Ulrich Pechstein

Transcranial High-frequency Repetitive Electrical Stimulation for Recording Myogenic Motor Evoked Potentials with the Patient under General Anesthesia

OBJECTIVE To demonstrate the feasibility of transcranial high-frequency electrical stimulation with the patient under general anesthesia with propofol and alfentanil. This method may be a useful tool for intraoperative monitoring of the motor pathways during cerebral and spinal operations. METHODS A short train from one to eight monopolar anodal electrical pulses was applied transcranially to the motor cortex with a frequency from 100 to 500 Hz in 10 patients. Myogenic motor evoked potentials (MEPs) were recorded from forearm flexor muscles, thenar, and hypothenar. Amplitude and latency of MEPs were evaluated after different stimulation parameters. This combination of anesthetic and transcranial stimulation technique allows recording of myogenic MEPs during general anesthesia, which was found not to be possible with single pulse transcranial stimulation. RESULTS To elicit myogenic MEPs from the target muscles, stimulation had to be applied within the following parameters: minimum threshold intensity was 60 mA for forearm flexor and thenar and 80 mA for hypothenar; minimum number of pulses was two for forearm flexor muscles and thenar and three for hypothenar; minimum stimulation frequency was 200 Hz for thenar and hypothenar and 100 Hz for forearm flexor muscles. CONCLUSION Transcranial high-frequency repetitive electrical stimulation seems to be a new method for monitoring the motor tract. With this method, it is possible to monitor the motor function without interfering with the surgical team or with the surgical treatment of infratentorial and spinal lesions. Monitoring of relaxation is necessary to compare the amplitude of MEPs.

Motor Evoked Potential Monitoring with Supratentorial Surgery

OBJECTIVE To assess feasibility and clinical value of motor evoked potential (MEP) monitoring with surgery close to supratentorial motor areas and pathways. METHODS Functional mapping by somatosensory evoked potential phase reversal and continuous MEP recording after high-frequency repetitive electrical cortex stimulation was performed during 182 operations in 177 patients. Significant MEP changes were reported to trigger surgical reaction. Intraoperative surgical and electrophysiological findings were documented prospectively. Patient files were reviewed for clinical data. MEP monitoring results were correlated with motor outcome. RESULTS MEP recording was successful in 167 cases (91.8%). Inadequate electrode placement was an important reason for failed recording in the remaining patients, whereas preoperative paresis and anesthesia had no significant effect. Permanently disabling new motor deficit occurred in 8 cases (4.9%), whereas transient and nondisabling weakness was frequent (27.4%). Significant MEP changes occurred during 64 operations (39%). Irreversible MEP loss always predicted new, usually permanent, paresis. Unaltered MEP recordings indicated unimpaired motor function in the monitored muscle groups, except for rare transient deficit because of late edema and rebleeding. Irreversible MEP deterioration without loss and reversible changes could be associated with new paresis, which was transient in most patients. No major complications were observed, except for intraoperative generalized seizure in one epilepsy patient under insufficient anticonvulsant therapy. CONCLUSION MEP monitoring with supratentorial surgery is feasible and safe. It may help to maximize resection within the limits of preserved motor function. Further evidence is needed to confirm these results.

Isoflurane plus nitrous oxide versus propofol for recording of motor evoked potentials after high frequency repetitive electrical stimulation

The goal of this study was to test the influence of two widespread techniques of general anesthesia on motor evoked potentials (MEP) in response to transcranial and direct cortical high frequency repetitive electrical stimulation. Total intravenous anesthesia (TIVA) based on propofol and alfentanil was examined in 17 patients (group A), and balanced anesthesia (BA), based on nitrous oxide, isoflurane and fentanyl, was studied in 13 patients (group B). Distinct motor responses were available in 15 of 17 patients (88%) of group A, and in one of 13 patients (8%) of group B. Amplitudes increased significantly with increasing stimulus intensity and number of pulses under conditions of TIVA. At the same time, latencies decreased significantly with increasing stimulus intensity and decreasing interstimulus interval, but not with increasing number of pulses. It is hypothesized that propofol suppresses corticospinal I-waves at the cortical level, resulting in a conduction block at the level of the alpha-motoneuron, and that this effect may be overcome by high frequency repetitive stimulation. In contrast, nitrous oxide and isoflurane seem to have an additional suppressive effect on corticospinal D-waves, which may be overcome by higher stimulation intensity. In conclusion, transcranial high frequency repetitive stimulation and TIVA provide a feasible setting for intraoperative MEP monitoring, while higher doses of nitrous oxide and isoflurane are not compatible with recording of muscular activity elicited by the stimulation technique as described.

Electrophysiological Considerations Regarding Electrical Stimulation of Motor Cortex and Brain Stem in Humans

OBJECTIVE To provide information about activation of descending motor pathways in humans, motor evoked potentials were obtained from 16 patients without any sensorimotor deficit, after both cortical and brain stem stimulation. METHOD Total anesthesia was achieved in all patients through intravenous administration. Short trains of one to five electrical pulses were delivered separately to the motor cortex and the brain stem. Compound muscle action potentials were recorded from the contralateral upper extremity. Threshold intensity, stimulus polarity, latencies, and effect of increased stimulus intensity on latencies were analyzed. RESULTS The threshold intensity was significantly lower when stimulating the brain stem than when stimulating the cortex. A monophasic anodal stimulus was better for cortical stimulation than for brain stem stimulation. Conversely, a monophasic cathodal stimulus was more effective for brain stem stimulation. The rate of unsuccessful stimulations was higher with brain stem stimulation and with increased stimulation intensity. The variability of latencies was so high that a calculation of the conduction velocity of the motor pathways was not possible. CONCLUSION The results indicate that cortical surface and brain stem stimulation act on different nervous elements. Because of the condensation of motor pathway fibers at the brain stem level, much less stimulus intensity for eliciting compound muscle action potentials was necessary. On the other hand, the higher rate of unsuccessful brain stem stimulations may be caused by a block of conduction at either the anterior horn cell pool or the neuromuscular junction. Thus, for cortical and for brain stem stimulation, different stimulating parameters seemed to be necessary with the patient under general anesthesia.

