A comparison of myogenic motor evoked responses to electrical and magnetic transcranial stimulation during nitrous oxide/opioid anesthesia

Transcranial motor evoked potentials (tc-MEPs) are used to monitor spinal cord integrity intraoperatively. We compared myogenic motor evoked responses with electrical and magnetic transcranial stimuli during nitrous oxide/opioid anesthesia. In 11 patients undergoing spinal surgery, anesthesia was induced with i.v. etomidate 0.3 mg/kg and sufentanil 1.5 microg/kg and was maintained with sufentanil 0.5 microg x kg(-1) x h(-1) and N2O 50% in oxygen. Muscle relaxation was kept at 25% of control with i.v. vecuronium. Electrical stimulation was accomplished with a transcranial stimulator set at maximal output (1200 V). Magnetic transcranial stimulation was accomplished with a transcranial stimulator set at maximal output (2 T). Just before skin incision, triplicate responses to single stimuli with both modes of cortical stimulation were randomly recorded from the tibialis anterior muscles. Amplitudes and latencies were compared using the Wilcoxon signed rank test. Bilateral tc-MEP responses were obtained in every patient with electrical stimulation. Magnetic stimulation evoked only unilateral responses in two patients. With electrical stimulation, the median tc-MEP amplitude was 401 microV (range 145-1145 microV), and latency was 32.8 +/- 2.3 ms. With magnetic stimulation, the tc-MEP amplitude was 287 microV (range 64-506 microV) (P < 0.05), and the latency was 34.7 +/- 2.1 ms (P < 0.05). We conclude that myogenic responses to magnetic transcranial stimulation are more sensitive to anesthetic-induced motoneural depression compared with those elicited by electrical transcranial stimulation. IMPLICATIONS: Transcranial motor evoked potentials are used to monitor spinal cord integrity intraoperatively. We compared the relative efficacy of electrical and magnetic transcranial stimuli in anesthetized patients. It seems that myogenic responses to magnetic transcranial stimulation are more sensitive to anesthetic-induced motoneural depression compared with electrical transcranial stimulation.     

Noxious stimuli do not modify myogenic motor evoked potentials by electrical stimulation during anesthesia with propofol-based anesthesia

PURPOSE: To investigate whether motor evoked potentials (MEP) to transcranial electrical stimulation under constant blood propofol concentration are affected by the arousing effect of surgical noxious stimuli. METHODS: Twenty patients who underwent elective spinal surgery were studied. Patients were anesthetized with 50% nitrous oxide in oxygen, fentanyl, and propofol to maintain the bispectral index (BIS) score around 50. MEP in response to a multipulse transcranial electrical stimulation at stimulus sites of C3-C4 were recorded over the right abductor pollicis brevis muscle. Changes of peak-to-peak amplitude and onset latency of MEP, BIS score before and after surgical stimuli were evaluated. Propofol plasma concentration was measured at the same time points. RESULTS: Both MEP amplitude and latency did not change significantly after surgical stimuli although BIS increased significantly (48 +/- 6 to 58 +/- 5; P < 0.05). Plasma propofol concentration was maintained at the same level between the two measurement points (3.3 +/- 0.7 to 3.3 +/- 0.7 micro g*mL(-1)). There was no relation between BIS change and changes of MEP amplitude and latency, and propofol plasma concentration. CONCLUSION: MEP to the transcranial electrical stimulation under a constant and clinically appropriate blood propofol concentration are not affected by surgical noxious stimuli.     

Tetanic stimulation of the peripheral nerve before transcranial electrical stimulation can enlarge amplitudes of myogenic motor evoked potentials during general anesthesia with neuromuscular blockade

