Adaptive phenomena in internuclear ophthalmoplegia: Introduction

Abduction nystagmus and hypermetric abduction saccades are the main abnorma lities on the non-paretic eye in internuclear ophthalmoplegia (INO). Their origin is still debated and includes an adaptively increased phasic innerva tion adjusted to the paretic medial rectus muscle (adaptation hypothesis). This hypothesis predicts a relation between the degree of the adduction par esis and the degree of the abduction overshoot and the occurrence of an abd uction nystagmus in opposite eye. The present study investigates these pred ictions. Material and Methods: Direct-current electro-oculography and analy sis of horizontal saccades in 50 consecutive patients with unilateral and 2 5 with bilateral INO. Results: All patients had overshooting abduction sacc ades on the eye opposite to the adduction paresis. In patients with unilate ral and bilateral INO, linear regression showed negative correlations betwe en adduction saccade velocity on the paretic eye and the degree of abductio n overshoot and the duration of the phasic innervation on the other eye (co rrelation coefficients between r = -0.515 and -0.675). The frequency of abd uction nystagmus increased with increasing degree of the adduction paresis. Patients with abduction nystagmus had a significantly higher degree of abd uction overshoot and significantly longer duration of the phasic innervatio n of abduction saccades. Conclusions: These findings add further evidence t o the existence of an adaptively increased phasic innervation adjusted to t he paretic medial rectus muscle in patients with unilateral and bilateral I NO. Such adaptation explains overshooting abduction saccades and abduction nystagmus present in most patients with INO on the non-paretic abducting ey e.