HARVARD MEDICAL SCHOOL.
July 8, 2006.
Question: As I've gotten older, I've noticed that I can't see as well at night, especially when I'm driving. Why does this happen, and is there anything I can do about it?
Answer: Diminished night vision is a very common problem. Starting about age 40, most people feel less comfortable driving at night than they did when they were younger. And a certain number of falls and stumbles at home are probably related to the inability of the older eye to see well in dim light.
Several changes in the eye explain why we find it harder to see in darkness. One has to do with the iris, the pigmented ring that gives our eyes their distinctive color. The iris has tiny muscles that control the size of the pupil. Like many muscles in the body, those in the iris get weaker and less responsive with age. As a result, the pupil shrinks from a diameter of about 5 millimeters when we're young adults to about 3 millimeters in old age. A smaller pupil means less light can enter the eye. People don't necessarily notice the difference during daylight hours or in a brightly lit room. But in dim light or darkness, the smaller pupil has an effect comparable to wearing sunglasses.
Changes in the iris are also part of the reason older eyes have a harder time adjusting to changes in light - going from darkness into bright light and vice versa. The muscles don't react as quickly, so the pupil is slower to constrict in reaction to bright light and to dilate in reaction to darkness.
Lens changes also can impair night vision. With age, the eye's lens becomes less transparent, allowing less light to pass through, which worsens night vision. When part of a lens becomes very opaque, it is called a cataract. The main symptoms are blurry vision and difficulty with glare, but night vision suffers, too. Cataract surgery is very common, safe and effective these days. One very helpful side benefit is better night vision.
Changes in the retina are yet another reason for worsening night vision. The retina (the membrane that lines the back of your eye) contains two types of light-gathering cells: cones and rods. The cones are responsible for color vision, while the rods provide only black-and-white images. But the rods are extra sensitive and therefore crucial for night vision. Recent research suggests that although people retain their cones as they age, they might lose almost a third of the rods in certain areas of the retina.
Driving at night is hard for many reasons. One is making the adjustment back to relative darkness after being blasted by the headlights of an oncoming car. Eye doctors call the ability to see in the dark after exposure to bright light "dark adaptation." It usually takes longer for older eyes. Having fewer rods might contribute to the problem. More likely, though, dark adaptation gets slower because the light-sensitive pigment in the rod cells doesn't regenerate as quickly in older eyes.
When people think about improving their night vision, often the first thing that comes to mind is carrots. Most of us grew up being told that carrots help you see better at night, and there's some truth to that. Carrots are rich in vitamin A. Our bodies need a molecule derived from vitamin A to regenerate the pigment in rod cells after they've been exposed to bright light. But the typical American diet is rarely short on vitamin A. Multivitamins also tend to include ample amounts of vitamin A. So, for well-nourished Americans, eating carrots or taking vitamin A supplements probably won't have much effect on night vision.
However, experts say eating a good overall diet that's rich in fruits and vegetables and low in saturated fat is important. Just like in your heart and blood vessels, deposits of cholesterol and other waste products can cause problems in the eye. This buildup can prevent vitamin A from replenishing pigment in the rods.
Aside from cataract surgery, there's no medical intervention that will fix night vision. When possible, limit yourself to daytime driving.
From the the faculty of HARVARD MEDICAL SCHOOL. Submit questions at www.health.harvard.edu/adviser.
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