BACKGROUND: Neuromuscular blockade can suppress myogenic motor evoked potentials (MEPs). The authors hypothesized that tetanic stimulation (TS) of the peripheral nerve before transcranial stimulation may enhance myogenic MEPs during neuromuscular blockade. In the current study, the authors evaluated MEP augmentations by TS at different levels of duration, posttetanic interval, neuromuscular blockade, and stimulus intensity. METHODS: Thirty-two patients undergoing propofol-fentanyl-nitrous oxide anesthesia were examined. Train-of-five stimulation was delivered to C3-C4, and MEPs were recorded from the abductor hallucis muscle. In study 1, TS with a duration of 1, 3, or 5 s was delivered at 50 Hz to the tibial nerve 1, 3, or 5 s (interval) before transcranial stimulation, and the effects of TS on MEP amplitude were evaluated. In study 2, TS-induced MEP augmentations were evaluated at the neuromuscular blockade level (%T1) of 50% or 5%. In study 3, MEP augmentations by TS at stimulus intensities of 0, 5, 25, and 50 mA were evaluated. RESULTS: The application of TS significantly enlarged the amplitudes of MEPs at the combinations of duration (3, 5 s) and interval (1, 3, 5 s) compared with those without TS. TS-induced MEP augmentations were similarly observed at %T1 of both 50% and 5%. TS-induced MEP augmentations were observed at stimulus intensities of 25 and 50 mA. CONCLUSIONS: The results indicate that TS of the peripheral nerve before transcranial stimulation can enlarge the amplitude of MEPs during general anesthesia with neuromuscular blockade. TS of the peripheral nerve can be intraoperatively applied as a method to augment myogenic MEP responses.     

Effect of nitrous oxide on myogenic motor evoked potentials during hypothermia in rabbits anaesthetized with ketamine/fentanyl/propofol

Background. A number of authors have reported that anaestheticssuppress myogenic motor evoked potentials (MEPs). However, theinfluence of hypothermia on these effects is unknown. Thereforewe investigated the effects of hypothermia on nitrous oxide-inducedsuppression of myogenic MEPs. Methods. Twenty-two rabbits anaesthetized with ketamine, fentanyland propofol were randomly allocated to one of three groups,with oesophageal temperatures of 40°C (n=8), 35°C (n=7)and 30°C (n=7). Myogenic MEPs in response to electricalstimulation of the motor cortex with a train of five pulseswere recorded from the soleus muscle. Following the controlrecording, nitrous oxide was administered at concentrationsof 30%, 50%, and 70% in random order, and MEPs were recorded.Control MEP amplitudes and percentage of control MEP amplitudes(%MEP amplitude) during the administration of nitrous oxidewere compared between the three groups. Results. Control MEP amplitudes were similar between the threegroups. Nitrous oxide suppressed MEPs in a dose-dependent mannerin all groups. During the administration of nitrous oxide, %MEP amplitudes at 35°C and 30°C (hypothermia) were significantlylower than those at 40°C (normothermia). Conclusion. These results suggest that nitrous oxide-inducedsuppression of MEPs may be augmented during hypothermia. Br J Anaesth 2002; 88: 836–40     

急性脊髓损伤时经颅磁刺激运动诱发电位监测的实验研究

目的 通过观察经颅磁刺激运动诱发电位(TMS-MEP)在急性脊髓损伤时的表现及其与功能预后的关系,探讨TMS—MEP于临床手术时在脊髓运动功能监测中的应用。方法 32只家兔被随机分成4组,对照组了解麻醉及手术对实验的影响,另3组动物为脊髓损伤组,观察脊髓损伤后动物TMS—MEP和运动功能的变化情况以及脊髓的病理学改变。结果 TMS—MEP在脊髓损伤后有不同程度的恶化但并未完全消失的动物,大部分动物的运动功能能基本恢复正常;脊髓损伤后TMS-MEP完全消失但其后1h内能有不同程度恢复的动物,其运动功能可以大部保存;对于TMS-MEP在脊髓损伤后完全消失而且后来一直未见有恢复迹象的动物,其运动功能的恢复大多较差。结论 TMS—MEP监测对脊髓的损伤非常敏感,伤时TMS-MEP的表现与其后运动功能的恢复有着很好的相关性,可以有效地应用于临床术中监测。     

Facial nerve motor evoked potentials elicited by transcranial electrical stimulation for intraoperative monitoring

