Optimization of Transcranial Motor Evoked Potentials in Supratentorial Surgeries

Background: During supratentorial brain surgery there is a risk of iatrogenic injury resulting in post-operative motor deficits. Intraoperative neurophysiological monitoring (IONM) can help surgeons avoid this outcome, while also expanding evidence of its prognostic and preventive benefit (1, 2). However, there are still pending issues, such as standardization of optimal methodologies for data acquisition and interpretation as well as technical improvement.

There are two main IONM strategies: mapping to localize nervous structures, and monitoring to provide real-time assessment of functional integrity (3). With respect to monitoring, muscle motor evoked potentials (MEPs) enable motor system assessment. They can be elicited by pulse train transcranial electric stimulation (TES) through scalp electrodes placed at standard 10-20 system sites (C1, C2, C3, C4, Cz-1cm, Cz+6cm), or at neuronavigation-guided scalp sites. The stimulation circuit consists of the user-selected anode and cathode sites chosen from the scalp electrodes (4). The stimulation parameters are user-selected pulse duration, interstimulus interval (ISI), number of pulses, and single train stimulation vs. double-train or multi-train facilitation. MEP threshold is the stimulation intensity required to elicit a certain number of MEPs with a predefined amplitude. The recording montage consists of user-selected recording electrode orientation and target muscles. Practitioner preferences for these technical aspects vary widely (5), and can influence monitoring efficacy.

During surgery, significant MEP deterioration from an earlier baseline indicates a warning based on various criteria such as amplitude reduction or threshold elevation (2). Warnings trigger rescue manoeuvres to recover MEPs. With reversible MEP deterioration (recovery to values above the warning level), a post-operative motor deficit might be prevented. Continuous real-time monitoring facilitates this favourable result by providing early warnings, and is therefore important. With irreversible MEP deterioration there is an increased risk of a motor deficit (1), and discontinuous intermittent monitoring may fail to help prevent this adverse outcome.

A major concern regarding TES MEPs is that the stimuli often produce movement of the surgical field, which entails the risk of the patient being harmed by surgical manoeuvres if they are not paused during the measurements (4). This can delay the surgical procedure and/or preclude real-time monitoring. Thus, there is a need to establish optimal TES MEP methodology that minimizes objectively assessed movement.

Objective: The hypothesis of this project is that MEP stimulation and recording paradigms can be optimized to minimize movement of the surgical field during supratentorial brain surgery. Consequently, the overall aim is to identify the most effective stimulation and recording techniques to maximize TES MEP performance by minimizing movement. The specific objectives are to determine: