A comparison of the sensitivity of epidural and myogenic transcranial motor-evoked responses in the detection of acute spinal cord ischemia in the rabbit

Monitoring motor-evoked responses to transcranial stimulation (tc-MERs) provides information about the functional status of the spinal cord during operations that pose the risk of spinal cord ischemia. Responses can be recorded from the epidural space (epidural tc-MERs) or from muscle (myogenic tc-MERs). In this study the relative sensitivity of epidural and myogenic tc-MERs to acute spinal cord ischemia was compared. Spinal cord ischemia was produced by infrarenal aortic balloon occlusion in nine anesthetized New Zealand White rabbits. Tc-MERs were evoked by transcranial electrical stimuli applied to the scalp. Responses were recorded from the lumbar epidural space and from the soleus muscle, and the effect of aortic occlusion was assessed. The peak-to-peak amplitude of the direct wave of the epidural response decreased gradually during aortic occlusion in eight animals and increased in one. The median (10th to 90th percentiles) time to a 50% reduction in amplitude was 11.3 (3-22) min. In contrast, myogenic responses disappeared within 2 min after the start of occlusion in all animals. Lower extremity ischemia as a cause of changes in myogenic tc-MER amplitude was excluded by ligating the right femoral artery and demonstrating that myogenic responses were preserved for 30 min, before occluding the aorta. We conclude that myogenic responses are more sensitive to acute spinal cord ischemia than epidural responses. The rapid detection of spinal cord ischemia with transcranial myogenic motor-evoked responses could be of clinical use in assessing the adequacy of spinal cord blood flow during operations where the spinal cord is at risk.