Figure 9-34

whether a pinhole improves vision (also a refractive error). The pinhole principle is illustrated in Fig. 9-35.

Fig. 9-34. The Rosenbaum hand-held vision card.

Fig. 9-35. The pinhole lens. The pinhole restricts light to the center of the cornea, where refraction (bending of the light) is unnecessary. Light remains in focus regardless of the refractive error of the eye. The next time you lose your eyeglasses in the woods, try viewing through a pinhole; vision will improve. Pinhole glasses would be used more frequently except that they decrease both the illumination and the field of vision, in addition to the weird cosmetic appearance.

For good vision, light must focus precisely on the retina. In nearsightedness (myopia: the patient sees near objects best), light end up focused in front of the retina (Fig. 9-36). In far-sightedness (hyperopia: the patient see far objects best), light focuses behind the retina. Fig. 9-37 illustrates why myopes have greatest difficulty with far vision, whereas hyperopes have greatest difficulty with near vision.

Fig. 9-36. Myopia and hyperopia.


Figure 9-36

Fig. 9-37. The closer an object (o) is to a convex lens, the farther is the image (i) focused. Hence, (see Fig. 9-36) for myopes, near objects are focused closer to the retina than far objects and are seen best. For hyperopes, far objects focus closer to the retina and are seen best.

Myopia commonly originates in youth as the eye grows. Typically, a grade school or high school student experiences difficulty in reading the blackboard from far, or in seeing a movie from the rear of the theater. The condition persists throughout life. Myopes always will require glasses (or contact lenses) to see distant objects clearly.

Hyperopia may be congenital. Presbyopia commonly arises and progresses from ages 45-60. Presby-

Hyperopia may be congenital. Presbyopia commonly arises and progresses from ages 45-60. Presby-

Figure 9-35

Figure 9-37

opes have difficulty seeing near objects clearly, as do hy-peropes, but the mechanism is different. The lens becomes less resilienc with age and does not thicken as readily on accommodation; hence, the accommodative effort is less effective in presbyopia and near vision is poor. At first the patient may try to compensate for the low lens resiliency by trying very hard to accommodate, supercontracting the ciliary muscles. He may even be successful in the effort, but at the expense of inducing headache or fatigue. He may try to compensate by holding the newspaper farther away, wishing his arms were longer. Eventually, though, reading glasses become necessary to provide the additional focusing power.

Fig. 9-38. Presbyopia.

The percentage of patients requiring glasses by age 60 in order to see perfectly for both near and far is close to 100%. The patient who never had a refractive error requires reading glasses (positive; convex lenses) for presbyopia as the eyes change between ages 45-60. The myopic patient may compensate for the need for reading glasses (plus lenses) by simply removing her myopic glasses (i.e. removes her negative, concave lenses). However, she still, and always will, require glasses to see far.

The hyperopic patient who sees far well may never require glasses for distance. At some point he will require them for near, and at an earlier age than the patient who has no refractive error.

CASE HISTORY: Sam was a smart young intern. He always carried around a small Rosenbaum near vision-testing card and routinely used it in his workups. The

Rosenbaum card, of course, is the "equivalent" of the Snellen chart. Although the patient stands 20 feet away from a Snellen chart and only 14 inches from the Rosenbaum card, the letters are proportionately smaller on the Rosenbaum card to compensate for the patient being closer. One day, Sam became extremely perplexed. He had a patient with a cataract. The patient had 20/100 vision (i.e. saw at 20 feet from the chart what a person with normal vision would see at 100 feet from the chart) in the affected eye, whether using the Snellen chart or the Rosenbaum card. There was nothing unexpected about this; the near and far charts were "equivalent." Sam noticed, however, that his own vision (without glasses) was 20/100 for far (the Snellen chart), but an excellent 20/20 for near (the Rosenbaum card). "Something must be wrong with these charts," Sam thought. "I used good illumination and was the proper distance from the charts. I should have equal vision with either chart because the charts are 'equivalent'." Sam consulted with his attending physician, who was not especially noted for keen judgment or knowledge. The attending tried the charts himself and, to his surprise, found that without his glasses he saw perfectly well for far but saw 20/100 for near. "Obviously", the attending commented "these charts work only on patients." How does one account for these discrepancies?

ANS: Fig. 9-39 illustrates why the Snellen and Rosenbaum charts are "equivalent."

Fig. 9-39 The basis for equivalency of the near and far vision charts. In order to see clearly, an image must not only be sharply focused on the retina but must be large enough. A microscopic object cannot be seen even when sharply focused. Patients may thus improve poor vision

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