Conventional Motor Evoked Potentials Research Articles

Intraoperative motor-evoked potential with tetanic stimulation changes pre- and post-hemispherotomy.

Careful examination of motor-evoked potential (MEP) findings is critical to the safety of intraoperative neuromonitoring during neurosurgery. We reviewed the intraoperative MEP findings in a pediatric patient who had undergone hemispherotomy for refractory epilepsy. The patient was a 4-year-and-2-month-old boy with extensive right cerebral hemisphere, drug-resistant epilepsy, left upper and lower extremity paralysis, and cognitive impairment. We examined intraoperative MEP results both before and after hemispherotomy. Post-hemispherotomy and MEPs were successfully elicited through transcranial electrical stimulation (TES) but not via direct cortical stimulation on the right side. Furthermore, TES on the right side, following hemispherotomy, led to a reduction in the MEP amplification effect resulting from tetanic stimulation of the left unilateral median and tibial nerves. Conversely, we observed the effects of MEP amplification during TES on the left side after tetanic stimulation of these nerves. Postoperatively, the patient underwent magnetic resonance imaging and electroencephalogram examinations, confirming the anatomical and electrophysiological completeness of the dissection. Notably, the seizures disappeared, and no apparent complications were observed. Collectively, our findings suggest that TES can still activate deep structures and elicit MEPs, even in cases where the corticospinal connections to the posterior limb of the internal capsule are entirely severed. Thalamo-cortical interactions may affect the MEP amplification, observed during tetanic stimulation. Injury to the corticospinal tracts of the white matter may be obscured on conventional MEP findings; however, it may be identified by MEP changes in tetanic stimulation.

Effects of intraoperative motor evoked potential amplification following tetanic stimulation of the pudendal nerve in pediatric craniotomy.

Monitoring the intraoperative motor evoked potentials (MEPs) in pediatric craniotomy is challenging because of its low detection rate, which makes it unreliable. Tetanic stimulation of the peripheral nerves of the extremities and pudendal nerves prior to transcranial electrical stimulation (TES) or direct cortical stimulation (DCS) amplifies the MEPs. The authors investigated the effects of MEP amplification following tetanic stimulation of the median and tibial nerve or the pudendal nerve in pediatric patients undergoing craniotomy. This prospective observational study included 15 patients ≤ 15 years of age (mean age 8.9 ± 4.9 years) undergoing craniotomy. MEPs were obtained with TES (15 cases) or DCS (8 cases)-conventional MEP without tetanic stimulation (c-MEP) and MEP following tetanic stimulation of the unilateral median and tibial nerves (mt-MEP) or following tetanic stimulation of the pudendal nerve (p-MEP) were used. Compound muscle action potentials were elicited from the abductor pollicis brevis, gastrocnemius, tibialis anterior, and abductor hallucis longus muscles. The authors compared the identification rate and the rate of amplitude increase of each MEP. For both TES and DCS, the identification and amplitude increase rates were significantly higher in cases without preoperative hemiparesis for p-MEPs than in those for c-MEPs and mt-MEPs. In comparison to patients with preoperative hemiparesis, p-MEPs displayed a higher identification rate, with fewer false negatives in DCS cases. In pediatric craniotomy, the authors observed the amplification effect of MEPs with pudendal nerve tetanic stimulation and the amplification effect of DCS on MEPs without increasing false negatives. These findings suggested the likelihood of more reliable intraoperative MEP monitoring in pediatric cases.

Impaired phase synchronization of motor-evoked potentials reflects the degree of motor dysfunction in the lesioned human brain.

The functional corticospinal integrity (CSI) can be indexed by motor‐evoked potentials (MEP) following transcranial magnetic stimulation of the motor cortex. Glial brain tumors in motor‐eloquent areas are frequently disturbing CSI resulting in different degrees of motor dysfunction. However, this is unreliably mirrored by MEP characteristics. In 59 consecutive patients with diffuse glial tumors and 21 healthy controls (CTRL), we investigated the conventional MEP features, that is, resting motor threshold (RMT), amplitudes and latencies. In addition, frequency‐domain MEP features were analyzed to estimate the event‐related spectral perturbation (ERSP), and the induced phase synchronization by intertrial coherence (ITC). The clinical motor status was captured including the Medical Research Council Scale (MRCS), the Grooved Pegboard Test (GPT), and the intake of antiepileptic drugs (AED). Motor function was classified according to MRCS and GPT as no motor deficit (NMD), fine motor deficits (FMD) and gross motor deficits (GMD). CSI was assessed by diffusion‐tensor imaging (DTI). Motor competent subjects (CTRL and NMD) had similar ERSP and ITC values. The presence of a motor deficit (FMD and GMD) was associated with an impairment of high‐frequency ITC (150–300 Hz). GMD and damage to the CSI demonstrated an additional reduction of high‐frequency ERSP (150–300 Hz). GABAergic AED increased ERSP but not ITC. Notably, groups were indistinguishable based on conventional MEP features. Estimating MEP phase synchronization provides information about the corticospinal transmission after transcranial magnetic stimulation and reflects the degree of motor impairment that is not captured by conventional measures.

