“Threshold-level” multipulse transcranial electrical stimulation of motor cortex for intraoperative monitoring of spinal motor tracts: description of method and comparison to somatosensory evoked potential monitoring

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Numerous methods have been pursued to evaluate function in central motor pathways during surgery in the anesthetized patient. At this time, no standard has emerged, possibly because each of the methods described to date requires some degree of compromise and/or lacks sensitivity.

Object. The goal of this study was to develop and evaluate a protocol for intraoperative monitoring of spinal motor conduction that: 1) is safe; 2) is sensitive and specific to motor pathways; 3) provides immediate feedback; 4) is compatible with anesthesia requirements; 5) allows monitoring of spontaneous and/or nerve root stimulus—evoked electromyography; 6) requires little or no involvement of the surgical team; and 7) requires limited equipment beyond that routinely used for somatosensory evoked potential (SSEP) monitoring. Using a multipulse electrical stimulator designed for transcranial applications, the authors have developed a protocol that they term “threshold-level” multipulse transcranial electrical stimulation (TES).

Methods. Patients considered at high risk for postoperative deficit were studied. After anesthesia had been induced and the patient positioned, but prior to incision, “baseline” measures of SSEPs were obtained as well as the minimum (that is, threshold-level) TES voltage needed to evoke a motor response from each of the muscles being monitored. A brief, high-frequency pulse train (three pulses; 2-msec interpulse interval) was used for TES in all cases. Data (latency and amplitude for SSEP; threshold voltage for TES) were collected at different times throughout the surgical procedure. Postoperative neurological status, as judged by evaluation of sensory and motor status, was compared with intraoperative SSEP and TES findings for determination of the sensitivity and specificity of each electrophysiological monitoring technique.

Of the 34 patients enrolled, 32 demonstrated TES-evoked responses in muscles innervated at levels caudal to the lesion when examined after anesthesia induction and positioning but prior to incision (that is, baseline). In contrast, baseline SSEPs could be resolved in only 25 of the 34 patients. During surgery, significant changes in SSEP waveforms were noted in 12 of these 25 patients, and 10 patients demonstrated changes in TES thresholds. Fifteen patients experienced varying degrees and durations of postoperative neurological deficit. Intraoperative changes in TES thresholds accurately predicted each instance of postoperative motor weakness without error, but failed to predict four instances of postoperative sensory deficit. Intraoperative SSEP monitoring was not 100% accurate in predicting postoperative sensory status and failed to predict five instances of postoperative motor deficit. As a result of intraoperative TES findings, the surgical plan was altered or otherwise influenced in six patients (roughly 15% of the sample population), possibly limiting the extent of postoperative motor deficit experienced by these patients.

Conclusions. This novel method for intraoperative monitoring of spinal motor conduction appears to meet all of the goals outlined above. Although the risk of postoperative motor deficit is relatively low for the majority of spine surgeries (for example, a simple disc), high-risk procedures, such as tumor resection, correction of vascular abnormalities, and correction of major deformities, should benefit from the virtually immediate and accurate knowledge of spinal motor conduction provided by this new monitoring approach.

Article Information

Address reprint requests to: Blair Calancie, Ph.D., The Miami Project, 1600 Northwest 10th Avenue, R-48, Miami, Florida 33136.

© AANS, except where prohibited by US copyright law.

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Figures

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    Recording of evoked responses to motor cortex stimulation in the same patient. A: Using a single-pulse magnetic stimulator in the awake patient, bilateral responses to a single stimulus (arrow) are shown in each of the tibialis anterior (TA) and abductor hallucis (ABH) muscles. The patient was attempting to contract his tibialis anterior muscles at the time the stimulus was applied (coil positioned at Cz). B: During surgery the following day, electrical stimulation (3 @ 2 msec) elicited responses in the patient's abductor pollicis brevis (APB) muscles bilaterally, but did not elicit responses in any of the lower-limb muscles studied. Stimulus intensity = 450 V, C3–C4 (anode—cathode). Vertical bar = 0.25 mV for A, 0.1 mV for B. L = left; Quads = quadriceps; R = right; TMS = transcranial magnetic stimulation.

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    Electromyogram recorded from six muscles in response to stimulation (delivered at Time 0) with either a single-pulse (A), 2 @ 2 msec (B), or 3 @ 2 msec (C) stimulus pattern. The site of stimulus delivery (C4—C3; anode—cathode) and stimulus intensity (400 V) was the same in all three cases. All records reflect the exact EMG responses recorded, without any type of signal averaging or postacquisition processing. Vertical bar = 0.5 mV. L = left; R = right.

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    Scatterplot displaying variability in the peak-to-peak amplitude (ordinate) and minimum latency (abscissa) of right abductor pollicis brevis responses evoked by TES in one patient during the entire surgical procedure. The stimulation site (C3—Cz) and pulse train (3 @ 2 msec) were unchanged. Stimulus intensity was either 175 V (circles) or 200 V (squares).

