gms | German Medical Science

Objective: Navigated transcranial magnetic stimulation (nTMS) is highly valuable to create patient-individual data with motor-eloquent areas visible for the operating surgeon during the resection of cerebral pathologies. To obtain reliable and reproducible results in navigated transcranial stimulation brain mapping, nTMS-examinations are conducted in a standardized way. Factors influencing stimulation parameters and results of nTMS-examinations are not completely investigated yet. The objective was to determine factors which influence nTMS-stimulation parameters and the quality of nTMS-data for neurosurgical procedures.

Methods: Patient records, imaging and nTMS-data from examinations of the upper extremity muscles were retrospectively reviewed. According to the ratio of positive to all applied stimulations, nTMS-data were judged as high- (more than 45% positive responses), intermediate- and low-quality (less than 25% positive responses).

Results: 65 nTMS-examinations in 61 patients (39.3% female) aged 59.95 years were evaluated. Resting motor threshold (rMT) was higher in patients with tumors in direct vicinity of the precentral gyrus (p=0.031), non-recurrent tumors (p=0.024) and in patients with motor deficits (p=0.049). Monitoring of more than two upper extremity muscles allowed mapping the hand area at lower intensities (p=0.015). Mapping conditions did not differ in patients with different tumor entities (all p>0.2). nTMS-mapping quality was user-independent. Applied stimulations and positive stimulations correlated significantly (r_s=0.66, p<0.001). The ratio of positive stimulations and stimulation intensity (minimum applied stimulator output: r_s=-0.14, p=0.26, maximum applied stimulator output: r_s=-0.15, p=0.25) showed no correlation. Patients with low-quality mapping data had lower rMT (p=0.049). Patients with arterial hypertension had lower ratios of positive stimulations (p=0.045). We did not observe differences in the ratio of positive stimulations among different tumor entities (p=0.54), vicinity to the primary motor cortex (p=0.47) or motoric deficits (p=0.56). High quality nTMS-data were associated with no postoperative deterioration in patients with highly motor-eloquent tumors (p=0.009).

Conclusion: nTMS-examinations are influenced by different conditions. Most of these conditions are patient-specific or can be derived from imaging studies. Nonetheless, stimulation intensity should be kept as low as possible during nTMS-motor-mapping.