic diseases, such as diabetes; a clot blocking blood flow to the retina; or retinal detachments. Blindness may also follow a stroke that damages the visual cortex.

There is no single standard of "good vision." The principal measures used are acuity, sensitivity to contrast, and defects in the visual field. Other tests measure hyperacuity (for instance, the ability to align two line segments), depth perception, color discrimination, dark adaptation, and the ability to assess facial expressions, not to mention reading speed and visually guided manipulation of objects.

Defects in the visual field are variously mapped. In hyperacuity and depth-adjustment tests of the fovea, the most discriminatory part of the retina, normal subjects can achieve thresholds corresponding to resolutions of 5 arc seconds or less. If the same tests are performed for the periphery, thresholds rise rapidly--at least three times as fast as the size increase in the smallest recognizable letter on the acuity chart.

Low-vision patients fall into several broad classes, depending on the types of defects brought on by eye disease.

• Lack of foveal development from extreme premature birth; albinism; an absence of cone receptors, causing aversion to light; and involuntary oscillating eye movements (nystagmus)--any of which degrades performance on any acuity-related task but which can be somewhat helped by magnification. Many patients also have problems with contrast sensitivity and illumination. Visual acuity may be low, in the 20/100 to 20/200 range. (The underlying convention is to assign 20/20 to the letter size a normally sighted person can just recognize at 6 meters.)
• Central field loss--generally due to macular degeneration but also linked with some retinal dystrophies and optic nerve disorders. In age-related macular degeneration, photoreceptors in the area are destroyed. In the disease's "dry" form, an area of atrophy slowly expands into a horseshoe-shaped blind region around the fovea, closes into a ring, and invades the fovea. In a "wet" form of the disease, new blood vessels grow under the retina. Acuity suffers badly, dropping to 20/70 to 20/200 or 20/400 in the wet form if treated, as low as 20/1000 if scarring continues. Most patients have only a milder form of the disease and less loss of vision. Even so, it forces them to rely on their peripheral vision, which, even with proper magnification,! has such poor text resolution that adjacent letters look jumbled together.
• Peripheral field loss--due to advanced glaucoma (pressure around the optic nerve fibers), optic nerve disorders, or diseases such as retinitis pigmentosa. A special case is the loss of much of the left or right field of vision after a stroke has affected the visual cortex. These patients often find it hard to orient themselves and avoid obstacles. In advanced stages (tunnel vision), the restricted field hampers reading, and acuity and sensitivity to contrast may also be reduced.
• Localized gaps in vision--known as scotomas, which may be the results of diabetic damage to the retina or blocking of retinal blood vessels. Nowadays they are often an unfortunate side-effect of laser surgery intended to prevent further damage from diabetic retinopathy or wet age-related macular degeneration.
• Reduced contrast sensitivity--often a secondary effect of a disease process that has been left undiagnosed. It has much the same effect on vision as a dense fog: the luminance in images vary and color looks washed out. Slight changes in brightness and hue across a scene become very difficult to detect. Not surprisingly, patients can find it hard to recognize faces.
• Illumination and adaptation problems--often the lot of patients with diseases of the photoreceptors. Many patients with age-related macula degeneration report a need for bright illumination, but this need can often be addressed by contrast enhancement.
• Distortions. Any distortion could point to a membrane growing under the retina. The standard explanation alludes to wrinkling of the retina, but it seems the cortical map of the visual field may reorganize itself around the projection of a small scotoma, such as a laser scar.
• Unexplained problems in sustained reading. These may be said to occur if a patient is unable to read running text at a print size a few times her acuity limit, but can "spot-read," say, a single word or number with the same print size. Here, the sufferer may have a small island of relatively good acuity surrounded by a scotoma, as in mid-stage "dry" age-related macular degeneration. Alternatively, a scotoma (blind area) to the right of the fixation point (sometimes called a leading scotoma) may deprive the patient of the ability to glance ahead, which presumably enables readers to choose the next fixation target. --G.D. and R.W.M.

(c) Copyright 1996, The Institute of Electrical and Electronics Engineers, Inc.