Mature Vision: Presbyopia
By early adulthood, the human eye is fully developed; at its maximum potential it includes various components that are not easily measured by traditional eye charts, such as depth perception, night vision and hand-eye coordination. However, vision does still change over time.
By about the age of 40, the symptoms of "presbyopia" begin to become noticeable, most commonly by difficulties reading fine print or using a computer for extended time periods. Presbyopia is one of the four "refractive errors", along with "myopia" (nearsightedness), "hyperopia" (farsightedness) and "astigmatism".
Presbyopia is almost universal in modern society; this condition causing blur when looking at reading materials is due to the crystalline lens inside the eye becoming thicker and less flexible over time.
Presbyopia (meaning “older vision”) does occur because we are aging, but not in the way most people think. Our eyes are not becoming weaker, and our focusing muscles are still working normally.
When we are younger, we change our focus from far away to close up without obvious effort, automatically. Muscles located behind the iris contract and relax, which changes the shape of the crystalline lens, moving the focal point of the eye from distance to near and back again. This happens without our being aware of it in most cases.
The crystalline lens inside the eye is one of the only tissues in the body that continues to grow throughout life; like a tree adds a ring to its growth pattern each year, the lens also adds material, becoming thicker, heavier, and harder to change shape when a focus change is needed. This is what presbyopia is: the difficulty almost all of us begin to notice in our middle years with reading materials held within arm’s length of our eyes. At first, we may find that holding the material farther away helps, but eventually this attempt to adapt to the changing optics in the eye becomes inadequate.
When presbyopia makes near vision difficult or impossible, we need help to focus on our reading, which is what reading glasses, bifocals and progressive lenses are meant to do. There are now even multifocal contact lenses.
No! Not BIFOCALS!
The word “bifocals” has a negative connotation in our society, because there is a perception that only “old people” need them; they are also perceived by some as making the person wearing them look older. In addition to these factors, many people have heard others talk about the trouble they may have had in learning to use them, and don’t want to try them for that reason.
Actually, when Benjamin Franklin, who is widely believed to be the inventor of the bifocal lens, knew what he was doing. He wanted to be able to see his reading material and still be able to look up and see clearly across the room or even down the street, without having to change his glasses. A bifocal, or any type of multi-focus lens, is simply a convenience item. Bifocal lenses just made his life easier by making it unnecessary to change his distance glasses for his reading glasses whenever he wanted to change the length of his gaze.
Something else most people may not realize is that eyecare practitioners often prescribe bifocal lenses for children, in order to help them focus more easily and read better. Others, young adults still in college or young professionals, benefit from bifocal lenses to relieve the stress that extended studying or computer use can cause. “Bifocal” does not always mean “old.”
Adapting to a multifocal lens isn’t very difficult, either; we recommend that patients with their first multifocal lenses wear them full time for about the first week, even if they do not need a prescription lens to see clearly in the distance. This allows the brain to figure out that simply moving the eyes downward lets near vision be clear, while straight-ahead gaze gives clear distance vision. In other words, it is necessary to learn to move our two-ounce eyeballs, rather than our fifteen-pound head. Once this becomes automatic, there should be no further problems with adapting to the use of a multifocal lens.. People who have great difficulty in using multifocals have usually not had them on long enough for the brain to make that connection.
People with presbyopia have several options in the types of lenses available, including single-vision reading glasses, regular bifocals that have the visible line and “no line” or progressive multifocals. There are also contact lenses available with multifocal correction. Each type of lens has advantages and disadvantages, and deciding which is right for you will depend on the activities you normally pursue and your overall lifestyle. The person with the right training and expertise to help with this is the optician.
All of the lens types discussed here (except perhaps the contact lenses) are available with various anti-reflective and anti-scratch coatings, as well as in those that change from light to dark with the amount of light. People with high amounts of prescription can also get their lenses in thin, lightweight designs.
Single-Vision Reading Glasses
This is a good choice for people who don’t need to change focus often; if you use a computer for hours during the day, or if you read for several hours, reading glasses give a nice, clear, full-field of clear vision. Contact lens wearers who want to keep using their current lenses can use readers over them for near tasks. Reading glasses in single vision are also less expensive and are available in a wide range of attractive frames in either full-frame or half-eye styles, which allow the person wearing them to easily look over them to see the clock down the hall or the calendar on the wall.
