'Early' and 'late' components of spinal evoked potentials in spinal cord trauma patients

Spinal somatosensory evoked potentials (EPs) were investigated in patients with different levels of spinal trauma during stimulation of tibial nerve in the popliteal fossa. Registration over lower thoracic and lumbar vertebrae revealed 'early' EPs with latencies of about 10 ms. Presence of H-reflexes in the gastrocnemius muscle did not reveal close relation to the 'early' EP emergence. Absence of direct muscle responses (M-responses) was accompanied by a lack of the EPs; only in one case, the 'early' EPs were seen in absence of the M-response. However, presence of the M-response did not by itself ensure possibility of EP registration. Asymmetry of the EPs evoked by stimulation of right and left tibial nerve was frequently seen. Stimulation of tibial nerve in spastic patients with middle thoracic trauma led in a number of cases to registration of 'late' EPs with latencies of 20-50 ms. Cauda equina lesions could lead to registration of delayed 'early' EPs with latencies increased by 2-3 ms. Possible sources of the 'early' EPs are discussed including discharges in Ia muscle- and skin afferents, ortho- and antidromic volleys in α-motoneurone axons, postsynaptic potentials in α-motoneurone bodies. According to the results, one cannot single out a pathway with exceptional importance for the 'early' EPs. Special attention is paid to the necessity of low dispersion of conduction velocity in the pathways under discussion. Different groups of 'late' EPs can probably be attributed to delayed signal conduction (only in cases of cauda equina lesions), synchronous afferent discharges may be due to stimulus-evoked muscle contractions, or to pathological reflexes of distant muscles.