Guyton DL. Retinoscopy: Minus Cylinder Technique, 1986; Retinoscopy: Plus Cylinder Technique, 1986; Subjective Refraction: Cross-Cylinder Technique, 1987. Reviewed for currency, 2007. Clinical Skills DVD Series [DVD]. San Francisco: American Academy of Ophthalmology.

Wunsh SE. The cross cylinder. In: Tasman W, Jaeger EA, eds. Duane’s Clinical Ophthalmology [CD-ROM]. Vol 1. Philadelphia: Lippincott Williams & Wilkins; 2006:chap 38.

Refining the Sphere

After cylinder power and axis have been determined using either the astigmatic dial technique or the cross-cylinder method, the final step of determining monocular refraction is to refine the sphere. The endpoint in the refraction is the strongest plus sphere, or weakest minus sphere, that yields the best visual acuity. The following discussion briefly considers some of the methods used.

When the cross-cylinder technique has been used to determine the cylinder power and axis, the refractive error is presumed to a single point. Add plus sphere in +0.25 D increments until the patient reports decreased vision. If no additional plus sphere is accepted, add minus sphere in –0.25 D increments until the patient achieves the most optimal visual acuity.

Using accommodation, the patient can compensate for excess minus sphere. Therefore, it is important to use the least minus sphere necessary to reach the best visual acuity. In effect, accommodation creates a reverse Galilean telescope, whereby the eye generates more plus power as minus power is added to the trial lenses before the eye. As this minus power increases, the patient observes that the letters appear smaller and more distant.

The patient should be told what to look for. Before subtracting each 0.25 D increment, tell the patient that the letters may appear sharper and brighter or smaller and darker, and ask the patient to report any such change. Reduce the amount of plus sphere only if the patient can actually read more letters.

If the astigmatic dial technique has been used and the astigmatism is neutralized (ie, if all the lines on the astigmatic dial are equally sharp or equally blurred), the eye should still be fogged; additional plus sphere only increases the blur. Therefore, use minus sphere to reduce the sphere power until the best visual acuity is achieved. Again, the examiner should be careful not to add too much minus sphere.

To verify the spherical endpoint, the duochrome test (also known as the red-green or bichrome test) is used (Fig 3-18). A split red-green filter makes the background of the visual acuity chart appear vertically divided into a red half and a green half. Because of the chromatic aberration of the eye, the shorter (green) wavelengths are focused in front of the longer (red) wavelengths. The eye typically focuses near the midpoint of the spectrum, between the red and green wavelengths. With optimal spherical correction, the letters on the red and green halves of the chart appear equally sharp. The commercial filters used in the duochrome test produce a chromatic interval of approximately 0.50 D between the red and green wavelengths. When the image is clearly focused in white light, the

eye is 0.25 D myopic for the green letters and 0.25 D hyperopic for the red letters.

Figure 3-18 Duochrome test. (Courtesy of Tommy Korn, MD.)

Each eye is tested separately for the duochrome test, which is begun with the eye slightly fogged (by 0.50 D) to relax accommodation. The letters on the red side should appear sharper; the clinician should add minus sphere until the 2 sides appear the same. If the patient responds that the letters on the green side are sharper, the patient is overminused, and more plus power should be added. Some clinicians use the RAM-GAP mnemonic—“red add minus; green add plus”—to recall how to use the duochrome test.

Because this test is based on chromatic aberration and not on color discrimination, it is used even with color-blind patients (although it may be necessary to identify the sides of the chart as left and right rather than red and green). An eye with overactive accommodation may still require too much minus sphere in order to balance the red and green. Cycloplegia may be necessary. The duochrome test is not used with patients whose visual acuity is worse than 20/30 (6/9), because the 0.50 D difference between the 2 sides is too small to distinguish.

Binocular Balance

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.

Fogging

When the endpoint refraction is fogged using a +2.00 D sphere before each eye, the visual acuity should be reduced to 20/200–20/100 (6/60–6/30). Place a –0.25 D sphere before first 1 eye and then the other, and rapidly alternate cover; the patient should then be able to identify the eye with the –0.25 D sphere before it as having the sharper image at the 20/100 (6/30) or 20/70 (6/20) level. If the eyes are not in balance, sphere should be added or subtracted in 0.25 increments until balance is achieved.

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 farther down the chart—as far as 20/70 (6/20), 20/50 (6/15), or even 20/40 (6/12)—with the +2.00 D fogging spheres in place. In this case, the refraction endpoints should be reconsidered.

Prism dissociation

The most sensitive test of binocular balance is prism dissociation. For this test, the refractive endpoints are fogged with +1.00 D spheres, and vertical prisms of 4 or 5 prism diopters (Δ) are placed before 1 eye (Fig 3-19). 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.