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Presbyopia, or Why do I need reading glasses over age 40?

Like last month's column, this one begins with a brief explanation of light, optics, and eye anatomy.

First, in order for light to be focused, rays of light must all converge on a single point, this is true whether it be in the eye, using a pair of binoculars or a camera, or any other optical system. Light travels in rays that move in straight lines in all directions. Objects beyond six feet away are considered at optical infinity, since at this distance the light rays are essentially parallel. When objects are closer than six feet, light rays are diverging to various degrees, so they must be bent to become parallel by the eye or optical system, so that they will be in focus.

The focusing mechanism in eyes is called, appropriately, the lens. The lens is suspended by many elastic fibers and is situated directly behind the iris. The elastic fibrils are attached to a muscle that can relax or contract, changing the shape of the lens. When any lens changes its shape, it alters the amount that it bends light. This is how the eye can change focus from looking at something from far away to looking at something up close. When it looks at something up close, the lens changes shape and adds enough power to bend those light rays.

Why does this mechanism begin to fail after age 40? Throughout life, fibers are added on to the lens. This happens over our entire lifespans, and as a result the lens gets progressively stiffer and more difficult for the muscle to change its' shape. When the shape of the lens can not be altered, it can not add power, so focusing on near objects is difficult. To remedy this, bifocals or reading glasses are prescribed with a little more power to help us focus when the lens fails. This is a normal part of aging, and everyone experiences it at some point.

Why can people who are nearsighted read without their glasses, but not with them, after the age of 40? Remember from my last entry that people with nearsightedness have too much power in their eyes, so their eyes are built for focusing up close naturally. When they put glasses on, they no longer rely on the natural physiology of their eye to read, but the focusing mechanism of the lens, which ages. Taking glasses off, however, relies only on the inherent focus system of their eye.

Why can't some people with low vision focus as easily for near work? Many people with vision problems from birth have some degree of amblyopia. This was covered in my first column. Amblyopia makes it more difficult for the brain to process visual data. This makes it harder for the brain to tell the lens of the eye where to focus, and as a result, some people have problems with near work. This can be remedied with reading glasses or a bifocal, so consult with your eye care provider if this is a difficulty for you or your child. Those who are over 40 with reduced vision experience the same changes that someone without vision loss do, and as a result the lens stiffens with age.

What are cataracts? The lens of the eye may not always be completely transparent. Any limitation to the transparency of the lens is called a cataract. There are actually two general kinds of cataracts, congenital and acquired. Congenital cataracts are present at birth, while acquired cataracts build up over a lifetime. These may be to any degree of opacity from not impacting vision at all to eliminating it entirely. Think of it as a head of hair. One gray hair does not make the entire head gray, any more than a very small amount of cataract limits vision. Just like gray hair increases in density, so can a cataract. When a cataract is removed, it is replaced with a silicone lens which focuses light much as the natural lens did, but unlike a natural lens, it can not be focused since it is not attached to the elastic fibers or muscle.

J. Hensil