The importance of latency correction for navigated TMS mapping, especially of the face and tongue area: A Technical Note

Facial mapping by navigated Transcranial Magnetic Stimulation (nTMS)^1–6 is often limited by direct stimulation effects of facial and trigeminal nerve fibres. Short latency MEPs recorded during mapping of the face and tongue area by nTMS may lead to false-positive results. We therefore analyzed MEP latencies in a preclinical trial.

Methods: We examined 10 healthy, right-handed subjects (5f/5m; age 24–34) on three days by single pulse nTMS. NTMS was applied at the dominant primary motor cortex at 110% of the resting motor threshold (RMT) using eXimia 3.2.2. MEPs were recorded from the abductor pollicis brevis muscle (APB; Figure 1), plantaris muscle, perioral muscles and the tongue. If the RMT could not be determined due to direct stimulation effects, mapping was performed at 95% of direct MT with muscular pre-contraction.

Results: Latencies directly correlated with human height, mostly for plantaris muscle mapping (figure 1). Mean latencies±1SD were 23±2ms for APB, 44±4ms for plantaris muscle, 10.8±0,6ms for mentalis muscle, 11.0±0.5ms for orbicularis oris muscle (OO) and 9.7±0.8ms for the tongue (figure 2). According to literature,^7–9 we regarded latencies below 7.0ms as artifacts (due to direct nerve stimulation). Mapping of the perioral region was feasible in 8/10 subjects only, interfered by direct stimulation effects. Mean latencies were independent from muscular pre-contraction (21 investigations at rest / 21 investigations at pre-contraction; mean latency mentalis at rest 10,69ms, mentalis pre-contracted 11,03ms, OO at rest 11,33ms, OO pre-contracted 10,96ms).

Conclusions: NTMS mapping of the facial and tongue muscles is technically rather difficult. It often requires muscular pre-contraction to lower excitability thresholds. Direct facial nerve stimulation effects often interfere with cortical TMS effects. We suggest to use latency correction (7.0–16.0ms for tongue/face mapping, independent from pre-contraction, height or age).

Fig. 1: In our population, MEP latencies showed a direct correlation with human height but not significantly correlated with age. The clearest correlation with human height was found for plantaris muscle mapping.

Fig. 2: MEP latencies for face and toungue mapping by nTMS were previously corrected by exclusion of latencies below 7ms. We hereby intended to avoid false-positive results due to direct stimulation effects. Further analysis showed that mean latencies ±1SD ranged from 8.5ms to 12ms and thus were far away from MEP latencies below 7ms.

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