Quantitative Subcortical Motor Mapping and Continuous Motor-Evoked Potential Monitoring during Surgery of Supratentorial Brain Tumors

Aim: Mapping and monitoring are increasingly used during the resection of tumors located close to the corticospinal tract (CST). The objective was to compare lowest subcortical stimulation thresholds (MT) and motor-evoked potential (MEP) monitoring signal abnormalities in correlation to motor function outcome.

Methods: An analysis of 70 patients who underwent tumor surgery from 2009 to 2011 (36 glioma WHO grade 1–3, 17 glioblastoma, 9 metastases, 5 cavernoma, and 3 others) was performed. Evaluation was done regarding the lowest subcortical mapping threshold (monopolar stimulation, train of five stimuli, ISI of 4.0 ms, impulse width of 500 µ) and stable monitoring of direct cortical stimulated motor-evoked potentials (DCS MEP) (same parameters) via a four contact strip electrode. Lowest MT was defined as the minimum stimulation intensity which elicited MEP from the target muscles at ≥30µV amplitude under total intravenous anesthesia. Motor function outcome was assessed according immediately after surgery, at day of discharge and at the 3-month follow-up visit.

Results: Lowest individual stimulation thresholds were as follows (MT in mA, number of patients): ≥15 mA n = 11; 14–10 mA n = 8; 9–6 mA n = 15; 5–4 mA n = 21; ≤3 mA n = 15. DCS MEP showed stable signals in 74%, unspecific changes in 19%, irreversible alterations in 3%, and irreversible loss in 4% of patients.

At the 3-month follow-up two patients had a reversible (lowest MT 5 and 4 mA) and three patients a permanent motor deficit (MT 13, 6, 1 mA). Of those five cases, severe DCS-MEP alterations were observed in four patients (one irreversible threshold increment, three MEP losses). Of the 15 patients with MT≤3 mA only one patient developed a permanent motor deficit. This patient had a MT of 1 mA and a sudden DCS-MEP loss.

Conclusion: There is an overlapping hierarchy between motor mapping and monitoring as warning sign for CST damage. Mapping provides an early warning sign and localizes motor tracts whereas monitoring has a high prediction of unchanged motor function when no signal alterations occur. Even a very low subcortical MT of 1–3 mA does not result in a new permanent neurological deficit provided that no alterations in continuous DCS MEP are present and subcortical mapping is repeated with high frequency and spacial coverage.

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