Intraoperative transcranial electrical stimulation evokes a motor response at short latency (SLR) of a few milliseconds in cranial nerves innervated muscles. In addition, a long latency response (LLR) is known for the laryngeal nerve. We present here the LLR in the tongue muscles.
In 22 neurosurgical interventions we have applied electrical stimulation pulses transcranially over the motor cortex of fully anesthetized patients. Stimulation sites were lateral (C3/C4 - Cz) or medial (C1-C2). We recorded motor evoked potentials from the tongue as the target muscle of the hypoglossal nerve.
Stimulation evoked a SLR of the tongue with a median (IQR) latency of 14 (13, 19) ms. In addition, a LLR appeared at 58 (54, 66) ms. Median (IQR) response amplitudes were 60 (32, 185) μV for SLR and 30 (20, 47) μV for LLR. While stimulation by C3/C4 - Cz evoked both the SLR and the LLR, stimulation by C1-C2 evoked preferably the LLR with a success rate of 16/18 (89%) surgeries (p =.0007).
Transcranial electrical stimulation robustly elicits a LLR in the tongue muscles of anesthetized patients.
We corroborated preliminary findings on the LLR of the tongue muscles in more patients.