Effect of Vergence Facility Testing on Horizontal Fusional Reserves

SIGNIFICANCE The effect of vergence adaptation on test sequences should be considered in clinical practice. PURPOSE This study aimed to compare the effect of vergence adaptation after different vergence facility (VF) testing methods on following horizontal fusional reserve measurement at near. METHODS This cross-sectional study was performed on 50 healthy subjects with a mean +/- standard deviation age of 21.18 +/- 1.41 years. The baseline horizontal fusional reserves were measured using base-in (BI) and base-out (BO) prisms ( increment ) at near. Then, VF was measured using 3 increment BI/12 increment BI flip prism at near, and then the measurement of fusional reserves was repeated. In the next step, fusional reserves were measured after VF testing using 8 increment BI/8 increment BO flip prism. The values of fusional reserves were compared with their baseline findings. RESULTS The results of negative fusional reserves after VF measurement using the 3 increment BI/12 increment BO prisms were significantly lower than the baseline values (blur [P = .03], break [P = .03], and recovery [P = .03]). There were no significant differences between the values of fusional reserves before and after VF testing using the 8 increment BI/8 increment BO prisms (P > .05). No correlation was observed between fusional reserves and VF responses (P > .05); however, the results revealed a positive correlation between the values of fusional reserves before and after VF testing using both methods (all P < .05). There was no significant difference in VF findings between the methods (P = .23). CONCLUSIONS The measurements of horizontal fusional reserves followed by VF testing were not affected except for the responses of negative fusional reserves, which decreased after VF determination using the 3 increment BI/12 increment BO method. Although the vergence adaptation does not seem clinically significant, it is recommended to evaluate horizontal fusional reserves before VF measurement or long-enough time after VF testing for the vergence system to return to its nonadapted state.