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

Abstract

at 10 cm was 6' coln~ared with 3" at 6 in. Using a virtual Aim: To compare the vertical fusional amplitudes in isometropic participants with normal binocular single vision at four distances of fixation: 33 cm, l m , 4 m , 6 m. Methods: Vertical fusion ranges (break point and recovery point) were measured with aS u l d e n 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 f 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, 6"; 1 m, 6"; 4 m, 5.5"; 6 m, 5.5A; and for the recovery points were: 33cm, 4A; l m , 4"; 4m, 3.SA; 6m, 3.5". The difference was significant between either of the near measures (i.e. 33 cm and l m) and either of the far measures (i.e. 4 m 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 4 m or 6 m is used.