CLINICAL ASPECTS OF DUCTION MEASUREMENTS

SUMMARY Ductions (prism vergences, fusional reserves, fusional amplitudes) measure the visual mechanism's resistance to induced diplopia, i.e., the “strength of the fusion compulsion reflex.” The eye movements induced during duction measurements are artificially produced fusion movements of positive or negative “fusional” convergence and are essentially reflex (involuntary) in nature, although they can be increased, in some individuals at least, by voluntary effort. Only the horizontal ductions measured at 33 cm. from the eyes (near ductions) will be considered here. Viewed somewhat differently, these measure the degree to which the accommodation-convergence relationship can be embarrassed (by placing horizontal prisms of increasing strength before the eyes) before diplopia becomes manifest. Since the break and recovery point readings differ by an average of 10Δ, somewhat different mechanisms seem to be involved. It would appear that the break point is a measure of the resistance to increasing amounts of induced diplopia, whilst the recovery point is more directly related to the “fusion compulsion reflex,” i.e., the binocular fixation reflex.