The effect of induced vertical divergence on horizontal fusional amplitudes

Aim: To find out whether horizontal fusion is compromised in the presence of slight vertical deviations induced with small vertical prisms. Methods: Eighteen fully binocular participants were recruited from a student population. Horizontal prism fusion range (PFR) was tested in the presence of 1D and 2D vertical prisms and with no vertical prism as a control. One eye remained occluded until horizontal fusion was to be tested, to reduce the effects of vergence adaptation. PFR was measured to break point under each of the three conditions. Results: As vertical divergence was increased, horizontal fusional amplitudes reduced (Friedman test: p < 0.0001). Wilcoxon signed rank tests showed that differences were statistically significant between 0D and 2D of induced vertical divergence, and between 1D and 2D of induced vertical divergence. When no vertical divergence was induced the median PFR was 33D (interquartile range 15D), for 1D vertical divergence it was 30D (interquartile range 17D) and for 2D vertical divergence it was 22D (interquartile range 12D). Conclusions: The greater the induced vertical divergence, the smaller the horizontal fusional amplitude recorded. This reduction was statistically significant with 2D of induced vertical divergence