Motor Evoked Potentials Double Train Stimulation: Optimal Number of Pulses per Train.

PURPOSE: In neurophysiologic intraoperative monitoring, double train transcranial electrical stimulation (dt-TES) for motor evoked potentials (MEP) consists of an initial, facilitating train of stimuli followed by a second, testing, train. The optimal number of pulses per train has not been determined. The authors wanted to determine the optimal combination of pulses per train that produced the highest MEP amplitude. METHODS: Dt-TES of 4 + 4, 2 + 7, and 7 + 2 pulses were attempted in 20 patients and compared with single train TES of nine pulses. Latencies of the MEP responses were also compared. Additionally, statistical comparisons were made of the differences between the amplitude and latency measurements within the patients. RESULTS: The mean age of the patients was 44.3 years (range, 9-84 years). Dt-TES of 2 + 7 pulses resulted in significantly higher mean amplitudes than dt-TES of 4 + 4 and 7 + 2 pulses and single train TES of nine pulses ( p < 0.001). The MEP latency with dt-TES of 2 + 7 pulses was longer than the latencies of single train TES of nine pulses and dt-TES of 4 + 4 and 7 + 2 pulses. CONCLUSIONS: Asymmetric dt-TES with 2 + 7 pulses improved MEP amplitudes. This type of stimulation may be preferable to single train TES and dt-TES with symmetric pulse trains.

"Quadripolar" Transcranial Electrical Stimulation for Motor Evoked Potentials.

PURPOSE: To determine if transcranial electrical stimulation (TES)-induced motor evoked potentials (MEPs) are of higher amplitude when using two electrodes as anodes and two as cathodes, known as "quadripolar stimulation." METHODS: Patients who underwent TES MEP monitoring in which control, bipolar stimulation and four variations of quadripolar stimulation were used were evaluated. The bipolar stimulation montage was C3-C4 (C3 was used as anode for stimulation first, then the polarity was switched to stimulate the contralateral side). Four quadripolar montages were used: C3/C1-C4/C2 (step 1), M3/M1-M4/M2 (step 2), C3/M1-C4/M2 (step 3), and M3/C1-M4/C2 (step 4). The area under the curve for the right foot TES MEP was compared for the various montages using descriptive statistics and Fisher exact test for proportions. RESULTS: Sixteen patients were retrospectively evaluated. The mean age as 51.6 years, range 4 to 80 years; 11 were female. The transcranial electrical stimulation MEP area under the curve for the right foot MEP was highest in the bipolar montage in 1 of 16 patients (6.3%). Meanwhile, it was highest in step 4 (M3/C1-M4/C2) in 9 of 16 patients (56.3%; P = 0.027). The highest right foot MEP area under the curve with one of the quadripolar montages was seen in 15 of 16 patients (93.8%; P = 0.0001). CONCLUSIONS: Quadripolar stimulation resulted in higher area under the curve for right foot MEP compared with conventional bipolar stimulation.

Lower Extremity Somatosensory Evoked Potential P37 Waveform Optimization.

Somatosensory evoked potentials (SEPs) using tibial nerve stimulation are used during neurophysiologic intraoperative monitoring (NIOM). These SEPs produce a P37 waveform that is recorded from scalp electrodes. In this study, we attempted to determine the best derivation for recording the P37 waveform. Surgical cases using tibial nerve SEP NIOM were reviewed. Only cases in which the P37 was recorded using all of the following derivations were analyzed: centroparietal ipsilateral-centroparietal contralateral (CPi-CPc), centroparietal midline-frontopolar midline (CPz-Fpz), and CPz-CPc. The amplitude of the P37 waveform was measured in each derivation. Descriptive statistics were obtained for the P37 waveform amplitude. The mean amplitude in each of the derivations was compared using a chi-square test. Data from 39 patients (78 lower limbs) were available for analysis. The mean age of the patients was 49.64 years (range: 4-87 years); 18 were female. The highest amplitude P37 waveform was recorded from the CPz-Fpz derivation in 29 (35.4%) limbs, whereas the CPz-CPc and CPi-CPc derivations showed the highest amplitude in 29 (35.4%) and 20 (24.4%) limbs, respectively. The mean amplitudes were not significantly different between the various derivations. In only 10 (24.4%) of patients was the best derivation the same for both left and right limbs. A single best derivation was not found for recording the P37 waveform. Multiple derivations should be used to record cortical channels whenever possible. If the number of available channels is limited, using at least the CPz-Fpz and CPz-CPc derivations is recommended.

Magnesium Sulfate-Induced Motor Evoked Potential Changes.

Motor evoked potentials (MEPs) are commonly used during neurophysiologic intraoperative monitoring. Anesthetic, homeostatic, surgical, and technical factors can cause a change in MEPs. The authors describe a case in which a bolus of magnesium sulfate resulted in a loss of amplitude of MEPs. Responses returned to near baseline about 20 minutes later. The patient did not have any new postoperative deficits. When MEP changes occur, in addition to evaluating other causes, magnesium sulfate administration should be considered.