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Updated: Aug 1, 2026
Non-invasive Assessment of Changes in Corticomotoneuronal Transmission in Humans
Published on: May 24, 2017
Does persistent crossover (ipsilateral) motor evoked potential (MEP) responses represent a technical failure for
Marshall F Wilkinson1, Kristine Pederson2, Philip Kawalec2
1Section of Neurosurgery Health Sciences Centre, GB 1 - 820 Sherbrook Street, Winnipeg, MB, R3A 1R9, Canada. mwilkinson@hsc.mb.ca.
Purpose:
Attention to motor evoked potential (MEP) stimulation intensity is necessary to avoid false negative MEP results during intracranial procedures. Observing ipsilateral (crossover) MEP responses has been hypothesized to indicate inappropriately strong stimulation intensity. We describe a case where persistent crossover MEP responses falsely suggested that stimulus intensity was too high and describe an alternative method to guide the selection of MEP stimulation intensity.
Methods:
A patient undergoing a suboccipital craniotomy for tumor resection had bilateral transcranial electrical MEP monitoring under total intravenous anesthesia. MEP results were obtained from left and right hand using C4-Cz and C3-Cz stimulation montages respectively. Selection of an appropriately superficial stimulus intensity was guided using MEP onset latencies.
Results:
MEP acquisition proceeded normally for contralateral left hand (C4-Cz montage). However, using the C3-Cz montage, persistent crossover responses were noted at stimulation intensities as low as threshold for contralateral right hand MEP (94 V/166 mA). Appropriate MEP stimulus intensity for subsequent monitoring (approximately 96 V/172 mA) was determined utilizing onset latency measurements from contralateral hand MEP responses. The stimulus intensity chosen was predicated on onset latency being ≥ 2 ms longer than latency at maximal stimulus level (shortest latency). A stimulus intensity-latency plot was generated offline to illustrate this important relationship for intracranial MEP use. MEP acquisition proceeded without incident and gross total resection was achieved without postoperative motor deficits.
Conclusion:
Despite crossover appearance contralateral hand MEP were quantitatively validated for intraoperative application using onset latency guidance.
Insights
Intracranial surgery requires careful motor evoked potential (MEP) stimulation intensity to prevent false negatives. Onset latency guidance, not crossover MEPs, reliably determined appropriate stimulus intensity in this case.
Area of Science:
- Neurosurgery
- Neurophysiology
- Anesthesia
Background:
- Motor evoked potentials (MEPs) are crucial for monitoring motor pathway integrity during intracranial procedures.
- Inappropriate stimulation intensity can lead to false negative MEP results, potentially compromising patient safety.
- Ipsilateral (crossover) MEP responses are often misinterpreted as indicating excessive stimulation intensity.
Purpose of the Study:
- To describe a case where persistent crossover MEP responses misleadingly suggested supramaximal stimulation intensity.
- To present an alternative method for guiding MEP stimulation intensity selection during intracranial surgery.
Main Methods:
- Bilateral transcranial electrical MEP monitoring was performed under total intravenous anesthesia during a suboccipital craniotomy.
- MEP responses from the left and right hand were recorded using specific stimulation montages (C4-Cz and C3-Cz).
- MEP onset latencies were utilized to guide the selection of appropriate stimulus intensity, aiming for a superficial threshold.
Main Results:
- Persistent crossover MEP responses were observed with low stimulation intensity (94 V/166 mA) using the C3-Cz montage.
- Onset latency measurements from contralateral hand MEPs were used to determine the optimal stimulus intensity (approx. 96 V/172 mA).
- This intensity was selected when MEP onset latency was at least 2 ms longer than the shortest latency at maximal stimulus intensity, allowing for successful surgery without motor deficits.
Conclusions:
- Contralateral hand MEPs can be reliably used for intraoperative monitoring even when crossover responses are present.
- Onset latency guidance provides a quantitative method to validate MEP stimulation intensity, ensuring safety and efficacy during intracranial procedures.
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