Tonic vs. Phasic Fiber Dysfunction in Isolated, Unilateral Sixth Cranial Nerve Palsy

Sixth nerve palsy is a relatively common cause of diplopia. We have observed an apparent non-linearity between the degree of deficit in abduction vs. static ocular misalignment among patients. We hypothesized that this disparity might be due to relative differences in impairment of phasic vs. tonic muscles fiber function. We selected isolated sixth nerve palsies because of the singular innervation of this cranial nerve to the lateral rectus muscle to examine the relationship between the degree of esotropia and limitation of abduction.