OBJECTIVE: To determine whether monitoring facial nerve motor evoked potentials (FNMEPs) elicited by transcranial electrical stimulation during skull base tumor surgery is useful for predicting facial nerve outcome. METHODS: We analyzed FNMEP findings in 31 patients with skull base tumor. Surgery was performed twice in 2 of the 31 patients because of tumor regrowth. FNMEP monitoring was conducted 33 times in the present study. Corkscrew electrodes positioned at C3 or C4 and Cz were used to deliver supramaximal stimuli (140-550V). FNMEPs were recorded from the orbicularis oculi and oris muscles. Correlation between the final-to-baseline FNMEP ratio and postoperative facial nerve function (House & Brackmann grade) was examined. RESULTS: Valid FNMEPs were obtained in 26 of the 33 (78.8%) recordings from the orbicularis oculi muscle and in 31 of the 33 (93.9%) recordings from the orbicularis oris muscle. Facial nerve function correlated significantly with the FNMEP ratios in the orbicularis oculi (r = -0.52 N=26, p < 0.01) and orbicularis oris (r = -0.60, N=31, p < 0.001) muscles. An FNMEP ratio of -50% consistently predicted immediate postoperative facial palsy, although the degree of palsy differed among the patients. CONCLUSIONS: Intraoperative FNMEP monitoring is useful for predicting facial nerve function after skull base surgery.     

Influence of propofol concentrations on multipulse transcranial motor evoked potentials

Background. Motor evoked potentials can be affected by propofolanaesthesia. We studied how increasing target concentrationsof propofol altered transcranial motor evoked potentials (tcMEP)during scoliosis surgery. Methods. Fifteen patients undergoing surgery for scoliosis wereanaesthetized with remifentanil and propofol without nitrousoxide or neuromuscular blocking agents (BIS<60). tcMEP wereelicited by transcranial electric multipulse stimulation ofthe motor cortex and recording of compound action potentialsfrom the anterior tibialis muscle. tcMEP were obtained beforesurgery with propofol target values set from 4 to 8 mg litre^–1,and then during surgery. Arterial propofol concentrations weremeasured for each tcMEP recording. Results. Before surgery, increasing propofol reduced tcMEP amplitudein a dose-dependent manner, with no effect on latency. Duringsurgery, at equivalent propofol concentrations, tcMEP were notstatistically different from those obtained before surgery.In all except one patient, tcMEP signals were present duringthe entire procedure. In this patient the loss of tcMEP wasunfortunately related to an anterior spinal cord lesion, whichwas confirmed by a wake-up test. Conclusion. We found that, although propofol had a dose-dependenteffect on tcMEP amplitude, anaesthesia could be maintained withremifentanil and propofol to allow recording and interpretationof tcMEP signals. Br J Anaesth 2003; 91: 493–7     

The effects of volatile anesthetics on intraoperative monitoring of myogenic motor-evoked potentials to transcranial electrical stimulation and on partial neuromuscular blockade during propofol/fentanyl/nitrous oxide anesthesia in humans

The aim of the present study was to compare the influence of volatile anesthetics on transcranial motor-evoked potentials (tcMEP) in humans anesthetized with propofol/fentanyl/nitrous oxide and on partial neuromuscular blockade (NMB). The authors studied 35 ASA I and II patients who were undergoing elective craniotomy and brain tumor resection. The patients were randomized to one of three groups to receive halothane (HAL), isoflurane (ISO), or sevoflurane (SEV). Anesthetic depth was initially adjusted using the bispectral index to 40+/-5, and NMB was adjusted to 40%-50% of one twitch of train of four (T1) after recovery from intubation. MEPs with train of five square-wave pulses were elicited using screw electrodes placed in the skull over C3-C4. After craniotomy, the inhalational agent was introduced at 0.5 MAC and then 1.0 MAC (20 minutes each), and the effects on MEPs, NMB, and hemodynamic variables were studied. A decrease in BIS and systolic blood pressure was observed with all agents. Both SEV and ISO at 1.0 MAC significantly decreased train-of-four ratio from 38.4+/-18.1 at control to 19.0+/-9.7 and from 35.3+/-12.4 to 26.1+/-13.7, respectively (P<0.001), but not HAL at 1.0 MAC. The amplitudes of tcMEPs were significantly reduced by all agents at 1.0 MAC, with the effect being less in HAL at 0.5 MAC. We have shown that HAL had a lesser suppressive effect on MEPs than either ISO or SEV at 0.5 MAC, which was partially due to a lesser degree of NMB.     

Low concentrations of isoflurane abolish motor evoked responses to transcranial electrical stimulation during nitrous oxide/opioid anesthesia in humans.

