Peripheral ischemia as a complicating factor during somatosensory and motor evoked potential monitoring of aortic surgery

Abstract

HE incidence of paraplegia following aortic surgery is reported to vary between 2% and 25%.lm4 This reported incidence has made intraoperative spinal cord monitoring an area of interest.5-9 Two monitoring modalities, somatosensory evoked potentials (SEP) and motor evoked potentials (MEP), can detect ischemia of nervous system structures. Of these, the SEP, although controversial, is used most for both experimental and clinical monitoring of aortic aneurysm surgery.5-i0 The SEP monitors physiologic integrity of the peripheral nerve, from the site of stimulation to the cerebral cortex, via the dorsal columns of the spinal cord. The most sensitive area of the spinal cord to ischemia is the anterior horn area, as shown by the distribution of postoperative spinal cord injury.” Although some investigators have suggested that a steal of blood from the posterior to anterior spinal cord may occur during aortic cross-clamping, changes in SEPs will, at best, indirectly monitor ischemia of the motor areas1*J3 Hence, SEP monitoring has yielded low specificity and sensitivity to postoperative neurologic deficits in the absence of shunt or partial bypass use during these operations.r4J5 As suggested by McNulty et a1,r3 use of these operative adjuncts, however, does in part improve the predictive value of SEP monitoring for postoperative neurologic deficits. Recent investigations have used MEPs induced by electrical stimulation to monitor spinal cord function during aortic occlusion.16-ia Monitoring MEPs may be more sensitive than SEPs to spinal cord ischemia. Clinical examples using magnetic transcranially induced MEPs in humans during aortic aneurysm surgery have yet to be reported. An often neglected factor in spinal cord monitoring is that an insult to any part of the nervous system pathway that is monitored will result in a change in the evoked potentials.19 Ischemia to the peripheral nervous system is an important factor that must be addressed during future clinical investigations involving evoked potential monitoring of aortic aneurysm surgery. Two cases are presented, one in which SEPs alone and one in which both SEPs and MEPs were monitored during aortic surgery. Both cases demonstrate the importance of peripheral ischemia as a possible explanation for changes in SEPs and/or MEPs during aortic surgery.