The main disadvantage to single-vision reading glasses is that when you are wearing them and raise your head and eyes from their near tasks, the distance will be significantly blurred. Some people have good tolerance for this, and some don’t like to experience any blur at all, at any distance.
Single-vision reading glasses are available over-the-counter (OTC) in a variety of retail outlets. These will be even less expensive, but the power of the lenses will be the same in both eyes; if you have different amounts of prescription for each eye, these may not work well for you. Bring a sample of the size of print you read most and look through the lenses for several minutes to be sure the frame is comfortable and that there is no “pulling” sensation or fatigue of our eyes, which can be caused by the lenses being set into the frames with the centers either too far apart or too close together to match the distance between your own pupils.
Flat-Top Bifocals and Trifocals
Many people use lenses with visible segments without problems, either in bifocal or trifocal form. This type of lens will allow clear vision through the top part of the lenses (above the line) and clear vision for close work (below the line) as well. Sometimes people need to see clearly at intermediate distances, too far away for the near lenses and too close for the distance ones; in this case, trifocals will make all three distances clear.
Flat-top style bifocals and trifocals have a long and proven history and work very well for the most part. They are more expensive than single-vision reading glasses, but less expensive than progressive or “no line” bifocals.
A "progressive addition lens (PAL)" uses optics that are different from either single vision or flat-top multifocals; they provide a continuum of clear vision, from close up at the bottom of the lens, to the intermediate as the eye moves upward, to distance clarity when looking straight ahead.
Also known as invisible bifocals or no-line bifocals, PALs are optically superior to flat-top styles because they allow clear vision at all distances, near, far and in between, without jumping from one to the other. Many people find that their vision seems more natural because there is no abrupt change from one segment to another, just a smooth transition from distance to near and back.
PALs are more expensive than either single-vision readers or flat-top styles of multifocals, but most people find the extra expense to be worthwhile for the ease of their use and the naturalness of their vision.
When PALs were first fabricated in the late 1970s, the optics were not perfected and people did have more adaptation problems from that and the fact that proper placement of the lenses in the frame is extremely important, however, today these lenses have amazingly good optics and most people adapt to them almost immediately.
There are several different types of multifocal contact lenses available, usually in one of two basic designs.
One type has a reading area, which is located in the bottom part of the lens; in this type, the contact lens must have some way of stabilizing itself from rotating on the eye. This can be achieved by making the lens heavier on the bottom with the addition of prism ballast, or, alternatively, by making both the top and bottom areas of the contact lens thinner, so the eyelids hold it in position.
A second type of multifocal contact lens has a ring-shaped or annular design, so that it doesn’t matter how the lens rotates on the eye.
The type of lens that will work best for an individual is a decision to be made between the practitioner and the patient, taking into account the amount of reading prescription needed, the shape of the eye opening in normal gaze, the tightness of the lid margins and the size of the pupils in normal lighting, among other things.
On the Horizon
New technology in the form of corneal surgeries has made some progress recently, most notably in the Kamra Inlay, thin plastic rings placed in the front part of the cornea to provide near clarity are an example.
Most people will develop cataracts in the crystalline lens of the eye and may have them removed and replaced with implant lenses that provide near focus as well as far are being used presently, and further advances and new lens designs are in the beginning stages. The future will undoubtedly bring even better options.
As the lens within the eye continues to thicken, it is normal for people to need increasing amounts of multifocal power; this does not continue indefinitely, however, but levels off at about age 60 or thereabouts when it stabilizes.
Presbyopia isn’t the end of clear vision for reading, and using a multifocal lens to help keep our eyes in focus is obviously not the worst thing that can happen as we age. With the choices available now with modern lens fabrication methods, computer-aided designs and vastly improved optics, we have come a long way from Ben Franklin’s crude bifocals.
As we age, it is perhaps more important to realize how lucky we are to have all these options and to be able to choose from among them. Our focusing help, whether in the form of simple reading glasses or the latest in high-tech PAL designs, allows us to see easily and clearly all day long.