Slowed abduction saccades in bilateral internuclear ophthalmoplegia

Abstract

Horizontal eye movements were investigated in 65 patients with bilateral internuclear ophthalmoplegia utilizing direct current electro-oculography. Abduction saccades were slowed in 35 patients (53.8%), being hypermetric in 65.7% of them. Slowing of abduction saccades is attributed to impaired inhibition of the tonic resting activity of the antagonistic medial rectus muscle. Experimental data indicate that this slowing results from a lesion of an uncrossed connection between the pontine reticular formation and the oculomotor nucleus. The prevalence of hypermetric abduction saccades increased with increasing severity of adduction paresis on the opposite eye. This confirms the view that medial rectus paresis induces an increased phasic innervation proportional to the deficit in adduction. A commensurate increase in the phasic innervation of the agonistic lateral rectus muscle causes hypermetric abduction saccades.