Night Blindness

Eyes Need Vitamin A

Night blindness is a complex subject. Doctors now know that it can result from nutritional factors, genetics, uncorrected nearsightedness or an eye disease such as cataracts, macular degeneration or retinitis pigmentosa. And anything that affects vitamin A metabolism, such as liver disease, intestinal surgery, malabsorption or alcoholism, can also cause the problem.

Some forms of night blindness can be corrected with a pair of new glasses, according to Mitchell Friedlaender, M.D., director of cornea and refractive surgery at the Scripps Clinic and Research Foundation in La Jolla, California. Other forms require surgery. Still other forms of night blindness respond to something as simple as vitamin A.

The Vitamin Connection

What does vitamin A have to do with vision?

The answer is complex, says Elias Reichel, M.D., assistant professor of ophthalmology at Tufts University School of Medicine in Boston. But it involves the part of the eye called the retina.

"The retina is that part of the eye that acts like the film in a camera," he explains. "It's used to perceive light." On the retina there are structures called rods and cones. These house four kinds of pigments, one of which binds to an eye-friendly form of vitamin A. When you enter a sunny room and light enters your eye, the pigment transforms. Instantly, the vitamin A in your eye changes shape and, in that process, excites nerve endings to start transmitting electrical impulses to your brain to let it know what's going on--that you've entered a sunny room.

When you enter a dark room, the vitamin A changes shape again and helps your eyes realize that you've entered a dark room.

But nothing is ever completely light or completely dark. That's why you have more than 130 million light- and dark-sensing eye structures that make the minute adjustments necessary to perceive light and dark. And they're all dependent on vitamin A to do their jobs.

Despite this heavy demand for vitamin A, it still is pretty hard to develop a deficiency of this nutrient in the United States, where the foods in which it is contained are plentiful. Common staples such as milk and margarine are fortified with vitamin A, and orange and yellow foods such as sweet potatoes and carrots are rich sources of beta-carotene. (Beta-carotene is a precursor of vitamin A and converts to vitamin A in the body.) We need to depend on outside sources for vitamin A because the body can't make its own.

Besides, since a healthy liver is usually able to store up to a year's supply of vitamin A, you'd have to be chronically deprived of vitamin A food sources for quite a while for it to affect your sight, as is the case with millions of children in developing nations.

"Night blindness resulting from vitamin A deficiency is very, very, very, very rare in individuals who live in America," says Dr. Reichel. And even if it does develop, it can frequently be reversed within an hour by injections of vitamin A.

Most people with night blindness have eyes that mobilize vitamin A so slowly that it takes a while to adjust to the dark, says Dr. Reichel. People notice it most often when they're going into theaters or driving at night.

Slowing Genetic Damage

Not only is vitamin A an effective treatment for deficiency-induced night blindness, but it may also slow night blindness that is induced by several hereditary conditions, usually grouped under the name retinitis pigmentosa.

Retinitis pigmentosa is considered rare in the United States, occurring in somewhere between 50,000 and 100,000 people. But it is the most common genetic eye disease, striking those who have a genetic mutation that slowly destroys the light-sensing structures in their eyes. The mutation is inherited from a parent, who frequently is unaware that he carries a gene that can imperil his child's vision.

Unfortunately, it does. People with retinitis pigmentosa usually begin to develop a loss of daytime side (peripheral) vision in young adulthood that progresses to tunnel vision and eventually to loss of vision in midlife. Without treatment, most people with retinitis pigmentosa will have significantly reduced day vision between the ages of 50 and 80.

Fortunately, research has shown that vitamin A can slow the retinal damage that can cause night blindness in adults with retinitis pigmentosa.

In a study of nearly 600 people between the ages of 18 and 49, researchers at the Massachusetts Eye and Ear Infirmary in Boston reported that 15,000 international units of vitamin A in supplement form each day, added to the approximately 3,000 international units a day generally available in a well-balanced diet, could slow the progression of retinal degeneration that can cause night blindness as a result of retinitis pigmentosa. The study was led by Eliot L. Berson, M.D., professor of ophthalmology and director of the Berman-Gund Laboratory at Harvard Medical School.

The study showed that those who had the lowest intakes of vitamin A had the most progression of retinal degeneration, while those who had intakes of 18,000 international units a day had the least retinal degeneration. The 18,000 international units came from both supplements and foods: 15,000 international units from a supplement, plus a regular dietary intake of 3,000 international units.

Although it isn't a cure, the daily supplement of 15,000 international units of vitamin A is estimated to provide seven additional years of useful vision to the average person with retinitis pigmentosa who starts this therapy at age 32.

But not all types of vitamin A will do the job, says Dr. Berson. Although there are several forms of vitamin A, they all have different functions in the body and cannot be used interchangeably. The form used in the study was vitamin A palmitate.

There are no reports of otherwise healthy adults with retinitis pigmentosa becoming ill from 15,000 international units of vitamin A palmitate daily, says Dr. Berson. But that doesn't mean that it's okay to take more than this amount or that more is better. Vitamin A can cause liver damage when taken in doses exceeding 25,000 international units a day. And doses of 10,000 international units in early pregnancy have been linked to birth defects.

The therapeutic dose of 15,000 international units greatly exceeds the Daily Value of vitamin A, which is 5,000 international units. If you'd like to try this therapy, discuss it with your doctor first, especially if you are a woman of childbearing age. Women who are pregnant should not use this therapy.

Vitamin E Accelerates Vision Loss

Although vitamin A has been shown to slow the course of retinitis pigmentosa, researchers have data to suggest that vitamin E does exactly the opposite: In high doses, it destroys light-sensing cells by inhibiting the transport of vitamin A in the retina.

In Dr. Berson's study, for example, people with retinitis pigmentosa who took 400 international units of vitamin E every day appeared to lose their vision faster than those not taking this dose of vitamin E. Based on this information, the researchers estimated that if vitamin E supplements were started in someone with retinitis pigmentosa at age 32, the course of the condition could be accelerated by as much as five years.

The researchers noted that they had no evidence to suggest that small amounts of vitamin E, such as those found in