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.
Table 4-2 Cross Cylinder Refraction Summary
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 (but also “darker,” which the patient may misinterpret as “clearer”).
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 4-18). 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.
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. When in doubt, it is preferable to err on the side of the more-plus or less-minus alternative (“leave ’em in the red”). 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 20/40 (6/12), because the 0.50 D difference between the 2 sides is too small to distinguish.
Figure 4-18 Diagram of duochrome test. A, Starting point with eye blurred using excess plus power—red letters should be clearer. B, Decreasing plus power (or increasing minus) until both sides of the chart have equal clarity. C, Continuing to decrease plus (increase minus) power until the green side of the chart is clearer.
(Illustration developed by Thomas F. Mauger, MD.)
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.