Torsional diplopia and its perceptual consequences: A 'user-friendly' test for oblique eye muscle palsies

Purpose: (I) To present a simple clinical test of torsional diplopia, based on the observation that a person with acquired incomitant cyclotorsion will perceive nonparallel double images of a horizontal rod held before the eyes. The apparent image crossing will determine laterality. (2) To study changes in visual perception induced by cyclotorsion. Methods: Ten subjects with unilateral or bilateral fourth nerve palsy and one subject with isolated superior branch third nerve palsy were studied. Simulation of image displacement in cyclotorsion with a computer graphic device was performed on three normal subjects to analyze the perceptual consequences of torsional diplopia, Results: In cyclotorsion, displacement of the retinal image could be separated into two vectors, each parallel to the eye's main meridians. Vertical displacement led to vertical diplopia because of the small fusional amplitude for vertical disparity, while fusion of the horizontally displaced retinal image was often possible. This led to changes in depth perception. Cyclotorsion, therefore, induces complex perceptual changes. These changes could be simulated in the laboratory in normal subjects. Conclusions: Cyclotropia acquired after childhood usually produces torsional diplopia. The subjective horizontal rod test is a convenient way of analyzing such misalignment. Visual disturbances in cyclotorsion are more complex than has previously been appreciated.