Anesthetic Fade in Intraoperative Transcranial Motor Evoked Potential Monitoring is Mainly Due to Decreased Synaptic Transmission at the Neuromuscular Junction by Propofol Accumulation.

Background and Study Aims We previously reported that normalization of motor evoked potential (MEP) monitoring amplitude by compound muscle action potential (CMAP) after peripheral nerve stimulation prevented the expression of anesthetic fadeanesthetic fade (AF), suggesting that AF might be due to reduced synaptic transfer in the neuromuscular junction. Methods We calculated the time at which anesthetic fadeAF began for each of craniotomy and spinal cord surgery, and examined whether anesthetic fadeAF was avoided by CMAP after peripheral nerve stimulation normalization in each. Similar studies were also made with respect to the upper and lower limb muscles. Results AF was observed in surgery lasting 160 minutes for craniotomy and 260 minutes or more for spinal surgery, and 195 minutes in upper-limb muscles and 135 minutes in lower-limb muscles. In all series, AF could be avoided by CMAP after peripheral nerve stimulation normalization. Conclusion In craniotomy, AF of MEP occurred as well as spinal cord surgery, and it was also corrected by CMAP after peripheral nerve stimulation. AF is considered to be mainly due to a decrease in the synaptic transfer of the neuromuscular junction due to the accumulation of propofol because of the avoidance by CMAP normalization. However, it may be partially due to a decrease in the excitability of pyramidal tracts and α-motor neurons, because AF occurred earlier in lower-limb muscles than in upper-limb muscles.

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The authors declare that they have no conflict of interest.