Intraoperative motor evoked potentials to transcranial electrical stimulation during two anaesthetic regimens

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Abstract

Objectives: To study motor evoked potentials (MEPs) to multi-pulse transcranial electrical stimulation (MP-TES) during orthopaedic spinal surgery under different anaesthetic regimens.

Methods: MEPs to MP-TES were recorded from tibialis anterior and abductor hallucis bilaterally in 50 operations. Anaesthesia was maintained with propofol and nitrous oxide in 29 operations and isoflurane (0.78±0.17% end-tidal) and nitrous oxide in 23 (two patients received both regimens). Analgesia was provided with fentanyl or remifentanil.

Results: Motor stimulation caused neither EEG changes nor seizures. MEPs were obtained in 97% of patients during propofol anaesthesia. The median amplitude and coefficient of variation (CV) at baseline (across all muscles) were 198 μV and 22%, respectively. Amplitudes throughout the operation paralleled the degree of neuromuscular block and were reduced after fentanyl bolus, isoflurane or morphine. Loss of MEPs or persistent amplitude decrements were associated with neurological complications in one patient and severe blood loss in another two patients. MEPs were obtainable in 61% of patients during isoflurane anaesthesia and became inconsistent for end-tidal concentrations >0.87±0.08%. Amplitudes were smaller (85 μV) and baseline variability higher (coefficient of variation 29%) than in the propofol group. The decrease in the number of recordings was greater for isoflurane than propofol when the number of pulses/train decreased from 4 to 2.

Conclusions: Muscle MEPs to MP-TES are a safe, sensitive and reliable method for monitoring motor pathways during propofol/nitrous oxide and fentanyl or remifentanil anaesthesia. MEPs are also obtainable in the majority of patients during isoflurane/nitrous oxide anaesthesia, but quantitative monitoring is not always possible with this regimen.

Introduction

Somatosensory evoked potentials (SEPs) are the most widely used method for intraoperative spinal cord monitoring. Following several reports of ‘false negatives’ with SEPs (Ginsburg et al., 1985, Lesser et al., 1986; Ben-David et al., 1987, Daube, 1989, Zornow et al., 1990, Mustain and Kendig, 1991), the importance of monitoring the integrity of the motor system has been increasingly recognized. Early studies of motor evoked potentials (MEPs) used transcranial stimulation of the motor cortex, using bipolar high voltage (Boyd et al., 1986, Zentner, 1989, Jellinek et al., 1991a, Jellinek et al., 1991b, Burke et al., 1992), unipolar low voltage electric stimulation (Pelosi et al., 1988), or magnetic stimulation (Schmid et al., 1992) and recordings from the spinal cord (Boyd et al., 1986, Pelosi et al., 1988, Zentner, 1991, Burke et al., 1992) or muscles (Pelosi et al., 1988, Zentner, 1989, Jellinek et al., 1991a, Jellinek et al., 1991b). Other authors stimulated the spinal cord and recorded from the muscles (Adams et al., 1993). Problems in these studies included the depressant effect of the general anaesthetics (particularly the volatile agents) on the excitability of both cortical and spinal motor neurones, or the muscle relaxants on the neuromuscular junction. The most successful methods combined high-voltage transcranial electrical stimulation with epidural spinal recording proximal to the corticospinal-motor neurone synapse (Burke et al., 1992). In this way, highly reproducible D waves having low inter-trial variability were recorded even during anaesthesia with volatile agents (Burke et al., 1995). This method however, has some limitations. It does not assess grey matter function, which is the most vulnerable to the effects of ischaemia, nor the lower spinal cord and cauda equina. Further, the motor pathways to the left and right leg cannot be examined separately. As epidural electrodes are usually in situ only during the period of time when the spine is exposed, this form of monitoring cannot be provided throughout the operation and is normally feasible only during procedures with a posterior approach (Burke et al., 1999). Electrodes in the operating field are at risk of been dislodged and recordings contaminated by artefact. Recording from muscle obviates these problems, but it has proved difficult to obtain muscle MEPs reliably during general anaesthesia using single pulse stimulation techniques. Inghilleri et al. (1990) first observed that paired electrical pulses to the motor cortex were more effective in eliciting MEPs than a single stimulus. Subsequent studies have reported a significant increase in the rate of successful intraoperative MEP recordings using multiple pulses (Jones et al., 1996, Pechstein et al., 1996, Pechstein et al., 1998, Taylor et al., 1993, Mochida et al., 1997; Calancie et al., 1998, Deletis and Kothbauer, 1998, Ubags et al., 1998).

In this study, we assessed the feasibility of recording muscle MEPs to multi-pulse transcranial electrical stimulation using different anaesthetic regimens during orthopaedic spinal surgery.

Section snippets

Patients

Motor monitoring was attempted in 50 spinal orthopaedic operations (29 posterior and 21 anterior procedures) in 38 patients (27 female, 11 male, aged 9–67 years, mean age 23.3±15.05 years) (Table 1). Patients with a history of epilepsy or skull fracture were excluded. The study was approved by the local Ethics Committee and all patients gave informed consent.

The majority of operations were for correction of spinal deformity, including idiopathic scoliosis in 19 patients, congenital scoliosis or

Results

MP-TES induced no CFAM changes apart from stimulus artefact. None of the patients had epileptic attacks during or after the procedure. No seizures were reported at clinical follow-up (4–24 months). No skin changes were reported or observed at stimulation sites.

Propofol

In this study, MEP monitoring was possible in 97% of operations with propofol and nitrous oxide anaesthesia. Background variability in the majority of operations and muscles was within the figures recommended for intraoperative somatosensory monitoring (Burke et al., 1999). This implies that it may be possible to adopt for the MEPs the criterion recommended for intraoperative SEPs and regard an amplitude decrease by more than 50% as indicative of a moderate risk of spinal cord damage (Burke et

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