Intraoperative neurophysiological monitoring for the anaesthetist

Abstract

The use of intraoperative neurophysiological monitoring (INM) during spinal orthopaedic and neurosurgical procedures provides a challenge to the attending anaesthesiologist. Since all anaesthetic agents affect synaptic function, the choice of agent will be determined by the type of surgery and the INM modality employed. Halogenated volatile agents decrease evoked potential (EP) amplitude and increase latency, and should be avoided in modalities that pass through cortical tracts. The effect on EPs is apparent at minimum alveolar concentrations of 0.3-0.5. Intravenous agents affect EPs in a dose-dependent manner, and should be titrated to response. Total intravenous anaesthesia with propofol and remifentanyl is the preferred technique. The risk of propofol infusion syndrome has not been shown to affect the choice of this agent. Compound muscle action potentials are abolished by barbiturates, and should be avoided during motor-evoked potential (MEP) monitoring. Although somatosensory-evoked potentials are unaffected by muscle relaxants, they prevent the monitoring of MEPs and should be avoided during multimodal use. When paralysis is required to ensure patient safety, the train-of-four ratio should be kept at 2/4 twitches and a T1 response at 10-20% of baseline, with use of a closed-loop system.