By Harvard Health Poor night vision is the result of changes in the pupil, lens and retina. Does eating carrots help? Probably not for well-nourished Americans.
It's been said that the eyes are the windows to the soul. At a more mundane level, they also say a lot about how old we are. With advancing years, we're prone to a number of serious -- and less-serious (but bothersome) -- conditions of the eye.
The prime example of a serious age-related eye problem is macular degeneration, a retinal condition that's a leading cause of legal blindness among people over age 55 in the developed world. Age-related nuisances include a falloff in tear production that results in drier and itchier eyes. Eyelids also droop and sag, like other body parts. And starting around the fifth decade, almost all of us suffer from presbyopia (Greek for "old sight"), an inability to focus on objects close to our eyes.
It doesn't get that much attention, but diminished night vision is one of the most common problems of the aging eye. Rare is the person who, starting around age 40, doesn't dread driving at night. Roadway crashes are the leading cause of on-the-job fatalities among older workers in the United States; safety officials say that poor night vision could be partly to blame. And a certain -- if unspecified -- number of falls and stumbles at home are probably related to the inability of the older eye to pierce the gloom.
Smaller PuilsSeveral changes in the eye explain why we find it harder to see in darkness. The iris is familiar as that pigmented ring that gives our eyes their distinctive color. But it's not there just to look pretty. The iris comprises a tiny set of muscles that control the size of the pupil. And 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 five millimeters when we're young adults to about three 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 that's 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 in darkness. Cloudier LensChanges in the lens of the eye also serve to impair night vision. In the salad days of youth, our lenses are quite flexible and transparent. With age, they stiffen, and the result is presbyopia -- and many pairs of reading glasses.
They also become 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, and one side benefit is better night vision. Fewer RodsChanges in the retina are yet another reason for worsening night vision. The retina -- the membrane that lines the back of your eye -- is somewhat analogous to the film in a traditional camera or perhaps to the sensor in a digital one. Light hitting the photoreceptor cells of the retina triggers biochemical changes in those cells, which send signals to the optic nerve. When those signals reach the brain, they are processed as images, and we experience the sensation of sight. These photoreceptors come in two varieties, cones and rods. Cones respond to light in the wavelengths associated with color, giving us color vision. They're also responsible for visual detail: It's your cones that let you read the words on this page. Rods are useless for color -- they only provide black-and-white images -- but they're exquisitely sensitive and are therefore crucial to good night vision.
The fovea, a tiny portion of the retina, contains only cones. In the macula, which surrounds the fovea, rods outnumber cones 9:1 in healthy young adults. Researchers at the University of Alabama in Birmingham conducted a study that involved counting the cones and rods in the retinas of recently deceased older adults. They found an abundance of cones in the macula and a diminished number of rods. This study and others indicate that we hang on to our cones, but may lose almost a third of the rods in the area right around the fovea in the macula. Darker AdaptationOur ability to see at night isn't just a matter of navigating in darkness. Often, we're asking our eyes to adjust back and forth between light that's suddenly very bright and then dim again. Driving at night is hard for many reasons, but certainly one of them is making the adjustment back to relative darkness after being blasted by the headlights of an oncoming car. Even a younger person will feel blinded for a few moments. Ophthalmologists call the ability to see in the dark after exposure to bright light "dark adaptation," and it usually takes longer for older eyes. Having fewer rods may contribute to the problem. More likely, though, dark adaptation gets slower because rhodopsin, the light-sensitive pigment in the rod cells, doesn't regenerate as fast in older eyes. Next: What can you do about it? >
Source: Health & Wellness
