The influence of depth of anesthesia on motor evoked potential response during awake craniotomy

Abstract

OBJECTIVE Motor evoked potentials (MEPs) are a critical indicator for monitoring motor function during neurological surgery. In this study, the influence of depth of anesthesia on MEP response was assessed. METHODS Twenty-eight patients with brain tumors who underwent awake craniotomy were included in this study. From a state of deep anesthesia until the awake state, MEP amplitude and latency were measured using 5-train electrical bipolar stimulations on the same site of the precentral gyrus each minute during the surgery. The depth of anesthesia was evaluated using the bispectral index (BIS). BIS levels were classified into 7 stages: < 40, and from 40 to 100 in groups of 10 each. MEP amplitude and latency of each stage were compared. The deviation of the MEP measurements, which was defined as a fluctuation from the average in every BIS stage, was also considered. RESULTS A total of 865 MEP waves in 28 cases were evaluated in this study. MEP amplitude was increased and latency was decreased in accordance with the increases in BIS level. The average MEP amplitudes in the > 90 BIS level was approximately 10 times higher than those in the < 40 BIS level. Furthermore, the average MEP latencies in the > 90 BIS level were 1.5-3.1 msec shorter than those in the < 60 BIS level. The deviation of measured MEP amplitudes in the > 90 BIS level was significantly stabilized in comparison with that in the < 60 BIS level. CONCLUSIONS MEP amplitude and latency were closely correlated with depth of anesthesia. In addition, the deviation in MEP amplitude was also correlated with depth of anesthesia, which was smaller during awake surgery (high BIS level) than during deep anesthesia. Therefore, MEP measurement would be more reliable in the awake state than under deep anesthesia.

Letter to the Editor: Comparison between surgery and Gamma Knife radiosurgery for brain AVMs

“…There is a wide range of propofol plasma concentration from 1 to 10 µg/ml when the BIS indicates an adequate level of anesthesia as its value of 50. 3 It is notable that MEP latency decreased in accordance with increases in the BIS value in the current series, 9 although there are several reports indicating that MEP latency did not significantly change following the administration of intravenous agents. 4,6,8 In the pyramidal tract, there are only 2 synaptic connections; one is between the upper motor neuron and the alpha motor neuron, and the other is located in the neuromuscular junction.…”

“…7 Our surgical results are similar to those of other reported cohort series and have been incorporated into pooled analyses. 3,9 Therefore, it is reasonable to assume that our results may be generalizable to other comparable centers. We also believe that it is reasonable to compare our results with those obtained from Starke and colleagues' combined cohort.…”

“…Therefore, it is highly suspected that the BIS values collected in the present study do not accurately correlate with the hypnotic state because the authors did not determine cutoff levels of BIS-EMG or BIS-SQI. 9 In particular, facial muscle activities may have contributed to higher BIS levels among the present population because the patients were asked to respond to intraoperative tasks during awake craniotomy. Additionally, we should remember that the relationship between BIS value and measured propofol concentration is nonlinear.…”

“…These cases, and the reasons for their exclusion, have been previously reported. 4,7 No patient with a Spetzler-Martin Grade I or II (Spetzler Ponce Class 9 [SPC] A) AVM was denied surgery because of perceived surgical risk, 6 of 205 patients with Spetzler-Martin Grade III (SPC B) AVMs were denied surgery because of perceived surgical risk, and 33 of 112 patients with Spetzler-Martin Grades 4 and 5 (SPC C) AVMs were denied surgery because of perceived surgical risk. 4,7 Another of the corrected biases in our series accounted for in the analysis is failure to progress from embolization to surgery because of complications occurring as a consequence of embolization.…”

“…In the current study, Ohtaki et al adopted a unique montage to assess the depth of anesthesia; a BIS self-adhesive EEG electrode strip was positioned from the contra-and ipsilateral nasal bones to the ipsilateral zygomatic bone. 9 Actually, there is another report in which the authors used an occipital montage for BIS monitoring; 12 however, as far as BIS heavily relies on frontal EEG patterns for its mathematical analysis, there is no guaranty that BIS-EEG from nonfrontal montages truly reflects the depth of anesthesia. Homology of raw EEG waveforms between frontal and nonfrontal montages should be validated when a nonfrontal montage is required for intraoperative BIS recording.…”