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Long latency response in laryngeal muscles evoked by mapping with nTMS and DCS of Broca's area - A patient case report

Aim: Mapping of the primary motor cortex (M1) and the Broca’s area was performed preoperatively by navigated transcranial magnetic stimulation (nTMS) and intraoperatively during awake brain surgery by direct cortical stimulation (DCS) in a 44-year-old female patient with an expansive tumor process (glioblastoma) in the fronto-temporal cortex of left hemisphere. This is the first patient in whom both methodologies were performed for detecting motor speech cortical areas. The nTMS methodology was originally developed on healthy subjects, while DCS methodology on patients. Methods: The patient underwent nTMS preoperatively and intraoperatively DCS over the left M1 i.e. muscles of the upper extremities, larynx, and over the Broca’s area in the posterior part of inferior frontal gyrus hemisphere. The motor evoked potentials (MEPs) were recorded from abductor pollicis brevis muscle (APB) and corticobulbar MEPs (CoMEPs) from laryngeal muscle by mapping the M1, while long latency responses (LLRs) were recorded from laryngeal muscle by mapping the Broca’s area. Preoperatively the patient was engaged in visual object naming task, with the stimulation time-locked at 0 ms after picture presentation. Single pulse nTMS was used for mapping the M1 for hand muscle representation, while stimulation protocol consisting of 4 bursts of 4 stimuli each, with an interstimulus interval of 6 ms, and a burst repetition rate of 4 Hz was used for mapping the Broca’s area. Intraoperative stimulation was performed as close to the nTMS makers as possible using the neuronavigation equipment. DCS paradigm of 5 monophasic pulses of 0.4 ms duration, repetition rate of 2, 2 Hz, intensity up to 15 mA was used for eliciting patient clinical response as well as recording CoMEPs, and LLRs. Results: Preoperatively, the latency of CoMEP was 9.14 ms and intraoperatively 5.62 ms. The latency of LLR preoperatively was 54.94 ms, while intraoperatively 37, 4 ms. CoMEP elicited by the stimulation of M1 for laryngeal muscles corresponded to induced dysarthria preoperatively and intraoperatively, while LLR elicited by stimulation of the premotor cortex in the caudal opercular part of inferior frontal gyrus corresponded intraoperatively to semantic paraphasia at the beginning of the surgery, and later on to speech arrest. Conclusion: Mapping of the Broca’s area was successfully performed in patient with both methodologies, nTMS as a part of preoperative neurosurgical planning and with DCS during awake brain surgery. Future reports are needed to establish normative data for nTMS and DCS mappings of Broca’s area on group of patients.

Long latency response; laryngeal muscles; Broca's area; primary motor cortex; TMS; DCS

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