Low Threshold Monopolar Motor Mapping for Resection of Motor-eloquent Brain Lesions in Children and Adolescents

Aims: Resection of lesions in proximity to the primary motor cortex (M1) and the corticospinal tract (CST) is generally regarded as high-risk surgery due to reported rates of post-operative severe deficits of up to 50%. Our objective was to determine the feasibility and safety of low-threshold motor mapping and its efficacy for increasing the extent of lesion resection in the proximity of M1 and the CST in children and adolescents.

Methods: We analyzed eight consecutive pediatric patients in which we performed nine resections for motor eloquent lesions within or proximal (≤10mm) to M1 and/or the CST. Monopolar high-frequency (HF) motor mapping with train-of-5 (TOF) stimuli (pulse duration = 500 microseconds; inter-stimulus interval = 4.0 milliseconds; frequency = 250 Hz) was used. The motor threshold was defined as the minimal stimulation intensity that elicited motor evoked potentials (MEPs) from target muscles (amplitude >30 mV). Resection was performed toward M1 and the CST at sites negative to 1-3 mA HF-TOF stimulation.

Results: M1 was identified through HF-TOF via application of varying low intensities. The lowest motor thresholds after final resection ranged from 1-9 mA in eight cases, and up to 18 mA in one case, indicating close proximity to motor neurons. Intra-operative EEG documented an absence of seizures during all surgeries. Two transient neurologic deficits were observed, but there were no permanent deficits. Post-operative imaging revealed complete radiologic resection in eight patients and minimal rest (<0.175 cc) in one patient.

Conclusions: HF-TOF with a minimal threshold of 1-3 mA is a feasible and safe procedure for resections in the proximity of the CST. Thus, low threshold motor mapping might help to expand the safety envelope for safe resection in pediatric patients with lesions located within the precentral gyrus and in proximity to the CST, and may be regarded as a functional navigational tool. The additional use of continuous MEP monitoring serves as a safety feedback for the functional integrity of the CST, especially because the true excitability threshold in children is unknown.