Transcranial electric stimulation (TES) motor evoked potential (MEP) monitoring is standard during surgery risking motor system injury. The stimuli are typically 5-pulse trains with a 4 ms interstimulus interval (ISI). The pulse duration (D) is often set to 50 or 500 µs. Both are effective, but setting D to the chronaxie would be physiologically optimal and limited data suggest that mean MEP chronaxie may be near 200 µs.

When necessary, one can obtain larger MEPs by increasing current (I) or D to increase stimulus charge (Q = I × D). However, this also increases patient movement that can interfere with surgery and reduce MEP acquisition frequency.

The main research question is whether increasing current or pulse duration when applying intraoperative neuromonitoring produces less patient movement during surgery. As such, the IOM ISIS System will be employed for neuromonitoring and an accelerometer will be used to quantify patient movement.

The constant-current TES stimulators will be used in this study with a high-precision oscilloscope. Total intravenous anesthesia (TIVA), surgery and TES MEP monitoring will proceed routinely without modification and normally involves acquiring many MEPs over several hours. The only departure from standard care will be the placement of two small accelerometers and a brief MEP sequence before skin incision to determine chronaxie and compare the effect of an equivalent increase of I or D on MEP amplitude and movement.

Transcranial electric stimulation (TES) motor evoked potential (MEP) monitoring is standard during surgery risking motor system injury. The stimuli are typically 5-pulse trains with a 4 ms interstimulus interval (ISI). The pulse duration (D) is often set to 50 or 500 µs. Both are effective, but setting D to the chronaxie would be physiologically optimal and limited data suggest that mean MEP chronaxie may be near 200 µs.

When necessary, one can obtain larger MEPs by increasing current (I) or D to increase stimulus charge (Q = I × D). However, this also increases patient movement that can interfere with surgery and reduce MEP acquisition frequency. Anecdotal observations suggest that increasing D may produce larger MEPs with less movement than increasing I, but there is no published support. If true, then the optimal D for monitoring may be above the chronaxie because less movement could facilitate more frequent MEP acquisition.

Finally, there is ongoing controversy about constant-current or constant-voltage TES. However, with stable resistance (R) there is no fundamental reason to prefer one or the other since they are related by Ohm's law I = V/R. Also, since threshold current and voltage vary with D their maximum levels should also vary with selected D, which could be confusing. Instead, because the International Electrotechnical Commission 50 mJ safety limit is based on energy (E = I2 × D × R = V2 × D/R), it may be more logical to apply constant-energy TES that would provide a consistent 0-50 mJ selection range at any selected D.

The primary objective is to assess whether intraoperative neuromonitoring with constant current or constant voltage produces less patient movement during surgery. The secondary objectives are to compare MEP amplitudes with the different setups, to estimate the true mean or median chronaxie and to develop the concept of constant-energy TES.

The main research question is whether increasing current or pulse duration when applying intraoperative neuromonitoring produces less patient movement during surgery. As such, the IOM ISIS System will be employed for neuromonitoring and an accelerometer will be used to quantify patient movement.

The constant-current TES stimulators will be used in this study with a high-precision oscilloscope. The calibration will assess 5-pulse trains with a 4 ms ISI and 100 mA output across a 1000 Ω resistor at 250, 500, and 1000 µs D. Measurements will include actual I and D of each pulse, and actual ISI.

Total intravenous anesthesia (TIVA), surgery and TES MEP monitoring will proceed routinely without modification and normally involves acquiring many MEPs over several hours. The only departure from standard care will be the placement of two small accelerometers and a brief MEP sequence before skin incision to determine chronaxie and compare the effect of an equivalent increase of I or D on MEP amplitude and movement.

Inclusion Criteria:

• Informed Consent signed by the subject
• The patient has a supra- or infra-tentorial lesion requiring surgery
• The patient is undergoing neurosurgery with the use of Intraoperative Monitoring (IOM) during surgery to protect functional tissue
• The patient is older than 18 years

Exclusion Criteria:

• No need for Intraoperative Monitoring (IOM)
• Vulnerable subjects (pregnant women, pregnant, impaired consciousness)
• People who do not want to participate in the study
• Emergency procedures in which no consent was obtained before the operation
• Multiple surgeries on the same patient
• Preoperative non-affected arm motor deficit (MRC <5), that is to say, no motor deficit of the arm ipsilateral to the surgery
• Inhalational anesthesia
• Persisting neuromuscular blockade