To study the feasibility of noninvasive monitoring of motor pathways in anesthetized patients, we evaluated the effect of isoflurane on motor evoked responses to constant-voltage transcranial electrical stimulation (tce-MERs). Reproducible tce-MERs were recordable from the tibialis anterior muscle during nitrous oxide/opioid anesthesia in 11 patients. Before the introduction of isoflurane, tce-MER onset latency was 30.8 +/- 1.9 ms, and amplitude ranged from 19 microV to 2.6 mV (median, 209 microV). Operating conditions necessitated neuromuscular blockade in three patients before administration of isoflurane. In the remaining eight patients, introduction of isoflurane in low concentrations resulted in an immediate increase in the latency and a decrease in the amplitude of tce-MERs. The tce-MERs were completely obliterated in all subjects at end-tidal isoflurane concentrations between 0.2% and 0.6% (median, 0.24%). After discontinuation of isoflurane, the tce-MER returned in all patients. The authors conclude that, during nitrous oxide/opioid anesthesia, with the stimulus and recording variables used, isoflurane even at very low concentrations precludes recording of tce-MERs.     

Differential effects of nitrous oxide and propofol on myogenic transcranial motor evoked responses during sufentanil anaesthesia

We have compared the effects of 50% nitrous oxide and propofol, each administered concurrently with sufentanil, on the amplitudes and latencies of the compound muscle action potential (CMAP) response to transcranial electrical stimulation. Using a crossover design, 12 patients undergoing spinal surgery were exposed to both 50% nitrous oxide and propofol, the latter in a bolus-infusion regimen. Six patients received nitrous oxide first and six received propofol first. CMAP were recorded from the tibialis anterior muscle in response to both single and paired transcranial electrical stimuli. With single pulse stimulation, median CMAP amplitude was significantly greater during administration of nitrous oxide than propofol (nitrous oxide 335 (10th-90th percentiles 35-849) microV; propofol 36 (0-251) microV) (P < 0.01). With paired stimulation, there was no significant difference in CMAP amplitude during the two regimens (nitrous oxide 1031 (296-1939) microV; propofol 655 (0-1867) microV). The results indicate that propofol caused more depression of transcranial electrical motor evoked responses than 50% nitrous oxide but that the difference was probably clinically unimportant when a paired stimulation paradigm was used.      

脊柱手术中经颅电刺激运动诱发电位监护的应用探讨

目的探讨脊柱手术中经颅电刺激运动诱发电位（transcranial electrical stimulation motor evoked potential,TES-MEP）监护的可行性和应用价值。方法2006年7月至2008年10月,在241例胸椎手术中对双侧胫前肌、足踇短屈肌、大鱼际肌或小鱼际肌（颈椎病变时）实施TES-MEP监护。术中全静脉麻醉58例,静脉麻醉＋七氟烷吸入麻醉（浓度〈1％）67例,静脉麻醉＋小剂量肌松剂116例。结果TES-MEP的检出率为89．2％,虽然3种麻醉方式的检出率无显著性差异,但各年龄组、不同靶肌肌力的检出率有显著性差异。术中TES-MEP阳性26例,其中不明原因的假阳性6例,真阳性20例,且均与手术操作有直接相关性。TES-MEP对脊髓运动功能监护的灵敏度为100％,特异度为97．9％,约登指数为0．979;对脊髓感觉功能监护的灵敏度为74．1％,特异度为97．9％,约登指数为0．72。结论异丙芬静脉麻醉＋七氟烷吸入麻醉（浓度〈1％）为首选方案,异丙芬静脉麻醉＋小剂量肌松剂为次选方案。TES-MEP不但能瞬间、直接、准确地监护脊髓的运动传导功能,而且能间接反映脊髓的感觉传导功能,是安全监护脊柱手术的新方法。     

The effect of hypothermia on myogenic motor-evoked potentials to electrical stimulation with a single pulse and a train of pulses under propofol/ketamine/fentanyl anesthesia in rabbits

