Night Vision Problems
Blinded By the Night
Itís been said that the eyes are the windows to the soul. At a more level, they also say a lot 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.
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 fourth or fifth decade, almost all of us develop presbyopia, 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. Often poor night vision is partly to blame for vehicular accidents, and many falls and stumbles at home can also be related to the inability of the older eye to pierce the gloom.
Several 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.
Taking a dim view
Changes 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.
Changes 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 of 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. Several years ago, 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.
Our 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 adaption," 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.
What can you do about it?
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, and a derivative of vitamin A (11-cw-retinal) is needed for dark adaptation because it regenerates the rhodopsin in rod cells after they've been exposed to bright light. Moreover, vitamin A supplementation - if not carrots specifically - has been shown to cure night blindness in people with vitamin A-deficient diets. This is however a problem seen only in extreme undernourishment. Most of us are rarely short on vitamin A. So, for the better fed amongst us, eating carrots probably won't have much impact on night vision.
In any case, solving a vitamin deficiency is a very different matter from super sizing vitamin doses to reverse a process that is, more or less, a natural part of aging. Moreover, high doses of vitamin A - as retinol, not beta carotene - can carry some risks. High retinol intake has been linked to osteoporosis and increased risk of fracture, especially of the hips.
Another possibility is boosting the macular pigments, such as lutein, which have antioxidant properties and which have been shown to help people with intermediate or advanced macular degeneration. But according to Dr. Gregory Jackson, one of the University of Alabama researchers who have studied night vision extensively, there's little evidence that shortages of macular pigment play a role in rod loss or dysfunction.
Aside from cataract surgery, there's no medical intervention that will fix night vision. Researchers have found that many older drivers voluntarily give up night driving. That's often a wise decision. So if your schedule permits, limiting yourself to daytime driving may be the only solution to the driving problems caused by poor night vision. A few night lights can illuminate your path at home, so your rods don't have to work as hard. In fact, researchers have found that older people tend to keep their homes darker than younger people - despite reduced sensitivity to light. Opening your shades and using bright lights is one way to help out the aging eye. Here, as in many other areas, the key to aging gracefully or successfully - or however you want to put it - is really a matter of working with a problem rather than trying to deny it or treat it into submission.