Incomplete spinal cord evoked injury potential in man

Intraoperative monitoring of spinal cord evoked potentials (SCEP) evoked by tibial nerve stimulation was performed in six patients who underwent junctional coagulation lesion of the dorsal root entry zone for relief of intractable pain. The pain was secondary to complete brachial plexus avulsion. Normal spinal cord evoked potentials showed an initial positive wave and two negative waves, then a group of high frequency conducted waves. On the avulsion side, incomplete spinal cord evoked injury potentials were recorded in all cases and in one on the normal side also. The incomplete spinal cord evoked injury potential consisted of a high-amplitude positive wave with a sharp rising phase and slower falling phase that led to a long lasting, low-amplitude, negative deflection. Several high-frequency components were superimposed onto the monophasic positive wave. The duration of these superimposed components was approximately the same as the duration of the normal spinal cord evoked potentials, but the incomplete spinal cord evoked injury potentials were longer than normal spinal cord evoked potentials for the negative afterwave. Incomplete spinal cord evoked injury potential amplitude was 3-11 times higher than the normal spinal cord evoked potentials. Our results suggest two possible sources of incomplete spinal cord evoked injury potential: the chronic subclinical spinal cord injury produced by the avulsion and the effect of subpial placement of the recording electrodes.