Introduction
Clinical refraction is the process of measuring the refractive status of the eyes of a patient. It is an essential tool in a great many eye examinations: refractive errors are present in nearly half of all adults and children. In most settings, “best-corrected visual acuity” is the gold standard for measuring visual performance. In this section of the Quick-Start Guide, we outline the basic technique of manifest refraction—the measurement of refractive errors through patients’ subjective descriptions of how well they can see.
Do not confuse manifest refraction with the prescribing of eyeglasses. The latter is a clinical art, for which refraction is only a first step. Guidelines for prescribing glasses are presented in Chapter 4.
Manifest refraction is a subjective process. Success is based on the ability to elicit the cooperation of the patient as much as the knowledge and skill of the examiner.
The steps in the process of manifest refraction follow this sequence:
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Perform the preliminaries (check visual acuity, obtain a visual history).
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Indications for refraction: which patients should be refracted?
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Occlude 1 eye.
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Make an initial estimate of the refractive error:
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Determine the best initial estimate of the refraction, if available, from prior records, old glasses, retinoscopy, or the auto-refractor, and place it in the phoropter or trial frame. If there is any known cylindrical correction, go to step 4; if not, go to step 5.
OR
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If there is no prior refraction information, retinoscopy, or auto-refractor data, begin by refracting only with spherical lenses to obtain the best-possible refraction using only spheres. Go to step 3c.
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Check for astigmatism by inserting a cylinder lens, adjusting the sphere to preserve the spherical equivalent, and rotating the cylinder—if this affects image clarity, you have detected some astigmatism. Leave the cylinder in the optimal position (this becomes the starting point for cross cylinder refinement—go to step 4); if rotating the cylinder has no effect on image clarity, assume there is no astigmatism. Remove the cylinder, restore the original sphere, and go directly to step 5.
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Refine the cylinder using the Jackson cross cylinder, first in the “axis position” to identify the correct cylinder axis, then in the “power position” to determine the optimal cylinder power. Preserve spherical equivalent when adjusting cylinder power by adjusting sphere power 1 click (0.25 D) in the opposite direction for every 2-click (0.50 D) change in cylinder power. Go to step 5.
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Refine the sphere power, first using a 0.50 D difference between the alternatives, bracketing the previous selection, then using a 0.25 D difference between the alternatives.
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Reverse the occlusion, and repeat for the fellow eye.
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Check for binocular balance.
Additional comments, set off in boxes, are interspersed between the step-by-step instructions. Most of the step-by-step instructions that we discuss here describe refraction performed with minus cylinder equipment. Equivalent instructions for plus cylinder equipment are labeled as directions and marked with a tinted background. Recommended language to use with patients is printed in boldface italics.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.