Dissociated vertical deviation: Etiology, mechanism, and associated phenomena

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