[Spinal and cortical SEP's in healthy subjects and paraplegics]

SEPs are evoked by electrical stimulation of tibial nerve in the fossa poplitea. Surface electrodes, located in S1, L4, L2, T12 with a reference in T6, can record lumbar evoked potentials and calculate a peripheral sensitive velocity. Bipolar leads between electrodes located in T12, T9, T6, T3 and C7 record medullary potential and calculate a medullary transit velocity. The cortical potential is monitored between C'z and a non-cephalic reference. 25 controls and 10 paraplegic patients are studied. In controls, sensitive peripheral velocity is 59 m/sec. The lumbar potential is composed of two negative waves, respectively due to the activation of sensitive roots and to the medullary potential. The medullary transit velocity, measured by the increase of the latency of the culmination of this negative wave along the spine, is 60 m/sec. The cortical potential is composed of two stable waves P30 N38 which are observed in every control; these waves are followed by a succession of positive and negative waves. In the 10 paraplegic patients, complete anesthesia observed in 5 cases is associated with an absence of cortical potential, and the hypoesthesia observed in 5 cases is associated with a cortical potential with a reduced amplitude (2 cases) or an increased latency (3 cases). In these last 3 cases, the medullary potential allows to specify the location of slowing of the transit velocity.