Methodology for intra-operative recording of the corticobulbar motor evoked potentials from cricothyroid muscles

OBJECTIVE To establish the methodology for recording corticobulbar motor evoked potentials (CoMEPs) from cricothyroid muscles (CTHY) elicited by transcranial electrical (TES) and direct cortical stimulation (DCS). METHODS Six healthy subjects and 18 patients undergoing brain surgery were included in the study. In six healthy subjects as well as in nine patients under general anaesthesia, CoMEP was obtained by TES. In three patients under general anaesthesia and in six patients during awake craniotomy, CoMEP was obtained by DCS. We used three methods of electrical stimulation for eliciting responses from CTHY muscles: (1) TES over C3/Cz or C4/Cz (in six healthy subjects and nine patients under general anaesthesia), (2) DCS with a strip electrodes placed over exposed cortex in two patients, and (3) DCS with a hand-held probe in seven patients. For recordings, we percutaneously placed hook-wire electrodes in CTHY muscle (76 μm diameter) passing through 27-gauge needles, guided by electromyogram (EMG) feedback. In anaesthetised patients, recording electrodes were placed before patients were put to sleep. RESULTS Recordings of CoMEPs by TES was successfully performed in all but one healthy subject and all patients. Either method of stimulation resulted in recordings of short- and long-latency responses (SLRs and LLRs) in CTHY muscle. After DCS, SLRs were recorded in all patients but two, in whom only LLR was obtained; in patient No. 1 (Table 3) LLR was elicited by a strip electrode, while in patient No. 4 (Table 3), LLR was elicited by a hand-held electrode. The possible explanation for eliciting only LLR in patient No. 1 was that the strip electrode did not cover the primary motor cortex. In patient No. 4, only LLR but not SLR was recorded, due to the underlying pathology which displaced M1 for cricothyroid muscle not being accessible to the DCS with the hand-held electrode. As each CTHY muscle is innervated from both hemispheres, CoMEPs were recorded from the left or right CTHY muscle during TES or DCS over either the left or right hemisphere. In healthy subjects, TES elicited SLR with a latency of 13.25 ± 1.38 ms, while in patients the latency was 12.73 ± 0.64 ms. In healthy subjects, TES elicited LLR with a latency of 40.53±4.66ms, and in patients, it elicited LLR with a latency of 44.70±4.61ms. The group of patients in whom the responses were elicited by DCS, SLR and LLR had similar latencies as responses elicited by TES; in the patient's group the values were 13.97±1.11ms versus 12.73±0.64ms and 45.91±3.88ms versus 44.70±4.61ms, respectively. CONCLUSIONS Together with the existing methodology for intra-operatively eliciting CoMEPs in the vocal muscles, a new methodology for eliciting CoMEPs in CTHY muscle has the potential to continuously monitor the functional integrity of the structures involved in conveying signals from the motor cortex to the CTHY muscle. It is highly probable that SLR is a neurophysiological marker for the primary motor cortex (M1) and it is equal to CoMEP for laryngeal muscle, while LLR is a marker for the opercular part of Broca's area. SIGNIFICANCE This new and rather simple method adds a new tool in exploration of the functional organisation of motor cortex and corticobulbar pathways for laryngeal muscles. Furthermore, it has great potential to intra-operatively monitor its functional integrity.