• Laura Säisänen, Petro Julkunen, Samuli Kemppainen, Nils Danner, Arto Immonen, Esa Mervaala, Sara Määttä, Anu Muraja-Murro, and Mervi Könönen.
• *Institute of Clinical Medicine, Faculty of Health Sciences and §Department of Applied Physics, University of Eastern Finland, Kuopio, Finland; ‡Departments of Clinical Neurophysiology, #Neurosurgery, and **Clinical Radiology, Kuopio University Hospital, Kuopio, Finland; ¶Department of Clinical Neurophysiology, NordLab Kajaani and ‖Kainuu Social and Health Care Joint Authority, Kainuu Central Hospital, Kajaani, Finland.
• Neurosurgery. 2015 Sep 1;77(3):394-405; discussion 405.

BackgroundNavigated transcranial magnetic stimulation (nTMS) has become established as an accurate noninvasive technique for mapping the functional motor cortex for the representation areas of upper and lower limb muscles but not yet for facial musculature.ObjectiveTo characterize the applicability and clinical impact of using nTMS to map cortical motor areas of facial muscles in healthy volunteers and neurosurgical tumor patients.MethodsEight healthy volunteers and 12 patients with tumor were studied. The motor threshold (MT) was determined for the abductor pollicis brevis and mentalis muscles. The lateral part of the motor cortex was mapped with suprathreshold stimulation intensity, and motor evoked potentials were recorded from several facial muscles. The patient protocol was modified according to the clinical indication.ResultsIn all healthy subjects, motor evoked potentials were elicited in the mentalis (mean latency, 13.4 milliseconds) and orbicularis oris (mean latency, 12.6 milliseconds) muscles. At 110% of MT of the mentalis, the motor evoked potentials of facial muscles were elicited mainly in the precentral gyrus but also from one gyrus anterior and posterior to it. The cortical areas applicable for mapping were limited by an artifact attributable to direct peripheral nerve stimulation. The mapping protocol was successful in 10 of 12 tumor patients at locating the representation area of the lower facial muscles. The MT of the facial muscles was significantly higher than that of the abductor pollicis brevis.ConclusionnTMS is an applicable and clinically beneficial noninvasive method to preoperatively map the cortical representation areas of the facial muscles in the lower part of the face. Instead of using the MT of the abductor pollicis brevis, the stimulus intensity during mapping should be proportioned to the MT of a facial muscle.

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