| Summary: | 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 1 Δ and 2 Δ 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 0 Δ and 2Δ of induced vertical divergence, and between 1 Δ and 2 Δ of induced vertical divergence. When no vertical divergence was induced the median PFR was 33 Δ (interquartile range 15 Δ), for 1 Δ vertical divergence it was 30 Δ (interquartile range 17 Δ) and for 2 Δ vertical divergence it was 22 Δ (interquartile range 12 Δ). Conclusions: The greater the induced vertical divergence, the smaller the horizontal fusional amplitude recorded. This reduction was statistically significant with 2 Δ of induced vertical divergence.
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