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    A: Scatterplot showing absolute threshold intensity values for evoking motor responses to single-pulse (closed circle), two-pulse (triangle down), and three-pulse (triangle up) TES, all determined for the abductor pollicis brevis muscle. Using single-pulse TES there were several cases in which the maximum stimulus intensity applied failed to elicit muscle responses: these are indicated by open circles. B: Bar graph showing the mean (± standard error of the mean) threshold intensities for evoking motor responses to three-pulse TES in those muscles that were most commonly studied. The thresholds for upper-limb muscles were lower than those for leg muscles. All threshold determinations summarized in this graph were made at the onset of the surgical procedure.

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    Summary data obtained in a 78-year-old man with C1–2 schwannoma, normal preoperative muscle strength, and a complaint of pain. A–E: The SSEP responses to median nerve stimulation, measured from subcutaneous electrodes positioned over C3 and C4, referenced to Fz. Stimulations of left and right sides are shown by arrows below panel E. During the surgical procedure there was no significant change in either the amplitudes or latencies of these waveforms from the initial (baseline) values. F–H: The TES-evoked responses from left- and right-side muscles. Responses from left-side muscles were lost before resection started (G) and did not recover during tumor resection (H) or by the conclusion of the procedure. The stimulus used was 3 @ 2 msec, C4—Cz, 400 V for each TES record shown. Vertical calibration for F–H: 0.5 mV for left (L/abductor hallucis) and 0.2 mV for other muscles. I: Scatterplot depiction. Absolute threshold intensity for TES-evoked responses determined during the course of the surgical procedure. Each symbol represents the threshold of the muscle shown. Open circles indicate those trials in which no response was evoked by TES at the stimulus intensity shown for that muscle. The time at which tumor resection was initiated is marked by a dashed line. FCR = flexor carpi radialis; L = left; R = right.

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    Summary data obtained in a 66-year-old woman with an L-1 lipoma and a tethered spinal cord. A–C: The SSEP responses to posterior tibial nerve stimulation; Cz—Fz recording montage. During the surgical procedure there was a significant decrease in the right SSEP amplitude while the mass was being resected (B); after closing the dura (C) this amplitude recovered to baseline levels. Left- and right-side stimulation as shown in Fig. 5. D–F: The TES-evoked responses at different times during the procedure. There were no significant changes in thresholds in this procedure. Stimulation parameters: C3–C4, 200 V, 3 @ 2 msec. Vertical calibration: 2 µV for abductor pollicis brevis, 100 mV for quadriceps, and 500 µV for tibialis anterior and abductor hallucis. G: Scatterplot displaying absolute threshold intensity for TES-evoked responses determined during the course of the surgical procedure. Each symbol represents the threshold of the muscle shown. H and I: Pre- and postoperative neurological evaluation of motor and sensory (dorsal column [DC] and lateral tract [LT]) function. The motor scores determined for each muscle and the sensory scores evaluated for each dermatome on the left and right sides are indicated by filled symbols. Asterisks denote those trials in which the outcome was limited either by pain or orthoses or attempts contravened by doctor's orders. R = right.

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    Summary data obtained in a 52-year-old woman with C-4 metastatic disease. A–C: The SSEP responses to posterior tibial nerve stimulation; Cz—Fz recording montage. There was a decrease in left-side response amplitude during the corpectomy, which recovered to within baseline limits by the conclusion of the procedure. Left- and right-side stimulation as shown in Fig. 5. D–F: The TES-evoked responses at different times during the procedure. A sudden increase in threshold and reduction in response amplitude immediately followed spine distraction and graft placement. Stimulation parameters: C3–C4, 250 V, 3 @ 2 msec. Vertical calibration: 50 µV for all traces except for panel D, abductor pollicis brevis (100 µV). G: Scatterplot displaying absolute threshold intensity for TES-evoked responses determined during the course of the surgical procedure. Each symbol represents the threshold of the muscle shown. Open circles indicate those trials in which no response was evoked by TES for the intensity shown in the right abductor pollicis brevis. H and I: Pre- and postoperative neurological evaluation of motor and sensory (dorsal column [DC] and lateral tract [LT]) function. The motor scores determined for each muscle and the sensory scores evaluated for each dermatome on the left and right sides are indicated by closed boxes. R = right.

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    Electromyogram recorded from different muscles in a single patient in response to three-pulse TES during a steady infusion of propofol (A) and immediately after bolus infusion of 2 ml propofol (B). The same stimulation (3 @ 2msec, 750 V) evoked widespread muscle recruitment (A) while the patient was not deeply anesthetized (that is, “light”) and only low-amplitude responses by the right soleus and abductor hallucis muscles (B) following intravenous infusion of a large bolus dose of propofol. HAMS = hamstring; L = left; R = right.

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    Graph depicting the current—voltage relationship of the stimulator determined for a single patient at different times during the surgical procedure. All measurements were obtained in the same stimulation configuration. For a given preset voltage, the amount of current actually delivered increased by a modest amount during the course of an extended surgical procedure. Each symbol represents a different sampling time.

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    Schematic representation of threshold-level TES under normal conditions (A) and after partial conduction block (B). For detailed description see text.

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