Simulated torsional disparity disrupts horizontal fusion and stereopsis

To investigate how increasing torsional disparity affects clinical measures of the quality of binocular single vision.

Synoptophore targets were adjusted to present varying degrees of torsional disparity (relative to the fusion-free rest position of the eyes) such as might be observed in a patient with excyclotropia. This disparity was increased in 2 degrees increments up to 10 degrees in 21 normal subjects. The fusional divergence and convergence amplitudes and random-dot stereoacuity were measured at each level of torsional disparity.

Horizontal fusional vergence was not significantly altered from the baseline measures at 2 degrees and 4 degrees of torsional disparity. However, both the divergence and the convergence amplitudes declined and were significantly different than the baseline measures at 6 degrees . A similar decline in the random-dot stereoacuity measurements was evident. Fine levels of stereopsis were decidedly affected at greater than 4 degrees of torsional disparity, while more gross levels were similarly affected beyond 6 degrees .

Although human binocular vision can demonstrate the sensory capacity for cyclofusion beyond 10 degrees , torsional disparity of 6 degrees or more significantly degrades horizontal fusional vergence and stereopsis. Torsion in excess of this degree might therefore be a barrier to asymptomatic and comfortable binocular single vision and may require appropriate attention in the planning of more successful surgery for superior oblique palsy and cyclovertical incomitant strabismus.