The Aging Eye

If the eye is a camera, its photographic components lose precision with age. Nearly all the structures that bend, guide, and transform light entering the eye change with age—some dramatically, some only slightly—reducing the amount of light reaching the retina. In fact, eye specialists have estimated that the retina of a 60-year-old typically receives one-third the light of a 20-year-old's retina.

Light may be limited even before it strikes the cornea. Baggy or droopy eyelids, which may occur after age 40, cut down on available light. Thinning of the eyelids with age, for example, may cause skin from the upper lid to protrude like an awning over the eyelashes, restricting the field of vision. Or the lower lid may turn inward, rubbing against the eye and causing pain and tearing. If these conditions are serious, plastic surgery may be recommended to alleviate them.

The aging cornea not only flattens, limiting the ability to focus, but may also be flecked with fatty deposits that reduce transmission of light. Increased scattering of light gives the cornea a yellowish tinge, reducing the luster of aging eyes. The cornea also loses sensitivity to touch with age, so injury to the eye's protective surface may occur without pain or other warning.

The smaller opening of the pupil with age, a condition known as senile miosis, markedly limits the amount of light falling on the retina. The disorder may be most serious in dim light, because the aging pupil's maximum diameter may only be one-fourth that of a younger eye. One possible benefit of the smaller diameter, akin to choosing the smaller opening of a camera lens, is enhancement of depth of field—objects both near and far are more likely to be in focus, although they appear dimmer. In fact, some older people may report improvement of eyesight, apparently due to smaller pupil size.

Yellowing or clouding of the lens is another factor in reducing illumination. In addition, the yellowing lens also acts as a filter, absorbing more blue and violet light. This changes the perception of colors: white objects may appear yellow, and the distinction between blues and greens is decreased.

Structural changes in the vitreous body, a large gel-like region that lies between the lens and the retina, have several implications for the elderly. In the aging eye, the vitreum liquefies and the gel portions clump together, causing spots, or floaters, to appear in the field of vision. The same change may trigger the vitreum to detach and pull away from the retina. The extra force may cause swelling of the macula, bleeding, or even detachment of the retina, resulting in severe impairment or loss of vision. Vitreous detachment is usually sudden, triggered by a jolt or other physical injury, and older workers exposed to large and frequent vibrations may be at high risk.

Light reaching the aging retina encounters fewer nerve cells than it does in younger eyes. For example, the number of light-sensing cones in the most sensitive area of the retina (the fovea) may decrease dramatically between age 40 and age 60. This is believed to affect the clarity of eyesight. Electrical measurements of visual activity of the brain, recorded from the scalp, appear to indicate that visual processing is slower in the elderly. Other age-related changes in the retina and the eye's nervous system are under study.

No older patient is typical. Although only about 17 percent of elderly people are severely visually impaired, quality of vision varies widely. For example, 60 percent of older people have near-perfect (20/20) vision with eyeglasses; about 40 percent have 20/40 vision or worse, discerning only at 20 feet what a person with normal vision can still see at double the distance.

A common problem among people over 40 is presbyopia, a gradual decline in the ability to focus on nearby objects and small print. The condition occurs when the lens of the eye hardens, losing the soft, pliable quality that enabled it to change shape for focusing. There is no cure, but eyeglasses and contact lenses help compensate for reduced vision. During their 40s and 50s people may change glasses every two years due to increased hardening of the lens.