Step 7: Accommodative Control and Binocular Balance
Accommodation is the ability of the eye to increase the focusing power of the crystalline lens, mainly to allow clear vision for near objects. In general, the primary goal of clinical refraction is to determine the refractive error, if any, with the accommodation fully relaxed. That means the eyes are focused for distant targets; the patient has the full use of the accommodative amplitude to focus over as large a range of nearer objects as possible. In addition, accommodation should be equally relaxed for the 2 eyes—a state referred to as binocular balance.
In practice, this fully relaxed state of accommodation may not occur spontaneously as patients sit for refraction, complicating the task of the refractionist. Many factors, such as hyperopic refractive errors (which can often be corrected by a patient’s own accommodative effort), awareness of near objects (including the refracting instruments and lenses), and “accommodative spasm,” may induce an increase in accommodative tone, steering the refractive process toward undercorrection of a hyperopic error or overcorrection of a myopic error. In some cases, patients seem to prefer the high-contrast, minified image seen by accommodating to compensate for a myopic overcorrection. It is thus important to make every effort to use accommodative control—techniques that relax the accommodation during the refraction process and guard against inadvertent stimulation of accommodation and overminusing the patient as a result. The use of a plus lens occluder (instead of an opaque occluder), and biasing the binary comparisons to offer the more-plus choice first, will help prevent spurious accommodation.
The “Rule-1” test is a quick and easy way to check accommodative control. You can perform this test for each eye separately or binocularly if the vision is comparable in the 2 eyes:
Dial in +1.00 D sphere power above the final refractive correction. Visual acuity should fall at least 2 lines on the eye chart. (For example, if the final refraction brings the acuity to 20/20, adding +1.00 D sphere to both eyes should reduce the acuity to 20/30 or worse.)
If the acuity does not drop 2 lines as expected, continue to increase the plus sphere power until the acuity is reduced by 2 lines.
Next, remove the excess added plus sphere power 1 click (0.25 D) at a time until the acuity returns to the optimal level. (Ask “Is this step definitely better, or are the letters just smaller and blacker?”) The final endpoint should be the last step that permits the patient to read additional letters. Disregard the patient’s enthusiasm for any additional minus sphere (or reduced plus sphere) power that does not allow more letters to be read.
If you perform this procedure for each eye separately, it also serves as a check of “binocular balance”—that is, assuring that the 2 eyes are accommodating to the same degree.
The Lancaster red-green (“duochrome”) test is recommended for patients with 20/40 or better best-corrected acuity. It can be performed monocularly or binocularly. Performed for each eye separately, this test functions as an effective check of binocular balance:
Bring up the duochrome slide or filter on your chart projector. This will display 1 side of the chart against a bright red background, and the other side against a bright green background
Ask “Which side of the chart has the clearer, crisper, blacker letters: the red side or the green side?” (If the patient is confused by the intense colors, add “Concentrate on the letters, not on the backgrounds. Look at the smallest letters you can read.”
If the patient reports that the letters seen against the red background are clearer, more minus sphere power is needed; if the green side is preferred, more plus sphere power is indicated.
When in doubt, leave the patient with the more plus (or less minus) sphere choice (ie, with a slight preference for the seeing against the red background), which will better relax the accommodation.
The duochrome test is based on the chromatic aberration of the human eye. This test works just as well in color-blind patients as in patients with normal color vision, so long as you describe the alternatives as left versus right instead of red versus green.
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.