Accommodative Convergence/Accommodation Ratio
Normally, accommodative effort is accompanied by a corresponding convergence effort. For practical purposes, the AC/A ratio is ordinarily expressed in terms of prism diopters of deviation per diopter of accommodation. Using this type of expression, the normal AC/A ratio is 3:1–5:1.
The AC/A ratio is relatively constant in a particular patient, but it should be noted that there is some variability among individuals. For example, a patient with an uncorrected 1.00 D of hyperopia may accommodate 1.00 D for clear distance vision without exercising a convergence effort. Conversely, a patient with uncorrected myopia must converge without accommodative effort to fuse at the far point.
The AC/A ratio can be measured by varying the stimulus to accommodation in several ways. These methods are described in the following subsections.
The heterophoria method involves moving the fixation target. The heterophoria is measured at 6 m and again at 0.33 m.
PD = interpupillary distance in centimeters
∆n = near deviation in prism diopters
∆d = distance deviation in prism diopters
D = amount of accommodation in diopters
The AC/A ratio can be measured in 1 of 2 ways with the gradient method. The first way is by stimulating accommodation. Measure the heterophoria with the target distance fixed at 6 m. Then remeasure the induced phoria after interposing a −1.00 D sphere in front of both eyes. The AC/A ratio is the difference between the 2 measurements.
The second way is by relaxing accommodation. With the target distance fixed at 0.33 m, measure the phoria before and after interposing +3.00 D spheres. The phoria difference divided by 3 is the AC/A ratio.
An abnormal AC/A ratio can place stress on the patient’s fusional mechanisms at one distance or another, causing asthenopia or manifest strabismus. Abnormal AC/A ratios should be accounted for when prescribing corrective lenses.
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.