Exploring the Divergence Range for Stereopsis Maintenance With a Computer-Simulated Troposcope in Patients With Intermittent Exotropia

PURPOSE. To investigate whether sensory input or motor signal of the extraocular muscle is the main activator of suppression in human intermittent exotropia (X(T)). METHODS. A case-control study was performed. Ten subjects with X(T) and 10 control participants were enrolled. The divergence range between both eyes when binocular vision was maintained was measured by using stereotests with a self-written computer program mimicking a troposcope. The break point, defined as the deviation angle at which stereopsis broke during eye deviation, was compared between the experimental and control groups by using a t test. RESULTS. The median near deviation angle in the experimental group was 42.5 prism diopters (PD) (mean, 44.5 +/- 10.82 PD). The mean break point was 40.45 +/- 10.79 PD in the X(T) group and 26.86 +/- 2.62 PD in the control group (P = 0.003). The mean ratio of the break point to the near deviation angle was 0.92 +/- 0.23 in the X(T) group, with the ratio close to 1 in 7 of 10 subjects. CONCLUSIONS. Binocular vision can be maintained if similar images are projected onto corresponding retinas during the tropic phase of X(T). The antidiplopic mechanism in X(T) patients (i.e., suppression) is evoked by sensory input from binocular rivalry rather than by motor signal of the extraocular muscle.