Comparison of motor-evoked potentials monitoring in response to transcranial electrical stimulation in subjects undergoing neurosurgery with partial vs no neuromuscular block

Kim WH, Lee JJ, Lee SM, Park MN, Park SK, Seo DW, Chung IS. Comparison of motor-evoked potentials monitoring in response to transcranial electrical stimulation in subjects undergoing neurosurgery with partial vs no neuromuscular block. Br J Anaesth 2013;110:567-76.

Motor-evoked potential (MEP) monitoring is commonly performed during neurosurgery to monitor the integrity of the motor pathways. While muscle relaxation is not desirable for intra-operative MEP monitoring during neurosurgery, some surgeons, neurophysiologists and anaesthesiologists still prefer to use the continuous infusion of neuromuscular blocking agents to maintain partial neuromuscular blockade.

Van Dongen et al.,[1] suggested that stable neuromuscular blockade aimed at 45-55% of baseline can provide reliable responses during intra-operative myogenic MEPs. However, there have been no evidence-based comparisons of MEP monitoring with no and partial neuromuscular block (NMB).

In the April 2013 issue of British Journal of naesthesia, a study was published to compare the effects of different levels of NMB including no NMB on MEP parameters.[2] This study was approved by the Samsung Medical Centre Institutional Review Board (2011-04-010) and registered at www.clinicaltrials.gov. Between June 2011 and February 2012, 120 patients were enrolled in this prospective randomised study if they were receiving MEP monitoring craniotomies for tumour or aneurysm and spinal laminectomies. Patients having ASA physical status classification of IIIor greater and who could not undergo MEP monitoringdue to central or peripheral neuromuscular diseases suchas cerebral palsy, myasthenia gravis, acute spinal injuryor neurologic shock were excluded from the study. Anaesthesia was induced by i.v.propofol with remifentanil through a target-controlled infusion pump and tracheal intubation was facilitated with rocuronium. Before rocuronium administration, the baseline twitch response was established with a neuromuscular transmission module.

The maximum electromyographic amplitude of T1 before rocuronium administration was considered to be the control response (Tc). Anaesthesia was maintained with propofol and remifentanil infusions. Subjects were randomly allocated into one of the four groups and were given doses of the neuromuscular blocking agent vecuronium adjusted every 15 min according to the group’s NMB target. Group A was to maintain two train of four TOF counts; Group B was to maintain a T1/Tc of 0.5; Group C was to maintain a T2/Tc of 0.5 (T1,2, first or second twitch height of TOF; Tc, control twitch height); Group D did not maintain NMB. The primary outcome measurement of the present study was the MEP amplitude, and also the co-efficient of variation (CV%) of all measured MEP amplitudes. Other variables measured and compared among the groups during surgery were (i) the incidence of patient spontaneous movements or respiration during MEP monitoring, (ii) any positive MEP changes during the surgery, (iii) the new onset of post-operative neurological dysfunction, (iv) the doses of anaesthetics administered and (v) the continuous end-tidal CO2 measurements. Allpatient characteristics and perioperative clinical variables were similar between the four groups except for the patients height in group C, remifentanil infusion dose in group D (higher than group A or C) and mean infusion dose of vecuronium in C (lower than A or B). The mean MEP amplitudes of the left arm and both legs were significantly higher in group D than groups A, B or C. The mean amplitude of the left arm and right leg was significantly higher in group C than groups A or B. The CVs of the four limbs were significantly smaller in group Dcompared with group A, B or C. Although the mean arterial pressure was significantly lowest in group D, there was no difference in incidence of hypotension, bradycardia and use of vasopressors between the groups. There were six cases of spontaneous respiration, although its incidence did not differ between the groups. The above study concluded that if NMB is used during MEP monitoring, a target T2/Tc of 0.5 is recommended. However, as the MEP amplitude was largestand least variable in the group with no NMB compared withany level of partial NMB used and as incidences of spontaneousmovement or increased vasopressor requirements did not increasewith no NMB, no muscle relaxation is strongly recommended over partial NMB during MEP monitoringin neurosurgery.

Previously Yamamato et al.,[3] devised a new technique of post-tetanic MEP (p-MEP) and found p-MEPs could be recorded at a T1 of 1 mV or %T1 of 10% with no or mild patient movement in response to transcranial stimulation. These strategies can be used as alternatives for improved surgery and patient monitoring.

No conflict of interest has been declared by the author(s).

• REFERENCES

• 1 van Dongen EP, ter Beek HT, Schepens MA, Morshuis WJ, Langemeijer HJ, de Boer A. et al. Within-patient variability of myogenic motor-evoked potentials to multipulse transcranial electrical stimulation during two levels of partial neuromuscular blockade in aortic surgery. Anesth Analg 1999; 88: 22-7
• 2 Kim WH, Lee JJ, Lee SM, Park MN, Park SK, Seo DW. et al. Comparison of motor-evoked potentials monitoring in response to transcranial electrical stimulation in subjects undergoing neurosurgery with partial vs no neuromuscular block. Br J Anaesth 2013; 110: 567-76
• 3 Yamamoto Y, Kawaguchi M, Hayashi H, Horiuchi T, Inoue S, Nakase H. et al. The effects of the neuromuscular blockade levels on amplitudes of posttetanic motor-evoked potentials and movement in response to transcranial stimulation in patients receiving propofol and fentanyl anesthesia. Anesth Analg 2008; 106: 930-4