Dissociated vertical deviation : Head and body orientation affect the amplitude and velocity of the vertical drift

Purpose: Five subjects with dissociated vertical deviation (DVD) were studied to determine if the amplitude or velocity of the vertical components of the DVD were affected by head/body orientation with respect to gravity. Methods: Deviations were measured in head upright, head supine, and supine positions, with head hanging postures using a binocular CCD video-based infrared eye tracker. Subjects were required to fixate a target presented in the primary position during alternate or cover/uncover tests. Results: Amplitude and velocity of DVD both in onset and recovery were affected by head/body orientation with respect to gravity. In four of five subjects, the amplitude of the DVD was asymmetric between the two eyes when the head was upright. When the head/body was moved from an upright to a supine with head hanging backward condition, the amplitude of the DVD in the two eyes inverted. The eye with the larger DVD in the upright position had a smaller DVD in the head-hanging orientation. A similar relationship existed between velocity and head/body orientation. We found that DVD velocity increased with amplitude. Conclusions: Passive effects of gravity on the eye-in-orbit do not influence DVD magnitude or frequency of occurrence.The data suggest, however, that otolithic and possibly neck afferent inputs play a role in DVD magnitude and may be a part of the etiology of the condition.