The final step of subjective refraction is to make certain that accommodation has been relaxed equally in both eyes. Several methods of binocular balance are commonly used. Most require that the corrected visual acuity be nearly equal in both eyes.
When the endpoint refraction is fogged using a +0.75 D sphere before each eye, the visual acuity should be reduced to 20/40–20/50 (6/12–6/15). Alternately cover the eyes and ask the patient if the chart is equally blurred. If the eyes are not in balance, plus sphere should be added to the better seeing eye until balance is achieved.
Figure 4-19 Binocular balancing by prism dissociation from patient perspective.
(Courtesy of Tommy Korn, MD.)
In addition to testing for binocular balance, the fogging method also provides information about appropriate sphere power. If either eye is overminused or underplussed, the patient should read smaller (better than 20/40) letters than expected. In this case, the refraction endpoints should be reconsidered.
The most sensitive test of binocular balance is prism dissociation (Fig 4-19). For this test, the refractive endpoints are fogged with+0.75 to +1.00 D spheres, and vertical prisms of 4 or 6 prism diopters (∆) are placed before 1 eye (or divided equally between the 2 eyes) using the Risley prisms in the phoropter (or in a trial lens frame). Some phoropters have a built-in 6∆ setting for this test. Use of the prisms causes the patient to see 2 charts, 1 above the other. A single line, usually 20/40 (6/12), is isolated on the chart; the patient sees 2 separate lines simultaneously, 1 for each eye. The patient can readily identify differences between the fogged images in the 2 eyes of as little as 0.25 D sphere. In practice, +0.25 D sphere is placed before 1 eye and then before the other. In each instance, if the eyes are balanced, the patient reports that the image corresponding to the eye with the additional +0.25 D sphere is blurrier. After a balance is established between the 2 eyes, remove the prism and reduce the fog binocularly until the best visual acuity is obtained.
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.