Preservation of motor maps with increased motor evoked potential amplitude threshold in RMT determination.

ABSTRACT

OBJECTIVE: Non-invasive pre-surgical mapping of eloquent brain areas with navigated transcranial magnetic stimulation (nTMS) is a useful technique linked to the improvement of surgical planning and patient outcomes. The stimulator output intensity and subsequent resting motor threshold determination (rMT) are based on the motor-evoked potential (MEP) elicited in the target muscle with an amplitude above a predetermined threshold of 50 µV. However, a subset of patients is unable to achieve complete relaxation in the target muscles, resulting in false positives that jeopardize mapping validity with conventional MEP determination protocols. Our aim is to explore the feasibility and reproducibility of a novel mapping approach that investigates how an increase of the MEP amplitude threshold to 300 and 500 µV affects subsequent motor maps. MATERIALS AND METHODS: Seven healthy subjects underwent motor mapping with nTMS. RMT was calculated with the conventional methodology in conjunction with experimental 300- and 500-µV MEP amplitude thresholds. Motor mapping was performed with 105% of rMT stimulator intensity using the FDI as the target muscle. RESULTS: Motor mapping was possible in all patients with both the conventional and experimental setups. Motor area maps with a conventional 50-µV threshold showed poor correlation with 300-µV (α = 0.446, p < 0.001) maps, but showed excellent consistency with 500-µV motor area maps (α = 0.974, p < 0.001). MEP latencies were significantly less variable (23 ms for 50 µV vs. 23.7 ms for 300 µV vs. 23.7 ms for 500 µV, p < 0.001). A slight but significant increase of the electric field (EF) value was found (EF 60.8 V/m vs. 64.8 V/m vs. 66 V/m p < 0.001). CONCLUSION: Our study demonstrates the feasibility of increasing the MEP detection threshold to 500 µV in rMT determination and motor area mapping with nTMS without losing precision.