Seesaw nystagmus. Role of visuovestibular interaction in its pathogenesis.

Elevation and intorsion of one eye and synchronous depression and extorsion of the other eye characterize a half cycle of seesaw nystagmus. Reversal of these movements constitutes the second half cycle, forming the "seesaw"-like movements. Based on analysis of the ocular oscillation characteristics of the cases of seesaw nystagmus reported in the literature, including the two new cases we present, we postulate that seesaw nystagmus is another type of ocular oscillation brought about by an unstable visuovestibular interaction control system. Nonavailability of retinal error signals to the inferior olivary nucleus essential for vestibuloocular reflex adaptation due to complete chiasmal dissection makes the system less stable. This system instability is further accentuated by the pursuit feedback element. The intact inferior olivary nucleus-nodulus connections in seesaw nystagmus would explain the 180 degrees phase difference that distinguishes it from the midline form of oculopalatal myoclonus, where these connections are likely disrupted.