Tetanic stimulation of the pudendal nerve prior to transcranial electrical stimulation augments the amplitude of motor evoked potentials during pediatric neurosurgery.

Reportedly, tetanic stimulation prior to transcranial electrical stimulation (TES) facilitates elicitation of motor evoked potentials (MEPs) by a mechanism involving increased corticomotoneuronal excitability in response to somatosensory input. However, the posttetanic MEP following stimulation of a pure sensory nerve has never been reported. Furthermore, no previous reports have described posttetanic MEPs in pediatric patients. The aim of this study was to investigate the efficacy of posttetanic MEPs in pediatric neurosurgery patients and to compare the effects on posttetanic MEP after tetanic stimulation of the sensory branch of the pudendal nerve versus the standard median and tibial nerves, which contain a mixture of sensory and motor fibers. In 31 consecutive pediatric patients with a mean age of 6.0 ± 5.1 years who underwent lumbosacral surgery, MEPs were elicited by TES without tetanic stimulation (conventional MEPs [c-MEPs]) and following tetanic stimulation of the unilateral median and tibial nerves (mt-MEPs) and the sensory branch of the pudendal nerve (p-MEP). Compound muscle action potentials were elicited from abductor pollicis brevis (APB), gastrocnemius (Gc), tibialis anterior (TA), and adductor hallucis (AH) muscles. The success rate of monitoring each MEP and the increases in the ratios of mt-MEP and p-MEP to c-MEP were investigated. The success rate of monitoring p-MEPs was higher than those of mt-MEPs and c-MEPs (87.5%, 72.6%, and 63.3%, respectively; p < 0.01, adjusted by Bonferroni correction). The mean increase in the ratio of p-MEP to c-MEP for all muscles was significantly higher than that of mt-MEP to c-MEP (3.64 ± 4.03 vs 1.98 ± 2.23, p < 0.01). Subanalysis of individual muscles demonstrated significant differences in the increases in the ratios between p-MEP and mt-MEP in the APB bilaterally, as well as ipsilateral Gc, contralateral TA, and bilateral AH muscles. Tetanic stimulation prior to TES can augment the amplitude of MEPs during pediatric neurosurgery, the effect being larger with pudendal nerve stimulation than tetanic stimulation of the unilateral median and tibial nerves. TES elicitation of p-MEPs might be useful in pediatric patients in whom it is difficult to elicit c-MEPs.

Trending algorithm discriminates hemodynamic from injury related TcMEP amplitude loss.

Jasiukaitis and Lyon (J Clin Monit Comput, https://doi.org/10.1007/s10877-018-0181-9, 2018) described an motor evoked potential (MEP)amplitude trending system to detect MEP amplitude loss against a background of MEP variability. They found that the end of case value of a running R2 triggered by a set MEP amplitude loss criterion appeared to discriminate new injury from non-injury in a small sample of three patients. The present study examines the predictive capability of the running R2 in a larger sample of patients (21 injured and 19 non-injured). It also varies the amplitude loss criterion (50%, 65% and 80%) for triggering the running R2 and the numbers of points used in the moving linear regression (8, 12 and 16). 40 patients who had undergone correction for lumbar deformity were retrospectively examined. 21 of these woke up with a newly acquired radicular injury, 19 did not but were characterized by hypovolemic hemorrhage. All 40 patients had sufficient MEP amplitude loss sometime during their procedure to cause the monitoring specialist to report this to the surgeon and anesthesia. End-of-case running R2s were significantly larger in the injury group. Using an 80% amplitude loss criterion to trigger the running R2 proved to be too stringent, causing reduced sensitivity. The running R2 appeared to have equivalent sensitivity to that of conventional MEP amplitude loss ratios, but superior specificity within this monitoring challenged sample. The different number of points for the moving regressions did not have any significant effect. End-of-case R2 values greater than 60% appeared to be highly predictive of new post-operative deficit, while values less than 40% appeared to insure no new deficit. The proposed trending system can discriminate injury from non-injury outcomes when compressive radicular injury during correction for lumbar deformity is involved. This discrimination appears to be successful even when MEP amplitude loss for non-iatrogenic reasons (i.e., hemorrhage) is also occurring.

