From The Eye Book published by The Johns Hopkins Press
The word presbyopia is derived from Latin and translates literally—though rather unflatteringly to those of us who develop it—as “old eyes.” The term describes the phenomenon that eventually befalls everyone: the loss of our ability to focus up close. Presbyopia generally starts anywhere from age thirty-five to fifty. You may first notice it when you’re trying to read the label on a bottle of aspirin or the fine print of a magazine ad.
This is what happens: Remember the accommodative system, the muscle and lens system within the eye that enables us to focus on near objects? The ciliary muscles, part of this system, sit just behind the iris. When these muscles contract, they cause the lens to bulge forward and change the focus of the eye, so that we can see something up close.
Now, remember how the lens in our eye is always growing? Well, by the time we reach our mid-thirties, this constant thickening has taken its toll; the lens has lost the flexibility needed to adjust its shape. Consequently, we can’t accommodate—in other words, we can’t see close up—nearly as well as we used to. Over time, the closest point at which the eye can focus moves farther and farther away—which is why one day we start holding the newspaper at arm’s length to read it. But our arms are only so long, and eventually they’re just not long enough to let us read without the help of corrective lenses.
Because presbyopia is part of the normal growth and development process of the lens, it isn’t really what we think of as an “age-related degenerative change” to our body. That is, the ciliary muscles aren’t weakening, and no tissue is breaking down. This ongoing evolution of the lens is simply an inevitable part of aging, something we can’t change or control. There are no exercises or treatments that can stave off presbyopia, or even slow it down.
How will presbyopia affect you? It depends. If you’re mildly to moderately nearsighted, presbyopia means you can read comfortably without your glasses, because your myopia does the job of focusing for a near object in place of your ciliary muscle and lens system. Because you can read more easily without glasses, presbyopia might not even trouble you until your late forties or early fifties. If you’re farsighted, problems with presbyopia will likely occur much sooner; your ciliary muscles already have extra work just compensating for the hyperopia as well as focusing on objects up close (see above). People with astigmatism often compensate by bringing reading material closer, which makes it relatively larger and easier to see. When presbyopia begins, you probably won’t be able to compensate as well for your astigmatism, and you might need a presbyopic prescription sooner.
The presbyopic prescription is described as the lens addition or the add, and it’s usually the same in both eyes. That is, your distance prescription (if you need one) balances out your vision, so that you see as well as you can with either eye by itself and with both eyes together. Because both eyes receive the same message from the brain to accommodate for up-close viewing, once your distance vision is balanced, you’ll probably need an even amount of lens power added to both eyes to help you see to read. You can think of the distance prescription as the basic foundation for an eyeglass correction. The reading prescription is added on to this to give the eyes the extra power they need to see up close.
Refractive Changes That Occur with Age
With each of the refractive errors discussed above, the changes in eyeglass prescriptions that occur throughout our lives tend to follow certain basic patterns. However, no two people are exactly alike; in fact, no two eyes are exactly alike! That’s why, even though both eyes follow the same general trends as we get older, it’s not unusual for one eye to change at a different pace than the other. But here are some changes you can expect, depending on your particular eyesight.
Aging and Nearsightedness
Here is the typical lifetime course of myopia: As our eyes grow, over the first twenty-five years or so, nearsighted people tend to become more nearsighted. Also, because so much of these early years is spent in school, the many hours a day spent reading and focusing up close probably add to the problem. All of the accommodation (discussed above) necessary for this focusing creates a situation in which the ciliary muscles are constantly contracted and the lens is constantly focused at near; eventually the eye can adapt to that contracted state—in other words, by getting used to focusing mostly at a reading distance—and become still more myopic.
Between ages twenty-five and thirty-five, the nearsighted prescription usually doesn’t change much. However, as you might expect, those of us whose work involves extensive reading, writing, or other up-close focusing may still need increasingly stronger prescriptions during this time.
