Contact Lenses Care

Eventually everyone experiences presbyopia. This is the normal aging process of the eye, which begins around the age of forty for most people, and is caused by the gradual loss of elasticity in the natural crystalline lens of the eye. The lens loses its ability to change shape the way it used to, and fails to bring the light rays of near objects into sharp focus. Of course a nearsighted person over the age of forty can see near objects clearly with the naked eye, but will have difficulty doing so if he is wearing glasses or contact lenses that correct his myopia.

The presbyope has “trombone vision“—that telltale gesture of moving objects farther and farther away from the eyes in order to see them clearly. Most people opt for bifocal glasses or simply resign themselves to placing on a separate pair of glasses to read, sew, and do other tasks that require good close-up vision. However, there are several ways to correct this condition with contact lenses: bifocal contact lenses, “monovision” lenses, “compromise” contact lenses, having two pairs of contact lenses, and wearing reading glasses over contact lenses.

Bifocal Contact Lenses

Currently in the U.S. bifocal contact lenses are available in two types: conventional hard lenses and the newly released soft lenses. There are available basically two designs of either form of bifocal contact lens. Both of these, like spectacles, have two distinct powers, or prescriptions: one for near, one for far.

Crescent bifocal contact lenses most resemble bifocal eye-glasses. They have the prescription for distance at the top part of the lens, and the crescent-shaped segment at the bottom contains the prescription for close-up vision. As you gaze ahead normally, the distance power is used; as you gaze down, the near power goes into effect.

Because contact lenses naturally rotate on the eye during wear, some means of keeping bifocal lenses stabilized is necessary. Otherwise the crescent at the bottom would gradually drift toward the top where it would do no good—unless you held your newspaper over your head. These lenses are often weighted at the bottom with a thicker segment (prism), which utilizes gravity to keep the lens oriented. Crescent bifocals may also be truncated, a process in which the bottom edge is cut off to align it parallel with the lower eyelid margin. Another less effective modification that stabilizes bifocal lenses features orientation grooves cut into the top edge of the lens, providing a pathway for the inner surface of the upper lid to prevent rotation of the lens.

Like crescent lenses, concentric bifocal contact lenses also have two distinct powers, or prescriptions, but with a structural difference: the near prescription segment is in the shape of a ring acting as the outer rim of the lens; the distance power is a circle in the middle. With this lens, which resembles a bull’s-eye target, there’s no worry about its rotating on the eye. As with crescent bifocal lenses distant vision is effected by looking through the center, and near vision by looking down through the outer rim.

The best candidates for bifocal contact lenses are those who have been successfully wearing single-vision contacts for some time to correct myopia, hyperopia, or astigmatism. They will have the least problem with adjusting; but even so the success rate is low. The recent advent of the soft bifocal lens, however, should improve these statistics for the millions of presbyopes in this country, many of whom would no doubt be delighted to be able to wear bifocal contacts.

Fitting bifocal contact lenses is difficult (such factors as pupil size and lower lid position complicate the fitting process); manufacture of the lenses is more intricate than for the single-vision contacts; delivery time is slow; and the cost is necessarily higher than for conventional lenses. Many wearers report that bifocals are less comfortable than single-vision contacts, usually because the prism that prevents rotation adds bulk and makes the lens heavier.

New contact lens wearers not only have to adjust to lens wear in general but also have to adjust to the means of wearing a bifocal prescription. As in bifocal spectacles this involves learning how to adjust to the “jump” in vision that occurs every time you shift your gaze from near to far, or vice versa. Initially vision may be frustratingly poor —especially with hard lenses—during the adaptation period because the sensation of having a foreign body in the eye causes ocular physiological changes that lift the lower lid and the lens. Thus the bifocal segment is higher than it should be. Since this bifocal segment is so very small, there is very little tolerance in the proper positioning of the lens. As difficult as it is to achieve, perfect centering of the contact lenses is absolutely crucial for successful bifocal hard contact lens wear.

In response to these problems manufacturers are experimenting with design modifications. Multifocal or variable- focus contacts (available in hard and gas-permeable lenses) have not two, but many powers that gradually blend into each other, avoiding the annoying jump in vision. The lens contains all the different powers needed by the wearer to see clearly at any distance: near, far, and intermediate. The bifocal contact lens candidate will have to be patient and well motivated because it usually takes a few weeks until the lenses can be worn with comfort and provide good vision. Also some lens modifications and adjustments are usually the rule.

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