Dissociated vertical deviation: an exaggerated normal eye movement used to damp cyclovertical latent nystagmus.

Abstract

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, sometimes seen only with magnification. The cyclovertical component of the latent nystagmus, when present, is similar to normal vestibular nystagmus induced by dynamic head tilting about an oblique axis. Such vestibular nystagmus characteristically produces a hyperdeviation of the eyes. In the case of cyclovertical latent nystagmus, the analogous hyperdeviation will persist unless corrected by a vertical vergence. A normal, oblique-muscle-mediated, cycloversion/vertical vergence is called into play. This occurs in the proper direction to correct the hyperdeviation, but it occurs in an exaggerated form in the absence of binocular vision, probably as a learned response. The cycloversion/vertical vergence helps damp the cyclovertical nystagmus (a cyclovertical "nystagmus block-age" phenomenon), aiding vision in the fixing eye. But this mechanism also produces unavoidable and undesirable elevation and extortion of the fellow eye, which we call DVD.