After about age thirty-five, the eye changes that cause accommodation problems begin to affect our degree of nearsightedness as well. As the up-close focusing system starts to fail, myopia that was brought on by excessive accommodation actually starts to get better; often this trend continues into our late fifties and early sixties. However, because this decrease in nearsightedness is a sign of diminishing accommodation, this seemingly happy turn of events may simply turn out to be the first step on the road to needing bifocals. Thus, for many people in their sixties there comes, after years of enjoying milder prescriptions, a reality check of sorts: becoming more nearsighted again. The most common type of cataract, nuclear sclerosis, is a consequence of years of growth and subsequent hardening of the lens. (For more on cataracts, see chapter 7 in The Eye Book.) In people with myopia this hardening of the lens at first may tend to increase its focusing power, and once again, we start getting more nearsighted.
Aging and Farsightedness
With hyperopia, again, there’s a typical lifetime cycle of progression. Up to about age twenty-five, farsightedness sometimes gets better on its own, as our eyes grow and develop. (If, for instance, the problem is that the eye is too short for light to be focused appropriately, this might resolve itself when the eye simply gets bigger.) Often there’s no need to correct this problem in someone younger than twenty-five, because the accommodative system has a tremendous ability to compensate for this kind of anatomical shortcoming.
But the eye changes that cause us to need bifocals can manifest themselves much earlier in someone who’s farsighted, and they can begin to affect our ability to read and focus up close as early as our twenties. As we get into our late thirties and early forties, when we begin holding the newspaper at arm’s length, we also begin to lose the ability to accommodate for our hyperopia. We begin having trouble with our distance vision and may require corrective lenses to help us to see better. Glass prescriptions for distance vision often get stronger in our late fifties and early sixties.
For farsighted people the dawn of cataract formation may cause the opposite effect of the change that occurs with nearsighted people: a shift in the power of the lens within the eye causes an increase in the eye’s focusing power, which results in a decrease in hyperopia. That is, in a farsighted person, distance vision without eyeglasses can actually improve with early cataracts. This is known as second sight (see below and chapter 7).
Aging and Astigmatism
Astigmatism doesn’t change as much with age as myopia or hyperopia do. After the eye stops growing, astigmatism levels off; decades may go by before you need a new prescription. Then along comes presbyopia—and this plus changes in lid tension across the cornea can cause changes in astigmatism that may be for better or worse, depending on your particular case. Early cataract formation can also cause changes, as the cataract subtly changes the shape of the lens within the eye.
Aging and Presbyopia
After its onset, presbyopia produces a classic pattern of change over the next fifteen to twenty years. Initially your glasses prescription is doing part of the focusing for you. As your accommodative system changes, the glasses prescription increases, until eventually the prescription does all of the focusing for you.
Usually the first symptom of presbyopia is that we can’t seem to bring an object as close to our eyes, and keep it in focus, as we used to. Also, it takes longer to bring a near object into focus and then to shift our focus to look at something across the street or even across the room. (The cause of this delay is the ever-thickening, increasingly less flexible lens, which doesn’t do its job as fluidly and effortlessly as it once did.)
As presbyopia progresses, the point at which we can focus moves farther and farther away, until eventually anything inside of twenty feet looks blurry. While your presbyopia progresses to the point where your reading addition to your distance prescription does 100 percent of the focusing for you, your near range of focus tends to diminish until your vision is really only clear from about twelve to twenty inches.
What’s happening is that as you need more lens power in your glasses to compensate for your presbyopia, your ciliary muscles and lens are less flexible for range of vision, and your near working distance keeps inching closer, to where the spectacle lens focuses.
Those of us who spend a lot of time viewing objects at intermediate distances of twenty to forty inches—the computer monitor, for example—often need additional lens prescriptions. For many people the best solution is either a progressive addition, “no-line” bifocal that allows for multiple working distances, or a trifocal that adds another lens to your spectacles, designed particularly for intermediate-distance viewing.
Early cataract formation, or nuclear sclerosis, also affects presbyopia. Because this causes a shift in vision that can make people either more nearsighted or less farsighted, some of us suddenly find ourselves able to read without our bifocals (a delightful phenomenon often referred to as second sight). Unfortunately, this is a short-term improvement; as the cataract progresses, vision at all levels tends to get worse.
