NTMS Facial Mapping: Mental Muscle superior to Orbicularis Oris Muscle: A Technical Note

Aims: Facial mapping by navigated Transcranial Magnetic Stimulation (nTMS) 1–6 is often limited by direct stimulation effects of facial and trigeminal nerve fibres. This direct effect is dose-dependent; thus, stimulating at the lowest amplitudes possible should be intended. We therefore compared the most frequently used muscles for mapping of the mouth area, i.e. the mental muscle and the orbicularis oris muscle concerning test-retest-reliability and feasibility. Methods: We examined 11 healthy subjects on 3 different days within 6 weeks using eXimia 3.2 for simultaneous mapping of the orbicularis oris muscle and the mental muscle (stimulation at rest, 110% output compared to resting motor threshold [RMT]; if the RMT could not be determined due to direct stimulation, mapping was performed at 95% of direct motor threshold with muscular precontraction; single pulse TMS, registration mismatch <3mm, surface EMG, latency accepted from 7–16ms 7 ). Results: Facial mapping of the mouth was feasible in 8/11 subjects. In 3/11 subjects reproducible mapping was not possible due to direct stimulation effects. The mean latency was 10–11ms, independent from muscular precontraction (21 investigations at rest / 21 investigations at precontraction; mean latency mentalis at rest 10.69ms, mentalis precontracted 11.03ms, orbicularis oris at rest 11.33ms, orbicularis oris precontracted 10.96ms). Stimulating both muscles simultaneously, the RMT was usually lower for mental muscle (75% of cases), the number of positive stimuli (amplitude >50uV) was significantly higher using mental muscle (mean/session: mental muscle 380 stimuli / orbicularis oris muscle 273 stimuli) and the mean amplitude was higher (mentalis muscle: 458uV / orbicularis oris muscle: 342uV; figure). Intersession reliabilty was comparable. Conclusion: The mentalis muscle mapping seems to be technically superior to orbicularis oris muscle mapping and often requires muscular precontraction. Fig. 1: The mean MEP amplitudes of mentalis muscle compared to simultaneous recorded MEPs of orbicularis oris muscle were higher suggesting easier excitability of the mentalis muscle area by navigates transcranial magnetic stimulation (nTMS). Literatur: 1. Bennett CM, Miller MB. How reliable are the results from functional magnetic resonance imaging? Ann N Y Acad Sci. 2010 Mar;1191:133-55. 2. Havel P, Braun B, Rau S, Tonn JC, Fesl G, Bruckmann H, et al. Reproducibility of activation in four motor paradigms. An fMRI study. J Neurol. 2006 Apr;253(4):471-6. 3. Pouratian N, Bookheimer SY. The reliability of neuroanatomy as a predictor of eloquence: a review. Neurosurg Focus. 2010 Feb;28(2):E3. 4. Picht T, Mularski S, Kuehn B, Vajkoczy P, Kombos T, Suess O. Navigated transcranial magnetic stimulation for preoperative functional diagnostics in brain tumor surgery. Neurosurgery. 2009 Dec;65(6 Suppl):93-8; discussion 8-9. 5. Picht T, Schmidt S, Brandt S, Frey D, Hannula H, Neuvonen T, et al. Preoperative Functional Mapping for Rolandic Brain Tumor Surgery: Comparison of Navigated Transcranial Magnetic Stimulation to Direct Cortical Stimulation. Neurosurgery. 2011 Mar 23. 6. Aggarwal NT, Wilson RS, Bienias JL, De Jager PL, Bennett DA, Evans DA, et al. The association of magnetic resonance imaging measures with cognitive function in a biracial population sample. Arch Neurol. 2010 Apr;67(4):475-82. 7. Saisanen L, Julkunen P, Niskanen E, Danner N, Hukkanen T, Lohioja T, et al. Motor potentials evoked by navigated transcranial magnetic stimulation in healthy subjects. J Clin Neurophysiol. 2008 Dec;25(6):367-72.