Dissociation of muscle action potentials and spinal somatosensory evoked potentials after ischemic damage of spinal cord

In patients undergoing spinal fusion and Cotrel-Dubousset instrumentation we recorded compound muscle action potentials (CMAP) from the lower limb and spinal somatosensory evoked potentials (SSEP) from the caudal epidural space after direct stimulation of rostral spinal cord via epidural electrodes. In three of 30 patients tested, the derotation maneuver altered CMAP but not SSEP. In ten dogs, we observed similar dissociation with decrease or disappearance of CMAP amplitude and unchanged SSEP after ligation of the thoracoabdominal aorta or intercostal arteries at each level. In contrast, both CMAP and SSEP were unchanged by clamping the artery at the lumbar level. This is likely due to the lack of collateral vascular flow at the thoracic cord level, the anterior cord in particular, which is mainly supplied by a single large radicular artery (Adamkiewicz artery). These findings support that the CMAP and SSEP are mediated through two independent pathways located in the anterior and posterior spinal cord, respectively. We postulate that the dissociate alteration of CMAP and SSEP by derotation maneuver is due to greater vulnerability of the anterior cord or motor tract to ischemia caused by the displacement of anterior spinal or radiculomedullary artery. Therefore, the patients requiring major derotation procedure would benefit from CMAP monitoring, which provides more sensitive measure of anterior cord function that the conventional SSEP monitoring.