Quantifying the vertical fusion range at four distances of fixation in a normal population

Abstract

Aim: To compare the vertical fusional amplitudes in isometropic participants with normal binocular single vision at four distances of fixation: 33 cm, 1m, 4 m, 6m. Methods: Vertical fusion ranges (break point and recovery point) were measured with a Gulden vertical prism bar with the participant fixing a 6/12 Snellen equivalent letter, twice at each distance. Order effects were controlled with randomisation of both fixation distance and prism direction. Results: Twenty-seven participants were examined (aged 20.4?1.05 years). Base up and base down measurements were similar, therefore measurements were combined to give a total vertical range. Median values for the break points were: 33 cm, 6h; 1 m, 6h; 4m, 5.5h; 6 m, 5.5h; and for the recovery points were: 33 cm, 4h; 1 m, 4h; 4 m, 3.5h; 6 m, 3.5h. The difference was significant between either of the near measures (i.e. 33 cm and 1m) and either of the far measures (i.e. 4m and 6 m). Conclusions: The vertical fusion range appears to be slightly greater at near than distance. However, the difference is not clinically significant. Measurements for distance, in a normal population, appear to be the same whether a fixation distance of 4m or 6m is used.