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Figure 9-39

by using a magnifying glass. The Rosenbaum and Snellen charts are "equivalent" only in that they both project the same sized images on the retina. If order to clearly see the letters, however, the eyes must focus them. The attending was presbyopic and had difficulty focusing for near. Sam was myopic. He saw perfectly for near but could not focus for far. Both Sam and the attending had refractive errors. The patient had a non-refractive problem; focusing worked fine for the patient. The patient's problem was one of the cloudiness of the lens. One would expect his vision to be as poor for near as for far.

QUESTION: Which method should the physician use for routine visual testing, the near chart or distant chart?

ANS: If one wishes to be certain that a patient has no difficulty in visual acuity, both near and far must be tested. Certainly, testing far vision alone is insufficient for the middle age patient who reports headaches, fatigue and blurred vision on reading up close. Near testing must be performed to evaluate for presbyopia. Similarly, when a teenager reports difficulty seeing distant objects, near vision testing is insufficient. Sometimes, the physician is selectively interested in determining whether the patient has a non-refractive error (e.g., visual loss from acute ocular trauma). Under such conditions it may suffice to test either near or far vision as the finding of normal vision for either would rule out visual loss from such a condition. In general, it is best to test both near and far vision.

Astigmatism

A spherical lens bends light equally all around its circumference. A cylindrical lens bends light only along one axis, as only one of its axes is curved.

Fig. 9-40. Refraction through a spherical and a cylindrical lens. A spherical refractive error is erased by introducing a spherical lens of opposite refractive

Figure 9-40

properties. A cylindrical refractive error (astigmatism) is corrected by introducing a cylindrical lens of opposite refractive properties. Patients may have a combination of astigmatism and a spherical refractive error. In that case the physician may correct the problem by combining a spherical lens with a cylindrical lens placed along the appropriate axis.

Reading Prescriptions

The following is a sample eyeglass prescription:

This means: In the right eye (OD) the correction calls for a minus (concave) 2.00 diopter spherical lens, combined with a plus (convex) 0.75 cylindrical lens that is placed at an axis of 180°. The left eye requires a minus 1.75 spherical lens combined with a +0.25 cylinder at 180°. In addition, the patient has difficulty with near vision (probably presbyopia) and requires for each eye (ou) a plus 1.50 spherical lens added on to the rest of the prescription as a bifocal segment, for near vision. The bifocal add is generally placed at the bottom of the eyeglasses as it is used when the patient is looking down to focus on near objects, particularly in reading.

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