Pathophysiologic mechanism of convergence nystagmus

The convergent eye movements in convergence nystagmus are reported to be opposed adducting saccades under excessive convergence drive. What occurs is a horizontal saccade of the adducting eye and a quick reversal of the abducting eye immediately after initiation of the original saccade. Accordingly, the opposed adducting saccades of the abducting eye have been explained by the dynamic overshoot mechanism. However, by electro-oculographic analysis of eye movements in a patient with convergence nystagmus, we found that the opposed adducting saccades of the abducting eye occurred not only after but also just before initiation of the original saccade. Our observation indicates that the opposed adducting saccades can occur without dynamic overshoot as the starter mechanism. Possibly, cessation of the discharge of omnipause neurons may lead to firing of the burst neurons for the medial rectus subnucleus via activation by the excessive convergence drive.