Abduction nystagmus in internuclear ophthalmoplegia.

We tested the hypothesis that abnormalities of the abducting eye in internuclear ophthalmoplegia reflect an adaptive process that helps overcome the adduction weakness of the opposite eye. This response operates under the constraints of Hering's law of equal innervation: any attempt to increase the innervation to a weak muscle in one eye must be accompanied by a commensurate increase in innervation to the yoke muscle in the other eye. In 4 patients with internuclear ophthalmoplegia, we patched one eye for 1 to 5 days to allow time for the central nervous system to optimize innervation for the habitually viewing eye. We predicted that there would be a conjugate adjustment of innervation that would diminish the abduction overshoot and backward postsaccadic drift made by the habitually viewing eye. This was the case in 3 of our 4 patients. Our findings show that the abduction nystagmus is a manifestation of a normal adaptive response in some patients with INO.