Effect of peripheral nerve tetanic stimulation on the inter-trial variability and accuracy of transcranial motor-evoked potential in brain surgery.

The aim of this study was to evaluate and compare the advantages of post-tetanic motor-evoked potential (p-MEP) and conventional motor-evoked potential (c-MEP) in terms of MEP inter-trial variability and accuracy. c-MEP and p-MEP were quantified in subjects who underwent brain surgery. c-MEP was generated by transcranial electrical stimulation (TES). p-MEP was generated using a preconditioning process involving tetanic stimulation at the left tibial nerve followed by TES. The presence of significant MEP deterioration was monitored during major surgical process. An additional 5-8 MEP obtained after major surgical process were used to analyze amplitude parameters such as mean, standard deviation, range, coefficient of variation (CV), and range to mean ratio. When only irreversible MEP deteriorations were considered as positive results, the false-positive rate was identical for p-MEP and c-MEP. When total MEP deteriorations were considered as positive results, the false-positive rate of p-MEP was lower and p-MEP had higher specificity than c-MEP. The mean amplitude of p-MEP was significantly higher than that of c-MEP. The CV and range to mean ratio of p-MEP were less than those of c-MEP. The p-MEP technique is useful for augmenting MEP amplitude and reducing inter-trial variability. p-MEP has clinical significance as a useful technique for intraoperative monitoring.

Effects of tetanic stimulation of peripheral nerve on intracranial direct electrical stimulation motor-evoked potentials in patients undergoing cerebral functional area operation

Objective To investigate the effects of tetanic stimulation of peripheral nerve on intracranial direct electrical stimulation motor-evoked potentials (MEP) in patients undergoing cerebral functional area operation. Methods Eight patients undergoing elective brain tumor resection under propofol-fentanyl anesthesia with partial neuromuscular blockade were enrolled in the study. Both conventional MEP (C-MEP) monitoring and posttetanic MEP (P-MEP) monitoring were performed throughout the operation for each patient, and the two groups of data were recorded. For one group, direct electrical stimulation with a train of five pulses was delivered to motor cortex and pyramidal tract, C-MEP was unilaterally recorded from the abductor pollicis brevis, and P-MEP was obtained 1 s after tetanic stimulation (frequency 50 Hz, intensity 50 mA, duration 5 s) to the ipsilateral tibial nerve.For the other group, direct electrical stimulation with a train of five pulses was delivered to motor cortex and pyramidal tract, C-MEP was unilaterally recorded from the tibialis anterior, and P-MEP was obtained 1 s after tetanic stimulation (frequency 50 Hz, intensity 50 mA and duration 5 s) to the contralateral tibial nerve. Randomized crossover method was used for C-MEP and P-MEP recording in each group, with an interval of 120 s. The adverse effects were observed. Results Amplitudes of P-MEP from the abductor pollicis brevis and tibialis anterior were significantly higher than those of C-MEP. Three patients had body movement during intraoperative cortex stimulation, while there was no awareness during operation and other electrical stimulation-related nervous system impairment and complications. Conclusion The application of tetanic stimulation of peripheral nerve before direct electrical stimulation can augnent the amplitudes of MEP from the abductor pollicis brevis and tibialis anterior in patients undergoing cerebral functional area operation. Key words: Electric stimulation; Evoked potentials,motor; Motor cortex; Pyramidal tracts

The Application of Tetanic Stimulation of the Unilateral Tibial Nerve Before Transcranial Stimulation Can Augment the Amplitudes of Myogenic Motor-Evoked Potentials from the Muscles in the Bilateral Upper and Lower Limbs

