gms | German Medical Science

Objective: Presurgical functional brain mapping is important for optimized resection planning of eloquently located brain tumors of the primary motor region. Beyond functional MRI (fMRI), motor mapping by navigated Transcranial Magnetic Stimulation (nTMS) has attracted rising attention in the last years. Facial and tongue mapping by nTMS is often limited by direct stimulation effects of facial and trigeminal nerve fibers. Short latency MEPs recorded during the mappings may lead to false-positive results. We therefore analyzed MEP latencies in a preclinical trial and investigated the effect of latency-correction in presurgical, clinical mappings.

Methods: We examined 10 healthy, right-handed subjects on three days by single pulse nTMS (eXimia 3.2.2; surface electrode MEP recordings: abductor pollicis brevis muscle [APB], plantaris muscle [PM], orbicularis oris muscle [OO], mentalis muscle [MM] and tongue; stimulation at 110% of resting motor threshold [RMT] over primary motor cortex; if the RMT could not be determined due to direct stimulation effects, mapping was performed at 95% of MT and muscular pre-contraction). In addition to the preclinical setup, we analyzed the effect of latency correction in presurgical mappings of the tongue and face area in patients with eloquently located brain tumors.

Results: Latencies directly correlated with human height, mostly for PM mapping. Mean latencies ±1 SD were 23±2 ms for APB, 44±4 ms for PM, 10.8±0.6 ms for MM, 11.0±0.5 ms for OO and 9.7±0.8 ms for the tongue. According to literature, we regarded latencies below 7.0 ms as direct stimulation artifacts. Mapping of the perioral region was feasible in 8/10 subjects only, interfered by direct stimulation effects. The mean latencies were independent from muscular pre-contraction (mean latencies [N=21 each]: MM at rest 10.69 ms, MM pre-contracted 11.03 ms, OO at rest 11.33 ms, OO pre-contracted 10.96 ms). Without latency correction, false-positive stimuli due to direct stimulation effects were recorded, especially when stimulating over the frontolateral cortex / pterional region.

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 using latency correction (7.0 - 16.0 ms for tongue/face mapping, independent from pre-contraction, height or age).