[Comparison between dissociated and associated heterophoria]

Since Ogle (1949) dissociated heterophoria is distinguished from associated heterophoria. Dissociated heterophoria is defined as a deviation from the orthovergence position that occurs when no fusional contours are provided. Associated heterophoria is a deviation that occurs under prism correction of fixation disparity, i.e. under a viewing condition in which fusional contours are absent only in the centre of the visual field. Both dissociated and associated heterophoria have been used as indications for prismatic correction. It has been controversially discussed to what extent dissociated and associated heterophoria are comparable.

A total of 43 subjects were investigated using the white Maddox rod test for horizontal dissociated heterophoria and the cross test of Hans-Joachim Haase (a test similar to the fixation disparity test described by Ogle 1949) for horizontal associated heterophoria. In both tests subjects were asked to continually correct any deviation using Herschel prisms. Readings were taken after 1 min.

The correlation between the results obtained with the white Maddox rod test and the cross test was r = 0.89 (p < 0.0001) and the slope of the regression line was 0.92 (95% confidence interval from 0.77 to 1.07). One subject was unable to reach a match in the cross test because she had an "obligate" fixation disparity due to a staphyloma posticum.

Our study revealed a high correlation between dissociated and associated heterophoria. This result appears plausible since the feedback loop for fusion is disconnected in both cases, although in a different manner: When dissociated heterophoria is measured, fusionable contours are absent. When associated heterophoria is measured, any disparity of peripheral contours that could serve as an error signal is nullified by instantaneous prism correction. The high concordance of dissociated and associated heterophoria suggests that both measures are of equal value as an indication for prismatic spectacle correction. In rare cases of an "obligate" fixation disparity, however, associated heterophoria cannot be determined so that dissociated heterophoria remains as the only indication.