Some observations on abduction nystagmus in internuclear ophthalmoplegia

Abstract

The findings in 220 consecutive patients with internuclear ophthalmoplegia (INO) investigated by direct-current electro-oculography (EOG) are reported. Abduction nystagmus is significantly more frequent in patients with severe INO and increases with increasing lateral movements in the direction of the paretic medial rectus muscle. In a series of 12 unselected patients, additional recordings were made immediately after patching the paretic eye, and in six of them also immediately after patching the non-pa-retic eye. Patching the paretic eye was followed by a decreased pulse-step mismatch and a decreased pulse duration causing decreased amplitudes of abduction and adduction saccades in most patients. Patching the non-paretic eye was followed by an increased pulse-step mismatch and an increased pulse duration causing increased amplitudes of abduction and adduction saccades in most patients. The author's findings support the adaptation hypothesis of abduction nystagmus in most patients with INO, and are in accordance with Hering's law of equal innervation. The increased innervation is adjusted to the adduction paresis and generated immediately after onset of the paresis. Abduction nystagmus in INO depends on the medial rectus paresis and presumably is initiated by a visual error signal.