Background: During surgery, lower cranial nerve (CN) electromyography (EMG) may show spontaneous activity without surgical correlate. These episodes have been observed in association with sudden patient movement. In the study presented, this activity was quantified and correlated to the Bispectral Index (BIS) to evaluate the potential of lower CN-EMG for monitoring the depth of anesthesia.
Methods: Spontaneous EMG activity in muscles targeted by the CNs IX, X, and XII was quantified and correlated with the BIS measured in 23 patients operated on for posterior fossa pathology. In a blinded retrospective analysis, the time interval from beginning of build-up of the respective parameter (EMG activity and BIS) until extubation was marked. The resulting time intervals were then compared between BIS and EMG.
Results: EMG and BIS build-up was seen 12.3 and 5.9 minutes, respectively, before extubation in median. Thus, EMG provided a longer "warning time" (P=0.026). Isolated lower CN EMG channels preceded BIS in 53%, 62%, and 70% (CN IX, X, and XII). The earliest available EMG channel preceded BIS in 67% of the patients by a median time of 4.3 minutes. The beginning of EMG build-up in the earliest channel was found to be significantly earlier than BIS (P<0.001).
Conclusions: Spontaneous EMG of muscles targeted by lower CNs seems to correlate well with arousal reactions at the end of anesthesia. In many cases, this effect preceded BIS changes. Thus, lower CN EMG monitoring may be a valuable tool in monitoring adequate depth of anesthesia.