gms | German Medical Science

Objective: Identification of the motorcortex (M1) via monopolar stimulation is the essential and commonly used technique for motor function preserving resection in infiltrating tumors compromising the area of M1 in the asleep setting. However, due to the widespread electrical fields of monopolar stimulation technique, results are not always distinctive enough and can lead to ambivalent results e.g. two gyri are tested positive with similar current. In those cases we employed the more localized electrical fields induced by bipolar 60 Hz stimulation in asleep patients.

Methods: We screened all surgeries performed for resection of supratentorial tumors using any direct cortical stimulation (monopolar and or bipolar stimulation) between 01/19-09/21 (n=324) for cases in which M1 could not be clearly identified via monopolar cortical stimulation according to surgical notes and monitoring protocol. Data were screened for localization of cerebral lesion, awake vs. non-awake, stimulation parameters and occurrence of seizures. Response to bipolar stimulation was determined by EMG or visible motor response.

Results: We identified 5 subjects (1.5%) in whom monopolar stimulation could not explicit or at all identify M1. 4 patients had lesions located in the right hemisphere, 3 of them included parietal or fronto-parietal lobe lesions, one lesion was located fronto-temporal. One patient suffered from a left parietal lesion. 80% (n=4) of the patients were planned for awake surgery but were not yet awake at stimulation. Additional 60 Hz bipolar stimulation identified M1 in all patients. However, in one patient 60 Hz stimulation induced a focal seizure. Mean current used for monopolar stimulation was 7 mA, for bipolar stimulation 2.6 mA.

Conclusion: Bipolar 60 Hz stimulation might be helpful for identification of M1 in cases of indifferent monopolar stimulation results.