Abduction
Abduction is the relative power of the external muscles to turn the eyes outward. Prism power base in and apex out is employed. To determine abduction, or the amount of divergence of the external rectus muscle of the right eye, prism power with base in or toward the nasal side should be employed. The rotary prism will therefore remain in the same relative position as in making the adduction test ([Fig. 22]), with the two cyphers (0) or zero graduations vertical, but the indicator or red line should be rotated inward from zero, or towards the patient’s nose.
With the patient’s attention again directed to the large letter “E,” or the Greek cross, this inward rotation should be continued until diplopia or double vision occurs. Like the former, this test should be repeated several times, the refractionist continuing to strive for the highest prism power which the eye will accept. This will indicate abduction of the right eye and should be so recorded as designated in [Fig. 24]. The amount of abduction ranges from 3 to 10 prism diopters. The normal average is 8.
The ratio of adduction to abduction is normally rated at about three to one. In other words, it is conceded that the power of the eye to converge is normally three times as great as its power to diverge, the usual measurements being eight to twenty-four respectively. While applicable in most instances, this may vary in different cases.
Superduction
Superduction, sometimes termed sursumduction, is the relative power of the superior recti to turn the eyes upward. Prism power base down and apex up is employed. To test superduction, the rotary prism should be placed in position with the two cyphers lying horizontally, with the handle pointed vertically ([Fig. 23]). The patient’s attention should again be directed to the large letter “E”, and the indicator or red line should be rotated downward from zero. The highest prism power that the patient will accept before the object appears to double in the vertical plane will indicate the degree of right superduction. This should be recorded accordingly. Conditions of this kind do not usually exceed two or three degrees. The test, however, should be repeated several times before the final result is recorded, as indicated in [Fig. 24]. The amount of superduction ranges from 1 to 4 prism diopters. The normal average is 2.
Fig. 23—To test superduction, base down is required. Rotary prism’s line or indicator should be rotated downward from zero.
To test subduction, base up is required. Indicator should be rotated upward from zero.