Spinal Injuries Unit, Southport and Ormskirk Hospital NHS Trust, Town Lane, Southport PR8 6PN, UK
* Corresponding author. E-mail: John.Watt{at}southportandormskirk.nhs.uk
Accepted for publication August 15, 2004.
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Abstract |
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Methods. We applied three-pulse rTMS (TriStim) in 11 patients undergoing spinal column surgery after spinal column injury and recorded the latency and peak-to-peak amplitude of MEPs. Anaesthesia was maintained with propofol and remifentanil.
Results. MEPs were monitored successfully intraoperatively in all patients.
Conclusions. It is possible to monitor intraoperative MEP using rTMS during anaesthesia with propofol and remifentanil.
Keywords: anaesthetics i.v., propofol ; analgesics opioid, remifentanil ; monitoring, spinal
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Introduction |
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Methods and results |
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Transcranial magnetic stimulation was provided in bursts of three pulses, 2 ms apart, based upon previous work.4 The single-pulse Magstim 200 had been upgraded with a Bistim to provide a three pulse stimulus, which was considered to be nearly as effective as the four-pulse Quadropulse 500 (Magstim Company, Whitland, UK). The double-cone coil recommended for stimulating the lower limbs was generally used, but on occasion the circular 90 mm coil was easier to place over the skull within the confines of a halofixation system. The coils were handheld to allow localization of either the right- or left-sided muscle by slightly altering the direction of focus. After positioning the patient prone in the operating theatre, nitrous oxide was discontinued and the stimulus was repeated to find the best position and determine if nitrous oxide was interfering with the response.
Nine of the 11 patients were tested before the operation to record baseline motor evoked potentials (MEPs), limiting the output to 3060% to prevent jerking of the unstable spine. Patients who had partial neurological loss usually needed a stronger stimulus output and the responses could sometimes reveal that one side was more compromised than the other. The muscle selected was just below or at the neurological level of actual or potential deficit and when, during the course of this study period, a two-channel stimulator became available the homologous right and left muscles were both stimulated.
We recorded the selected responses with a Dantec Keypoint Workstation (Dantec Dynamics UK, Bristol, UK). This displays sequential MEPs until a successful response is obtained, which may then be printed out. Amplitude and latency are available either as hard copy or stored. Peak-to-peak voltages and latencies were recorded every 2 min during the operation when damage could occur, and compared with preoperative values where possible. As responses are reduced during anaesthesia, the Magstim output was increased to 100% during surgery. Possible neural damage was considered to be shown by an increase in latency of 5 ms6 or a decrease in amplitude of >50%.7
Six patients were premedicated with oral promethazine 50 mg. Three patients required awake fibre-optic intubation and were given i.m. glycopyrrolate 200 µg and Cyclimorph-10 1 ml. One patient was given oral lorazepam 2 mg for awake intubation, although benzodiazepines were generally avoided because of their potential to suppress the MEP. While the patient was placed prone we administered nitrous oxide and isoflurane. Once the patients were positioned safely, anaesthesia was maintained with infusions of propofol and remifentanil and monitoring was started.
In eight patients, propofol was infused at 10 mg kg1 h1 for 10 min, 8 mg kg1 h1 for the next 10 min and then at 6 mg kg1 h1. In the other patients we gave target-controlled infusions using a Diprifusor set to obtain a predicted plasma concentration of 3.54 µg ml1. We gave a dose of remifentanil 1 µg kg1, followed by infusion of 0.51 µg kg1 min1 to all patients.
Mivacurium was administered to all patients after induction of anaesthesia. Mivacurium infusion was continued for a period in five patients during diathermy to reduce muscle contractions, using an infusion of about 3 µg kg1 min1 to obtain two or three visible twitches in a train of four.8 Nine patients were ventilated with an airoxygen mixture, and two patients received 50% nitrous oxide. End tidal isoflurane was less than 0.2% in all patients once TriStim monitoring started.
Reliable MEPs were obtained, monitored bilaterally in six patients and unilaterally in five. Values for latency were within 2 ms of preoperative values in five patients, increased by 4 ms in two and decreased by 4 ms in one. No patient sustained neurological damage or worsening of neurological deficit. A typical intraoperative course of latency and peak-to-peak amplitude is shown in Figure 1. Amplitude tended to increase with noxious surgical stimulus during initial dissection and with the decompression of the spinal cord towards the end of the procedure. Because the EMG signal could vary with surgical stimulation, conditions of anaesthesia and variations in coil positioning, the interpretation of amplitude and latency of the signal had to be related to these factors. Table 1 summarizes the results of each patient.
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Comment |
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Total i.v. anaesthesia can cause systemic hypotension, so i.v. anaesthesia was not started until positioning was complete. Finding the best cranial position usually needs repeated stimuli, at a time when the concentrations of volatile agents and mivacurium are decreasing. Variation of MEP is expected using TMS, partly because positioning of the hand held coil requires experience and practice. Some prefer to keep the position constant with a clamp. A halo fixation device can impair placement of the figure-of-eight coil but positioning was easier once the patient was prone. There is also some variation in waveforms produced by TMS, partly from changes in neuronal threshold during a sequence of stimuli, and also because a muscle like the tibialis anterior has innervation from L4, L5 and S1. The peroneus longus could be more suitable because it has a predominantly L5 supply. Once stable responses are obtained, it is not necessary to stimulate continuously, but to stimulate only when neural damage could occur.
TMS, compared with transcranial electrical stimulation, has minimal risk of electric shock or burns. Unlike electrical cortical stimulation, baseline recording and assessment after operation are possible with comparison of right and left sides. Stimulation of deeper and less accessible nerves is also possible.
In addition to the recent report of transcranial electrical stimulation during i.v. anaesthesia,11 we report that repetitive magnetic stimulation allows assessment of long motor tracts during anaesthesia using propofol and remifentanil with no more than 25% nitrous oxide. By evolving a reliable method of anaesthesia that allows comparison of preoperative magnetic MEPs with repeated intraoperative responses, operative procedures can be carried out more safely.
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References |
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