Surgical management of brain-stem cavernomas

We present a series of seven patients who were operated on for symptomatic brain-stem cavernomas. The following approaches were used: medial suboccipital (N = 4), lateral suboccipital (N = 1), subtemporal-transtentorial (N = 1), and frontal transcortical-transventricular-subchorioidaltrans velum interpositum (N = 1). Intraoperative motor (N = 4) and somatosensory (N = 1) evoked potential monitoring revealed temporary changes in 3 patients. Immediately postoperatively, the following additional deficits were observed in 6 patients: oculomotor nerve paresis (N = 2), abducens nerve paresis (N = 3), facial nerve paresis (N = 2), deafness (N = 1), and increased ataxia (N = 3). One patient died due to septic complications not related to surgery. After a mean observation time of 2 years, 2 patients had improved, 3 were unchanged, and 1 patient deteriorated as compared to his preoperative status. In conclusion, surgical treatment of brain-stem cavernomas, although carrying a significant risk of temporary neurological deterioration is recommended in symptomatic patients in whom the cavernoma seems to reach the surface of the brain-stem. Intraoperative functional topographic mapping and monitoring have proven useful tools lowering the surgical risks in these patients.

Influence of nitrous oxide on motor-evoked potentials.

Study Design. Rabbits were used as an experimental model in the study of motor-evoked potentials. Objectives. To evaluate the effect of nitrous oxide on motor-evoked potentials while monitoring direct muscle and spinal cord responses. Summary of Background Data. Motor-evoked potential monitoring provides a promising tool for intraoperative assessment of descending pathways function. However, to date, this technique is still at an experimental stage, since its routine use is mainly limited because of intraoperative recording difficulties caused by the influence of anesthesia. Methods. Eight male rabbits weighing between 3000 g and 3500 g were studied. Motor-evoked potentials were recorded from the extremity muscles and from the epidural space of the thoracic cord in response to electrical stimulation of the motor cortex at baseline conditions and at increasing nitrous oxide concentrations (10-70 vol%). Results. The authors found a major suppressive effect of high nitrous oxide concentrations on the electromyographic responses. With 50 vol% nitrous oxide, electromyographic amplitudes were suppressed to 46% (fore leg) and 14% (hind leg) of the baseline values, whereas latencies did not change significantly. In contrast to muscular activity, spinal evoked responses representing neural activity were not affected by any concentration of nitrous oxide. Conclusions. Intraoperative monitoring of descending pathways by means of motor-evoked potentials during anesthesia of the rabbits based on nitrous oxide is feasible when neural activity is evaluated. Higher doses of nitrous oxide, however, are not compatible with recording of muscular activity.

Influence of fentanyl, alfentanil, and sufentanil on motor evoked potentials.

The effects of the opioids fentanyl, alfentanil, and sufentanil on motor pathways were studied in a total of 30 rabbits. Compound muscle action potentials (CMAP) were recorded from the extensor muscles of the upper extremity as well as evoked spinal cord potentials (ESCP) from the thoracic epidural space in response to electrical stimulation of the motor cortex. After establishing stable baseline values, an equipotent intravenous bolus of one of the three opioids was applied that abolished reflex motor response to noxious stimulation. Motor evoked potentials (MEP) were recorded from the time of bolus administration until recovery of MEP amplitudes and latencies. Afterwards, the opioids were administered continuously with cumulative dosage up to total absence of motor evoked response. Our results show a dose-dependent suppression of the CMAP: When reflex movement to noxious stimulation was extinguished, we found a significant (P < .001) reduction of the amplitudes to 34+/-18% (mean +/- SD) in the fentanyl group, to 43+/-24% in the alfentanil group, and to 53+/-20% of baseline values in the sufentanil group. Increasing opioid plasma levels were associated with complete extinction of the CMAP. We hypothesize that the descending volleys within motor pathways are mainly inhibited at a spinal level, because ESCP, particularly the number of spinal I-waves, are not severely affected even when CMAP are completely suppressed. In conclusion, intraoperative monitoring of descending pathways by means of MEP during anesthesia with opioids is feasible at anesthetic plasma concentrations maintaining a surgical level of analgesia. Even with high opioid plasma levels, a valid MEP monitoring could be performed evaluating neural activity of spinal MEP.

Intraoperative SEP monitoring in aneurysm surgery

Somatosensory evoked potentials (SEP) were monitored during surgical treatment of 282 aneurysms in 226 patients. Significant SEP changes were found in 32 aneurysms (11.3%) mainly referring to accidental or intentional vessel occlusion. Response to these changes included reapplication of aneurysm clips, repositioning of retractors, or removal of temporary clips as followed in 23 cases (8.1%). Stable SEP signals allowed the surgeon to proceed with the surgical course and coincided in all but 2 cases (0.7%) with an uneventful outcome. Despite the limitations of SEP monitoring in certain anatomical locations, it has been found to be helpful in the operative management of some cases such as multilobed aneurysms, giant aneurysms, trapping procedures, and procedures requiring temporary vessel occlusion.