gms | German Medical Science

Objective: Functional brain mapping of tumors adjacent to the primary motor cortex (M1) or the pyramidal tract is crucial to avoid postoperative motor deficits. Direct cortical stimulation (DCS) is considered the gold standard of cortical mapping. However, DCS – especially under general anaesthesia – is technically challenging and may lead to false-negative results, e.g., due to low cortical excitability as a result of medication. Particularly in case DCS is not possible intraoperatively for any reason, results derived from presurgical functional mapping are extremely helpful when planning the surgical approach and defining the extent of resection. In the past years, presurgical mapping of M1 by neuronavigated transcranial magnetic stimulation (nTMS) has become a useful alternative for presurgical mapping. We, therefore, investigated the robustness of nTMS compared to DCS.

Method: To date, 17 patients with M1-adjacent tumors were prospectively investigated by nTMS of the hand, foot and tongue M1 representation. The resting motor threshold (RMT) of each M1 area was determined (definition of RMT: lowest intensity level still leading to 3 out of 5 motor evoked potentials with an amplitude >50 uV). Motor-evoked potentials were from the respective muscles were acquired at 110 % RMT. Intraoperatively, monopolar direct cortical stimulation (mDCS) was performed (train of five, duration 500 us, 500 Hz, max. intensity 25 mA).

Results: With DCS, MEPs could be evoked in 64% of all stimulated areas. By contrast, the feasibility rate of nTMS was 95% for all areas. In case of negative DCS, the nTMS RMT always exceeded 35% of the maximum stimulator output. The mean RMT in DCS-negative cases was 50.3±14.5%, compared to 37.9±50.3% in DCS-positive cases (p=0.004). Moreover, the RMT of each body part representation was higher in DCS-negative cases than in DCS-positive cases (see table). Analyses of factors influencing the RMT revealed that subcortical edema adjacent to the corticospinal tract and high dosage of steroids correlated with lowered RMT whereas epilepsy and high dosage of levetiracetam correlated with elevated RMT levels.

Conclusions: For a number of patients, DCS did not evoke a measurable MEP, especially in patients with high RMT values. In contrast, nTMS revealed valid results in 95% of cases. Hence, nTMS appears to be more robust than DCS, especially in patients with high RMT. To a certain extent, the feasibility of DCS may be predictable by the RMT.