Vagal and recurrent laryngeal nerves neuromonitoring during thyroidectomy and parathyroidectomy: A prospective study

The aim of the study was to stimulate the vagal and the recurrent laryngeal nerves during and after thyroidectomy or parathyroidectomy, to record muscle responses, interpret the electrophysiological modifications and identify prognostic factors for postoperative vocal fold mobility.

A prospective study monitored 151 vagal nerves and 144 recurrent laryngeal nerves in 114 patients. Seven patients (14 vagal nerves) underwent continuous monitoring via an automatic periodic stimulation (APS^®) electrode. In 15 patients (21 vagal nerves), the stimulation threshold was studied. Muscle response was recorded on direct vagal and/or recurrent laryngeal nerve stimulation by a monopolar electrode or direct repeated stimulation via an electrode on the vagal nerve. In case of signal attenuation on the first operated side, surgery was not extended to the contralateral side.

The vagal nerve stimulation checked inferior laryngeal nerve integrity and recurrent status, without risk of false negatives. The vagal nerve stimulation threshold, before and after dissection, that induced a muscle response of at least 100μV ranged from 0.1 to 0.8mA. Similarity between pre- and post-dissection responses to supramaximal stimulation, defined as 1mA, on the one hand, and between post-dissection vagal and laryngeal recurrent nerve responses on the other correlated with normal postoperative vocal cord mobility. Conversely, muscle response attenuation below 100μV and increased latency indicated a risk of vocal fold palsy.

Vagal nerve stimulation allows suspicion or elimination of lesions on the inferior laryngeal nerve upstream of the stimulation point and detection of non-recurrent inferior laryngeal nerve. Intermittent monitoring assesses nerve function at the moment of stimulation, while continuous monitoring detects the first signs of nerve injury liable to induce postoperative recurrent nerve palsy. When total thyroidectomy is indicated, signal attenuation on the first operated side casts doubt on continuing surgery to the contralateral side in the same step.