Somatosensory evoked potentials following median and tibial nerve stimulation in patients with Friedreich's ataxia.

The changes in evoked potentials following median and tibial nerve stimulation in nine patients with clinically defined Friedreich's ataxia are reported and discussed. The response originating in the brachial plexus (Erb's point potential) was absent or reduced in amplitude with no prolongation of peak latency, and the response generated in the cauda equina (N18) was absent in all cases. Conduction time from the brachial plexus to cervical spine and medulla oblongata was normal, whereas the central conduction time (N13a/N20, N13b/N20) was delayed. There was moderate to marked attenuation of the primary cortical response to median nerve stimulation. In one patient N20 disappeared during the course of the disease as opposed to the persisting subsequent negative wave, the latter thus simulating a very marked delay in the primary cortical response. Accordingly the cortical response to tibial nerve stimulation, which was only present in two patients and was markedly delayed, might represent a later potential with the primary response absent. The findings are consistent with neuropathological descriptions of a dying back neuropathy with primary axonal degeneration concerning the 1st order sensory neuron. In addition there is evidence either of delayed conduction in 2nd and 3rd order sensory neurones or of abnormal synaptic transmission.