gms | German Medical Science

Objective: Navigated transcranial magnetic stimulation (nTMS) is an established non-invasive diagnostic tool to map the primary motor cortex (M1) before brain tumor surgery. The objective of this study is to examine whether nTMS can elicit motor-evoked potentials (MEPs) over non-primary motor areas (NPMA) that can clearly be separated from M1 responses.

Methods: In 10 patients with brain tumors, as well as in 3 healthy subjects, the cortical motor representation of the first dorsal interosseous muscle (FDI) was mapped with single pulse biphasic nTMS. The examination included the determination of the FDI resting motor threshold (RMT) and subsequent FDI area mapping at 105% RMT, as well as a mapping of the premotor and supplementary motor areas with 120% RMT and with the electric field (EF) oriented perpendicular to the nearest sulcus. The EF distribution and direction of MEP positive NPMA sites were analyzed in correlation to the M1 hotspot stimulation.

Results: The RMT for M1 was 69 V/m in patients / 62 V/m in the healthy controls. During NPMA mapping a total of 1676 stimulations in patients / 715 in volunteers were applied. Of these, 135 / 70 stimulations led to NPMA MEPs of > 50µV. Most of these were observed in the immediate vicinity of M1, in fact overlapping with the FDI area map. In all but three NPMA positive sites, the direction of the EF was almost identical to that over M1 (deviation < 45°). No significant difference was observed between NPMA vs. M1 latencies. The average M1 EF for the MEPs in NPMA territory was 71 V/m in patients and 60 V/m in healthy subjects, i.e. almost identical to the M1 RMT values. Of all NPMA MEP positive sites, 51 in the patient group / 27 in the healthy controls had an EF value over the M1 hotspot that was lower than the M1 RMT. In45 of these 51 / 23 of 27 MEP positive NPMA sites, the EF value was less than 20% lower than the M1 RMT EF. Overall, a significant correlation for MEP positive NPMA sites was observed between the EF at the M1 hotspot and the M1 RMT EF (p = 0.01).

Conclusion: In this study, the majority of MEPs detected outside the primary motor area with biphasic single pulse nTMS originated from the primary motor cortex based on the analysis of latencies, EF distribution and vector rotation. Future TMS examinations have to monitor closely the relationship between signals coming from M1 and NPMA to decide whether MEP positive sites outside the primary motor cortex can be surgically removed without functionaldamage.