Update on Anesthetic and Metabolic Effects During Intraoperative Neurophysiological Monitoring (IONM)

The influence of depth of anesthesia and blood pressure on muscle recorded motor evoked potentials in spinal surgery. A prospective observational study protocol

For high-risk spinal surgeries, intraoperative neurophysiological monitoring (IONM) is used to detect and prevent intraoperative neurological injury. The motor tracts are monitored by recording and analyzing muscle transcranial electrical stimulation motor evoked potentials (mTc-MEPs). A mTc-MEP amplitude decrease of 50–80% is the most common warning criterion for possible neurological injury. However, these warning criteria often result in false positive warnings. False positives may be caused by inadequate depth of anesthesia and blood pressure on mTc-MEP amplitudes. The aim of this paper is to validate the study protocol in which the goal is to investigate the effects of depth of anesthesia (part 1) and blood pressure (part 2) on mTc-MEPs. Per part, 25 patients will be included. In order to investigate the effects of depth of anesthesia, a processed electroencephalogram (pEEG) monitor will be used. At pEEG values of 30, 40 and 50, mTc-MEP measurements will be performed. To examine the effect of blood pressure on mTc-MEPs the mean arterial pressure will be elevated from 60 to 100 mmHg during which mTc-MEP measurements will be performed. We hypothesize that by understanding the effects of depth of anesthesia and blood pressure on mTc-MEPs, the mTc-MEP monitoring can be interpreted more reliably. This may contribute to fewer false positive warnings. By performing this study after induction and prior to incision, this protocol provides a unique opportunity to study the effects of depths of anesthesia and blood pressure on mTc-MEPs alone with as little confounders as possible.

Trial registration number NL7772.

Sensitivity and Specificity of Intraoperative Neuromonitoring for Identifying Safety and Duration of Temporary Aneurysm Clipping Based on Vascular Territory, a Multimodal Strategy

BACKGROUND

Patients who undergo clipping of cerebral aneurysms face an inherent risk for new postoperative neurologic deficits. Intraoperative neuromonitoring (IONM) is used often for early detection of ischemic changes, while it is still potentially reversible. However, the value, safety, and efficacy of temporary clipping and multimodal IONM to minimize risks are debated. Our retrospective series examined the sensitivity and specificity of IONM using transcranial motor evoked potentials and somatosensory evoked potentials and quantified the safety of temporary clipping by duration and vascular territory.

METHODS

Our prospectively collected database (2010-2013) included 123 consecutive patients who underwent clipping of 133 cerebral aneurysms with use of IONM. We determined postoperative deficit rate and sensitivity and specificity of monitoring to predict these changes intraoperatively. The rate of permanent deficit after temporary clipping was correlated with duration, vascular territory, and IONM findings.

RESULTS

Of 133 clipped aneurysms, 15 instances of IONM changes occurred, including 12 temporary without new postoperative deficit and 3 permanent with new postoperative deficit. Somatosensory evoked potential monitoring predicted one of the permanent deficits and transcranial motor evoked potentials predicted the other 2 deficits.

CONCLUSIONS

Multimodal IONM was highly specific and sensitive for detecting new deficits. Three patients with new deficits had temporary clipping, including 2 patients with IONM changes not temporally associated with clip placement. Our 1.1% rate of permanent neurologic deficit attributed to temporary clipping support its safety. Differences in patterns of IONM changes among vascular territories warrant further investigation.

Multimodality intraoperative neurophysiological monitoring during Onyx embolization of cerebrovascular malformations

General anesthesia prohibits neurological examination during embolization of cerebrovascular malformations when provocative testing prior to pedicle occlusion is needed. Intraoperative neurophysiological monitoring (IONM) has the potential to fill this gap but remains relatively unexplored. We conduct a retrospective review of consecutive patients with cerebrovascular malformations treated with Onyx (ethylene vinyl alcohol copolymer, dissolved in dimethyl sulfoxide) embolization under general anesthesia with IONM from 2009 to 2012. Somatosensory evoked potentials (SSEPs), transcranial motor evoked potentials (TcMEPs), visual evoked potential (VEPs), auditory brainstem response (ABR), and electroencephalography (EEG) were used selectively in all patients depending on the location of the malformation. Provocative testing combined with IONM was performed in 28 patients over 75 sessions. Three patients demonstrated changes in TcMEPs or ABR during provocative testing, which halted the planned embolization. Two patients demonstrated changes in baseline SSEPs after embolization, despite normal IONM during provocative testing, correlating with postprocedural contralateral weakness. Six patients developed visual deficits after arterial occlusion despite unchanged VEPs and occipital EEG during provocative testing and embolization. We therefore conclude that the sensitivity of TcMEPs and SSEPs is preferable to EEG, and we strongly caution against relying on occipital recorded VEPs to predict visual deficits.