Vergence adaptation and the clinical AC/A ratio

The accuracy of the gradient technique for measuring the clinical accommoda tive convergence to accommodation (AC/A) ratio is dependent upon obtaining veridical heterophoria measurements. However, previous studies have demonst rated that the sustained output of slow fusional vergence, which may take s everal minutes or even hours to decay, can bias heterophoria assessment. In the clinical setting, it is usual to estimate the AC/A ratio after just a few seconds of dissociation. This study investigated whether the slow fusio nal vergence response alters this crosslink ratio by comparing values of AC /A measured both before and immediately after a 1-hr period of continuous m onocular occlusion. Sustained occlusion produced a significant change in ne ar heterophoria in 10 out of the 21 subjects examined, but no significant c hange in AC/A was observed in either the adapting or non-adapting subgroups . Accordingly, while the sustained output of slow fusional vergence will in fluence clinical measurements of heterophoria, its presence does not alter the stimulus AC/A ratio significantly. (C) 2000 The College of Optometrists . Published by Elsevier Science Ltd.