Somatosensory Evoked Potential and Transcranial Doppler Monitoring to Guide Shunting in Carotid Endarterectomy

Objective Clamping of the internal carotid artery (ICA) during carotid endarterectomy (CEA) is a critical step. In our neurosurgical department, CEAs are performed with transcranial Doppler (TCD) and somatosensory evoked potential (SEP) monitoring with a 50% flow velocity/amplitude decrement warning criteria for shunting. The aim of our study was to evaluate our protocol with immediate neurologic deficits after surgery for the primary end point. Methods This is a single-center retrospective cohort study of symptomatic and asymptomatic ICA stenosis patients from January 2012 to June 2015. Only those cases in which CEA was performed with both modalities (TCD and SEP) were included. The Mann-Whitney U test was applied to evaluate TCD and SEP ratios based on immediate postoperative neurologic deficits. Results A total of 144 patients were included, 120 (83.3%) with symptomatic ICA stenosis. The primary end point was met by six patients (4.2%); all of them were patients with a symptomatic ICA stenosis. The stroke and death rate was 1.4%. Ratios of SEP amplitudes demonstrated significant differences between patients with and without an immediate postoperative neurologic deficit at the time of ICA clamping (p = 0.005), ICA clamping at 10 minutes (p = 0.044), and ICA reperfusion (p = 0.005). Ratios of TCD flow velocity showed no significant difference at all critical steps. Conclusion In this retrospective series of simultaneous TCD and SEP monitoring during CEA surgery of predominantly symptomatic ICA stenosis patients, the stroke and death rate was 1.4%. SEP seemed to be superior to TCD in predicting the need for an intraoperative shunt and for predicting temporary postoperative deficits. Further prospective studies are needed.

The Morphology of Somatosensory Evoked Potentials During Middle Cerebral Artery Aneurysm Clipping (MoSAC): A Pilot Study

Somatosensory evoked potential (SEP) monitoring is a standard tool during clipping of aneurysms of the middle cerebral artery (MCA), and the parameter used to detect a state of cortical ischemia is amplitude. We think that the sensitivity of SEP can however be improved by using other parameters. Our study moves in this direction via SEP morphology. In this pilot preliminary study, involving a small sample without postoperative neurological deficit, we aimed at investigating the value of SEP morphology (in the 15- to 35-ms time frame), in comparison with SEP amplitude (N20 peak-to-peak), as a measure of sensitivity to blood flow reduction. The changes in the SEP morphology of 16 patients undergoing clipping of an unruptured MCA aneurysm was studied. We applied the Morph-Fréchet index for each recorded SEP (at 30-second intervals), quantifying the pattern shape change with regard to the average SEP recorded after dura opening (baseline). We also compared 3 measurements of the SEP morphology, without and with GARCH-derived filter. Filtered Morph-Fréchet never exceeded the individual’s “normality” range in baseline but did so in 81% of the risk phase on average across the 16 subjects, which is more than that for amplitude (36%, P = .002). This pilot study indicates that a measurement derived from the networking nature of the brain was sensitive to blood flow reduction. The SEP morphology approach promises to improve SEP monitoring sensitivity during clipping of unruptured MCA aneurysms. New and Noteworthy. The higher sensitivity to blood flow reduction of SEP morphology than amplitude promises to improve the effectiveness of intraoperative monitoring during MCA aneurysm clipping procedures.

Amplitude Instability of Somatosensory Evoked Potentials as an Indicator of Delayed Cerebral Ischemia in a Case of Subarachnoid Hemorrhage

We describe a 55-year-old male patient with a subarachnoid hemorrhage (SAH) as a result of left middle cerebral artery (MCA) aneurysm rupture, who underwent continuous electroencephalogram (EEG) and somatosensory evoked potential (cEEG-SEP) monitoring that showed an unusual SEP trend pattern. EEG was continuously recorded, and SEPs following stimulation of median nerves were recorded every 50 minutes, with the amplitude and latency of the cortical components automatically trended. An increase in intracranial pressure required a left decompressive craniectomy. cEEG-SEP monitoring was started on day 7, which showed a prolonged (24 hours) instability of SEPs in the left hemisphere. During this phase, left MCA vasospasm was demonstrated by transcranial Doppler (TCD), and computed tomography perfusion (CTP) showed a temporo-parieto-occipital ischemic penumbra. Following intravascular treatment, hypoperfusion and the amplitude of cortical SEPs improved. In our case, a prolonged phase of SEP amplitude instability during vasospasm in SAH correlated with a phase of ischemic penumbra, as demonstrated by CTP. In SAH, SEP instability during continuous monitoring is a pattern of alert that can allow treatments capable of avoiding irreversible neurological deterioration.

Spinal Cord Monitoring

Continuous electrophysiologic monitoring of spinal cord or spinal nerve (or both) function intraoperatively can minimize potential damage that may occur during spine surgery. SEPs are easiest to use for monitoring function and have had the widest application. Unless spinal cord injury is caused by a vascular insult, with purely motor damage, SEP monitoring can identify the damage early enough to alert the surgeon. The addition of MEP monitoring further protects the motor pathways that may be at risk during some spinal procedure. Neurotonic discharges recorded from peripheral muscle are sensitive to nerve root irritation and, thus, can help surgeons recognize when and where damage may be occurring. These techniques appear reliable and with experience the neurophysiologist can acquire the skills to perform and correctly interpret these studies thus enhancing the neurologic and functional outcomes during the often complex procedures.