Recently, we reported a new technique to augment motor-evoked potentials (MEPs) under general anesthesia, posttetanic MEP (p-MEP), in which tetanic stimulation of the peripheral nerve before transcranial stimulation enlarged amplitudes of MEPs from the muscle innervated by the nerve subjected to tetanic stimulation. In the present study, we tested whether tetanic stimulation of the left tibial nerve can also augment amplitudes of MEPs from the muscles which are not innervated by the nerve subjected to tetanic stimulation. Thirty patients undergoing spinal surgery under propofol-fentanyl anesthesia with partial neuromuscular blockade were examined. For conventional MEP (c-MEP) recording, transcranial stimulation with train-of-five pulses was delivered to C3-4, and the compound muscle action potentials were bilaterally recorded from the abductor pollicis brevis, abductor hallucis (AH), tibialis anterior, and soleus muscles. For p-MEP recording, tetanic stimulation (50 Hz, 50 mA of stimulus intensity) with a duration of 5 s was applied to the left tibial nerve at the ankle 1 s before transcranial stimulation. Transcranial stimulation and recording of compound muscle action potentials were performed in the same manner as c-MEP recording. Amplitudes of c-MEP and p-MEP were compared using Wilcoxon's signed rank test. Amplitudes of p-MEPs from the left AH muscle innervated by the left tibial nerve with tetanic stimulation were significantly larger compared with those of c-MEPs. Amplitudes of p-MEPs from the bilateral abductor pollicis brevis and soleus muscles and right AH and tibialis anterior muscles, which were not innervated by the left tibial nerve with tetanic stimulation, were also significantly larger compared with those of c-MEPs. In patients under propofol and fentanyl anesthesia with partial neuromuscular blockade, the application of tetanic stimulation to the left tibial nerve augmented the amplitudes of MEPs from the muscles without tetanic nerve stimulation and those with stimulation.

Inhibitory conditioning stimulus in transcranial magnetic stimulation reduces the number of excited spinal motor neurons

Objective: To study the mechanisms of amplitude attenuation caused by a transcranial magnetic conditioning stimulus. Both conventional MEPs and the recently described triple stimulation technique (TST) were applied; the latter to improve the quantification of the response size decrease. Methods: TST uses a peripheral collision method to eliminate the effects of desynchronization of the transcranial magnetic stimulation (TMS) induced spinal motor neuron discharges. The attenuation of motor evoked potentials (MEPs) and responses to TST was studied in 10 healthy volunteers using the conditioning-test paradigm with 2 ms interstimulus intervals. Results: Conventional MEPs and responses to TST demonstrated a marked attenuation by the preceding conditioning stimulus in all subjects. The ratio of MEP to TST amplitudes was the same in conditioned and unconditioned responses. Conclusions: Our findings suggest that the transcranial conditioning stimulus does not change the degrees of desynchronization of spinal motor neuron discharges, but results in a reduced number of excited alpha motor neurons. This reduction can be estimated by both MEPs and TST.

A clinical study of motor evoked potentials using a triple stimulation technique.

Amplitudes of motor evoked potentials (MEPs) are usually much smaller than those of motor responses to maximal peripheral nerve stimulation, and show marked variation between normal subjects and from one stimulus to another. Consequently, amplitude measurements have low sensitivity to detect central motor conduction failures due to the broad range of normal values. Since these characteristics are mostly due to varying desynchronization of the descending action potentials, causing different degrees of phase cancellation, we applied the recently developed triple stimulation technique (TST) to study corticospinal conduction to 489 abductor digiti minimi muscles of 271 unselected patients referred for possible corticospinal dysfunction. The TST allows resynchronization of the MEP, and thereby a quantification of the proportion of motor units activated by the transcranial stimulus. TST results were compared with those of conventional MEPs. In 212 of 489 sides, abnormal TST responses suggested conduction failure of various degrees. By contrast, conventional MEPs detected conduction failures in only 77 of 489 sides. The TST was therefore 2.75 times more sensitive than conventional MEPs in disclosing corticospinal conduction failures. When the results of the TST and conventional MEPs were combined, 225 sides were abnormal: 145 sides showed central conduction failure, 13 sides central conduction slowing and 67 sides both conduction failure and slowing. It is concluded that the TST is a valuable addition to the study of MEPs, since it improves detection and gives quantitative information on central conduction failure, an abnormality which appears to be much more frequent than conduction slowing. This new technique will be useful in following the natural course and the benefit of treatments in disorders affecting central motor conduction.

Rapid cortical motor output map changes assessed by the triple stimulation technique.

The amplitudes of motor evoked potentials (MEPs) were mapped by transcranial magnetic stimulation (TMS) using the triple stimulation technique (TST) in 11 normal individuals. Stimuli were given while the subjects were (a) distracted, (b) concentrating on their target (recorded) hand, and (c) concentrating on their contralateral hand. Within seconds, the proportion of excited motor units increased, similarly in all subjects, by an average of 70% from (a) to (b), and by 48% from (a) to (c). At the optimal stimulation site, results obtained with the TST were compared to those of conventional MEPs. The TST proved superior in detecting the rapid changes of the motor output caused by the non-specific mental tasks studied.