Pseudo-internuclear ophthalmoplegia after surgical paresis of the medial rectus muscle.

Unilateral or bilateral limitation of adduction with jerk nystagmus of the abducting eye occurred after a weakening operation of the medial rectus muscle in five patients aged 9 to 39 years. The nystagmus waveform was indistinguishable and the clinical findings were similar to those in internuclear ophthalmoplegia. Persistence of the abducting nystagmus after occluding the adducting fellow eye in four of the five patients reflected a continued attempt of the central nervous system to increase neural activity to the paretic eye. This hypermetric response disappeared after several days of patching of one eye, only to reappear after removal of the patch. These observations suggested that the generation of abducting nystagmus in conjunction with medial rectus muscle palsy of the fellow eye does not necessarily depend on lesions in the median longitudinal fasciculus, but may also be caused by a gain-step mismatch of the saccadic generation system.