In the present study, we investigated the effect of hypothermia on myogenic motor-evoked potentials (MEPs) in rabbits. The influence of stimulation paradigms to induce MEPs was evaluated. Twelve rabbits anesthetized with ketamine, fentanyl, and propofol were used for the study. Myogenic MEPs in response to electrical stimulation of the motor cortex with a single pulse and a train of three and five pulses were recorded from the soleus muscle. After the control recording of MEPs at 38 degrees C of esophageal temperature, the rabbits were cooled by surface cooling. Esophageal temperature was maintained at 35 degrees C, 32 degrees C, 30 degrees C, and 28 degrees C, and MEPs were recorded at each point. MEP amplitude to single- pulse stimulation was significantly reduced with a re-duction of core temperature to 28 degrees C compared with the control value at 38 degrees C (0.8 +/- 0.4 mV versus 2.3 +/- 0.3 mV; P < 0.05), whereas MEP amplitude to train-pulse stimulation did not change significantly during the cooling. MEP latency was increased linearly with a reduction of core temperature regardless of stimulation paradigms. In conclusion, these results indicate that a reduction of core temperature to 28 degrees C did not influence MEP amplitudes as long as a train of pulses, but not a single pulse, was used for stimulation in rabbits under propofol/ketamine/fentanyl anesthesia. IMPLICATIONS: Intraoperative monitoring of myogenic motor-evoked potentials (MEPs) may be required under hypothermic conditions because of its neuroprotective efficacy. However, data on the influence of hypothermia on myogenic MEPs are limited. The results indicate that multipulse stimulation may be better than single-pulse stimulation when monitoring MEPs during hypothermia.     

术中经颅电刺激运动诱发电位监测在术前脊髓损伤患者中的应用

背景:经颅电刺激运动诱发电位(TceMEP)监测已经广泛应用于脊柱外科手术中,但是到目前为止,针对术前脊髓损伤(POSD)患者的神经监测有效性还存有争议。目的:探讨POSD患者术中TceMEP监测的可行性,并且分析Tce MEP信号快速消失的临床意义。方法:共收集2014年1月至2018年1月349例POSD患者的术中神经监测数据,其中332例患者获得了有效的术中TceMEP基线,17例神经功能障碍较为严重者(11例瘫痪和6例不完全截瘫)未获得。332例中,27例术中TceMEP信号显著消失,收集其临床特征,并严格随访术后3个月的神经功能变化。结果:在POSD患者中,TceMEP基线获得成功率为95.1%(332/349);27例患者出现了明显的TceMEP信号消失,其中23例为真阳性,4例为假阳性。其中21例患者术后出现新发脊髓损伤(13例为暂时性脊髓损伤,8例为永久性脊髓损伤)。预测脊髓损伤的术中TceMEP监测敏感度为100%,特异度为98.7%,阳性预测值为85.2%,阴性预测值为100%。与颈椎管狭窄症相比,先天性脊柱后凸、结核性脊柱后凸和胸椎管狭窄是TceMEP信号消失的高危诊断。结论:术中TceMEP监测对大多数POSD来说是可行的。在高危诊断和复杂的外科手术过程中,快速的TceMEP信号消失可能预示着新的脊髓损伤。     

The influence of nitrous oxide to supplement fentanyl/low-dose propofol anesthesia on transcranial myogenic motor-evoked potentials during thoracic aortic surgery

OBJECTIVE: Intraoperative monitoring of myogenic motor evoked potentials to transcranial electrical stimulation (tc MEPs) is a new method to assess the integrity of the motor pathways. The authors studied the effects of 50% nitrous oxide (N2O) and a low-dose propofol infusion on tc MEPs paired electrical stimulation during fentanyl anesthesia with partial neuromuscular blockade. DESIGN: Cross-over study. SETTING: St Antonius Hospital, Nieuwegein, The Netherlands. PARTICIPANTS: Ten patients scheduled to undergo surgery on the thoracoabdominal aorta were studied; 6 women aged 54 to 69 years and 4 men aged 68 to 77 years. INTERVENTIONS: After achieving a stable anesthetic state and before surgery, tc MEPs were recorded during four 15-minute periods: (I) air/oxygen (O2; F(I)O2 = 50%); propofol target blood concentration, 0.5 microg/mL; (II) N2O/O2 (F(I)O2 = 50%); propofol target blood concentration, 0.5 microg/mL; (III) N2O/O2 (F(I)O2 = 50%; propofol target blood concentration, 1.0 microg/mL; and (IV) air/O2 (F(I)O2 = 50%); propofol target blood concentration, 1.0 microg/mL. MEASUREMENTS AND MAIN RESULTS: Tc MEPs were recorded from the right extensor digitorum communis muscle and the right tibialis anterior muscle. The right thenar muscle was used for recording the level of relaxation; the T1 response was maintained at 40% to 70% of the control compound muscle action potential. There was no significant difference in onset latency among the four phases. The addition of N2O and doubling the target propofol infusion to 1.0 microg/mL resulted in a 40% to 50% reduction of tc MEP amplitude recorded in the extensor digitorum communis muscle and tibialis anterior muscle (p < 0.01). During each phase, tc MEPs could be elicited and interpreted, except in one patient, in whom no tc MEPs could be elicited in the leg because of technical problems. CONCLUSION: The data indicate that tc MEP monitoring is feasible during low-dose propofol, fentanyl/50% N2O in 02 anesthesia and partial neuromuscular blockade.     

