Peripheral and central nerve conduction in subacute myelo-optico-neuropathy.

Five patients with subacute myelo-optico-neuropathy (SMON) were studied by short-latency somatosensory evoked potentials (SEPs) in response to electrical stimulation of the median nerve and the posterior tibial nerve. SEPs with median nerve stimulation were normal in all cases, but SEPs with posterior tibial nerve stimulation were abnormal in two patients with severe sensory loss in the legs. Abnormalities suggested normal peripheral conduction but a marked attenuation of the cortical component and delayed central conduction. These findings were in conformity with postmortem morphometric analysis, which showed marked reduction of myelinated fibers in the gracile fascicle but only slight reduction of large myelinated fibers in the sural nerve. The pathophysiology of SMON appears to be mainly a central distal axonopathy.