The fourth instance of vascular compromise occurred during coiling of an anterior communicating artery aneurysm. Following placement of several coils, there was abrupt loss of tceMEPs from the right upper and lower extremities, as shown in Fig. 42.2. Responses from the left upper and lower extremities were unchanged from baseline, as were cortical SSEPs to interleaved stimulation of the left and right ulnar nerves, respectively. Cortical SSEPs to stimulation of the right posterior tibial nerve showed only a nominal, clinically insignificant 25 % attenuation from baseline, while those to stimulation on the left side were unchanged. Placement of additional coils was halted temporarily and an arteriogram performed. The arteriogram showed no embolic occlusion or evidence of obvious vasospasm at that time; however, it is not implausible that this acute right upper and lower myotomal tceMEP loss represented an early functional manifestation of developing vasospasm not yet appreciated on angiography. Following a brief surgical pause, tceMEPs on the right side reappeared and response amplitudes returned to baseline range. Placement of coils subsequently was resumed and completed successfully with no additional episodes of tceMEP amplitude attenuation.

Upper extremity positioning alerts occurred in five (3.7 %) of 135 procedures and typically involved compression of the ulnar nerve. In each case, repositioning of the affected arm resulted in prompt resolution of neurophysiologic changes, as shown in Fig. 42.3. During the early stages of the procedure, tceMEPs from left first dorsal interosseous (FD) muscle and cortical SSEPs to stimulation of the left ulnar nerve decreased in amplitude by greater than 60 % from baseline. At this time, there were no concomitant changes in any of the other tceMEPs, including those from left extensor carpi radialis muscle, which unlike the FD muscle is innervated by the radial and not the ulnar nerve.

In light of motor and sensory signal changes that appeared restricted to the left ulnar nerve, attention was directed to inspection of the left upper extremity. Following repositioning of the left arm, both the SSEPs and tceMEPs recovered to baseline amplitude.