24/03/2023
"Intraoperative neuromonitoring using motor evoked potentials (MEP) satisfactorily detects motor tract integrity changes during spinal surgery. However, monitoring is affected by “anesthetic fade,” in which the stimulation threshold increases because the waveform amplitude decreases with the accumulation of propofol.
CASE: During a cervical antero-lateral interbody fusión surgery the left TA MEP increased during decompression and removal of intervertebral defect (Photo 1).
Soon after the TA MEP amplitude collapsed (Photo 2).
Suitable changes were seen also in lower limbs SSEP (three ms increased latency in TN SSEP) and EEG (larger suppression periods).
We announce the incident but we let the surgeon to continue the surgery. After a minute we asked him to stop and we increased intensity of stimulation by 70-90 V (Photo 3). MEP were recovered and at the end of the surgery the amplitude showed left lower limb improvement (Photo 4).
True story:
The anesthesiologist injected a Propofol bolus because the patient was "coughing". The surgeon was not alarmed and the went very well.
Conclusions:
1) The use of multimodal approach is mandatory.
2) The MEP are sensitive to Propofol bolus, especially the larger CST fibers (TA, APB) as previously demonstrated in animals (Keller, 1992).
3) MEP decay should be quickly investigated, announced and checked for "False Positive" like in our case.