Isoflurane-induced attenuation of motor evoked potentials caused by electrical motor cortex stimulation during surgery

Dysfunction of spinal motor conduction during surgical procedures may not be reflected by changes in somatosensory evoked potential waveforms. A method of monitoring that allows direct and continuous assessment of motor function within the central nervous system during surgery would be useful. This paper describes one such method utilizing noninvasive electric cortical stimulation to evoke muscle activity (the motor evoked potential, or MEP) during surgery. The effect of isoflurane (superimposed on a baseline of N2O/narcotic anesthesia) on MEP's in response to cortical stimulation is specifically examined. Eight patients undergoing elective neurosurgical operations were included in the study. All patients received a background of general anesthesia and partial nondepolarizing neuromuscular blockade. The motor cortex was stimulated electrically via self-adhesive scalp electrodes. Electromyographic responses from multiple muscles were measured with subdermal electroencephalograph-type needle electrodes. Motor responses to stimulation were continually recorded on magnetic tape for off-line analysis. Once closing of the surgical incision was begun, a series of four to five stimuli of constant magnitude were applied to obtain "baseline" MEP responses. Patients were then ventilated with isoflurane for up to 8 minutes, during which time stimuli were continued every 15 to 20 seconds. Comparison was made of MEP responses for trials before, 1 minute after, and 5 minutes after the addition of isoflurane. All patients demonstrated reproducible motor responses to cortical stimulation during surgery. Addition of isoflurane [isoflurane)exp, less than or equal to 0.5%) to pre-existing anesthesia caused marked attenuation of MEP amplitudes in all patients within 5 minutes of its application, without affecting neuromuscular transmission as judged by direct peripheral nerve stimulation. It is concluded that: 1) monitoring motor system integrity and function with electric transcranial cortical stimulation during surgery is feasible when utilizing an N2O/narcotic anesthetic protocol; and 2) the quality of data obtained will likely suffer with the addition of isoflurane.     

Measurement of motor evoked potentials following repetitive magnetic motor cortex stimulation during isoflurane or propofol anaesthesia

Background. Isoflurane and propofol reduce the recordabilityof compound muscle action potentials (CMAP) following singletranscranial magnetic stimulation of the motor cortex (sTCMS).Repetition of the magnetic stimulus (repetitive transcranialmagnetic stimulation, rTCMS) might allow the inhibition causedby anaesthesia with isoflurane or propofol to be overcome. Methods. We applied rTCMS (four stimuli; inter-stimulus intervalsof 3, 4, 5 ms (333, 250, 200 Hz), output 2.5 Tesla) in 27 patientsand recorded CMAP from the hypothenar and anterior tibial muscle.Anaesthesia was maintained with fentanyl 0.5–1 µgkg^–1 h^–1 and either isoflurane 1.2% (10 patients)or propofol 5 mg kg^–1 h^–1 with nitrous oxide 60%in oxygen (17 patients). Results. No CMAP were detected during isoflurane anaesthesia.During propofol anaesthesia 333 Hz, four-pulse magnetic stimulationevoked CMAP in the hypothenar muscle in 75%, and in the anteriortibial muscle in 65% of the patients. Less response was obtainedwith 250 and 200 Hz stimulation. Conclusions. In most patients, rTCMS can overcome suppressionof CMAP during propofol/nitrous oxide anaesthesia, but not duringisoflurane anaesthesia. A train of four magnetic stimuli ata frequency of 333 Hz is most effective in evoking potentialsfrom the upper and lower limb muscles. The authors concludethat rTCMS can be used for evaluation of the descending motorpathways during anaesthesia. Br J Anaesth 2003; 91: 487–92     

