Preservation of motor maps with increased motor evoked potential amplitude threshold in RMT determination
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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.
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).
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.
KeywordsnTMS Motor mapping Resting motor threshold Latencies
The research of Giuseppe Lucente is supported by a Rio Hortega contract (ISCIII CM16/00016 and FEDER).
Compliance with ethical standards
All procedures were performed in accordance with ethical standards of the institutional research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards.
Conflict of interest
TP has served as a speaker for NexStim Oy. GL, HS and SL declare that they have no affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers’ bureaus; membership, employment, consultancies, stock ownership or other equity interest; and expert testimony or patent-licensing arrangements), or non-financial interest (such as personal or professional relationships, affiliations, knowledge or beliefs) in the subject matter or materials discussed in this manuscript.
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