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Poor Nighttime Vision a Sad Reality of The Aging Process

Editor's Note: This article is reprinted from the Guelph Mercury, July 8, 2006.

In 800 B.C. Homer wrote, "Now the night comes and it is wise to obey the night." In his time there were frequent muggings at night and home robberies. Not much has changed since then. But today we're even more vulnerable, by being involved in tragic car accidents as the sun goes down. Our eyesight may be to blame.

So what happens to our sight as we age, and how can we decrease the risk of death on the highway?

A report from the Harvard Medical School states that "roadway crashes are the leading cause of on-the-job fatalities among older workers in the United States and that poor vision is partly to blame."

Macular degeneration is one of the causes of these accidents. And for those over the age of 55 years it's also the main reason for being classified as "legally blind." The macula is a pin-sized area in the retina at the back of the eye that transmits images to the brain. Like a rifle bullet it can zero in on small, distant objects. And when the macula develops degenerative changes, nighttime driving becomes hazardous.

Several other changes occur in the eye as we age. Like other parts of the body, the eyelids begin to droop, providing less peripheral vision. The pigmented iris that makes us brown- or blue-eyed isn't present just to make us pretty. It also begins to sag as tiny muscles that control the size of the pupil weaken with age, making it more difficult to keep the iris open. During our early years the size of the iris is about five millimetres. But it shrinks to about three millimetres later in life. A smaller pupil allows less light to strike the retina making nighttime vision less sharp. This loss of vision is not detectable during the day. But driving a car with droopy eyelids and a narrowed pupil has the same effect as wearing sunglasses while driving at night.

Older drivers are also unable to adapt as quickly to changes in intensity of light. On occasion we've all complained, "Why didn't that idiot turn off his high beam?" An aging iris contracts more slowly to the blazing light. But once the car has passed, it's also slow in dilating to provide more light for the dark road ahead. And a light pigment called rhodopsin that helps adjust to night vision decreases as we age. Today, most people are aware of the risk of developing cataracts in the lens in later years. A less transparent lens decreases the amount of light striking the retina and worsens night driving. The final problem for the aging eye involves photoreceptors in the retina. Photoreceptors transmit the image of an upcoming car to the brain. Older adults can lose one-third of them.

So what can we do to decrease the hazards of driving at night? Mothers urged us to eat our carrots to maintain good vision and they were partly right. Carrots are rich in vitamin A needed to regenerate rhodopsin. But there's no sure way to prevent macular degeneration. Some doctors advise patients to take a daily pill containing lutein, a macular pigment. They believe that this antioxidant helps to slow down the disease. Others say eating a diet rich in fruits and vegetables makes more sense. The only way to improve night vision is cataract surgery. But there are several studies going back many years that rarely get much attention. They report that people who take vitamin C show decreased risk of developing cataracts. This continues to make sense as, unlike animals, humans do not manufacture this powerful antioxidant vitamin.

A sure way to survive night driving is to be realistic about your ability to see well in the dark. And good sense dictates that as visual acuity decreases, it's prudent to stay away from the steering wheel after sunset. Homer was right: obeying the night saves lives.

Gifford-Jones is the pen name of Toronto physician Ken Walker.

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