Intraoperative transcranial electrical motor evoked potential monitoring during spinal surgery under intravenous ketamine or etomidate anaesthesia

Summary Motor evoked potentials (MEPs), monitoring the motor function directly, are superior to somatosensory evoked potentials (SSEPs) in monitoring the motor system during spinal surgery. Reliable MEPs are difficult to elicit under normal anaesthesia. Using intravenous anaesthesia with either ketamine or etomidate infusion, we performed intraoperative MEP monitoring in 12 spinal operations for 11 cases from February 1992 to May 1992. For anaesthesia, ketamine was used in 5, etomidate in 7, fentanyl was supplemented in all, muscle relaxation at 30% to 50% of pre-anaesthetic muscle power was maintained with atracurium or vencuronium infusion. Transcranial bipolar electrical stimulation was used to induce MEPs. Concomitant SSEP monitoring was performed in 3. No significant anaesthesia related side effects were noted except one episode of unpleasant dream occurred in the ketamine anaesthesia group.Successful monitoring was achieved in 10 sessions. In 5 of which warning to the surgeons was made due to sudden MEP deterioration, which recovered followed by definite management in four and persisted in one. In the other 5 sessions, no warning was made due to stationary or gradual change in MEPs. Bilateral two-channel recordings were used in 3 sessions. In 2 of which unilateral transient change was noted. Loss of SSEPs was noted in one despite unchanged MEPs, in whom only new sensory deficits occurred postoperatively. Compared to the baseline MEPs in terms of latency and amplitude, the final MEPs improved in 5 sessions, did not change significantly in 4 sessions, deteriorated in one session, and were correlated well with the immediate postoperative motor status.In our small series, the intraoperative MEP monitoring showed neither false negative nor false positive result. It is concluded that the intraoperative MEP monitoring is feasible under intravenous ketamine or etomidate anaesthesia and valuable in spinal surgery.     

Comparison of transcranial motor evoked potentials and somatosensory evoked potentials during thoracoabdominal aortic aneurysm repair

OBJECTIVE: To compare transcranial motor evoked potentials (tc-MEPs) and somatosensory evoked potentials (SSEPs) as indicators of spinal cord function during thoracoabdominal aortic aneurysm repair. SUMMARY BACKGROUND DATA: Somatosensory evoked potentials reflect conduction in dorsal columns. tc-MEPs represent anterior horn motor neuron function. This is the first study to compare the techniques directly during thoracoabdominal aortic aneurysm repair. METHODS: In 38 patients, thoracoabdominal aortic aneurysm repair (type I, n = 10, type II, n = 14, type III, n = 6, type IV, n = 8) was performed using left heart bypass and segmental artery reimplantation. tc-MEP amplitudes <25% and SSEP amplitudes <50% and/or latencies >110% were considered indicators of cord ischemia. The authors compared the response of both methods to interventions and correlated the responses at the end of surgery to neurologic outcomes. RESULTS: Ischemic tc-MEP changes occurred in 18/38 patients and could be restored by segmental artery reperfusion (n = 12) or by increasing blood pressure (n = 6). Significant SSEP changes accompanied these tc-MEP events in only 5/18 patients, with a delay of 2 to 34 minutes. SSEPs recovered in only two patients. In another 11 patients, SSEP amplitudes fell progressively to <50% of control without parallel tc-MEP changes or association with cross-clamp events or pressure decreases. At the end of the procedure, tc-MEP amplitudes were 84 +/- 46% of control. In contrast, SSEP amplitudes were <50% of control in 15 patients (39%). No paraplegia occurred. CONCLUSION: In all patients, tc-MEP events could be corrected by applying protective strategies. No patient awoke paraplegic. SSEPs showed delayed ischemia detection and a high rate of false-positive results.