Effect of test sequence on fusional vergence ranges

Abstract

Background. Vergence adaptation or prism adaptation is a well-known phenomenon, but the effect of testing order on fusional vergence ranges has not been widely studied. Methods. Sixty-two patients had fusional vergence ranges taken with rotary prisms in either a base-in, base-out, base-in order or a base-out, base-in, base-out order. Results. The results showed statistically significant reductions in some fusional vergence range findings as a result of preceding fusional vergence range testing. Base-in findings were affected more often by base-out testing than vice-versa. The most consistent change was a reduction in the distance base-in break of 1 to 3 prism diopters when it followed base-out testing. Conclusions. This study supports the recommendation in some texts that the effects of vergence adaptation on fusional vergence test findings can be minimized by performing base-in tests before base-out. However, these effects are generally small in amount.