Purpose: The etiology and mechanism of dissociated vertical deviation (DVD) are explored.
Methods: In 6 young adults with DVD, the simultaneous horizontal, vertical, and torsional eye movements for both eyes were recorded by using dual-coil scleral search coils. Analysis of the simultaneous vertical and torsional movements that occurred during the DVD response identified the primary muscles acting in the vergences and versions involved.
Results: Typically, both horizontal and cyclovertical latent nystagmus developed upon occlusion of either eye. A cycloversion/vertical vergence then occurred, with the fixing eye intorting and tending to depress and the covered eye extorting and elevating. Simultaneously, upward versions occurred for the maintenance of fixation, consisting of various saccades and smooth eye movements, and this led to further elevation of the eye behind the cover. The cyclovertical component of the latent nystagmus became partially damped as the DVD developed.
Conclusions: In patients with an early onset defect of binocular function, the occlusion of one eye, or even concentration on fixing with one eye, produces unbalanced input to the vestibular system. This results in latent nystagmus with a cyclovertical component, sometimes only seen with magnification. A normal, oblique-muscle-produced, cycloversion/vertical vergence then comes into play, occurring in an exaggerated form in the absence of binocular vision, probably as a learned response. This cycloversion/vertical vergence helps damp the cyclovertical nystagmus (a cyclovertical "nystagmus blockage" phenomenon), aiding vision in the fixing eye. But this mechanism also produces unavoidable and undesirable elevation and extorsion of the fellow eye, which we call DVD.