Neuromonitoring of the Recurrent Nerve: Validation and Merits

Abstract

BACKGROUND: Neuromonitoring is being advocated in thyroid surgery to aid in the identification of the recurrent laryngeal nerve and to predict the postoperative vocal cord function. Here we documented and analyzed electromyographic responses to investigate the reliability of transligamental neuromonitoring under surgical conditions. METHODS: In a prospective study on 12 nerves at risk, the electromyogram was recorded with the computerized Neurosign100® system to (i) estimate the nerve conduction velocity, (ii) to compare the magnitude of the response after vagal and recurrent nerve stimulation and (iii) to document the effects of a nicotinic acetylcholine receptor antagonist. RESULTS: The nerve conduction velocity was estimated at 45 m/s. Half-maximum and near-maximum responses were elicited by 0.17 and 1 mA, respectively. The magnitude of the electrical field response (but not the conduction velocity) depended on the extent of neuromuscular blockade thus identifying a possible confounding intraoperative variable. The field response after contralateral stimulation was about 10-fold smaller than after ipsilateral stimulation. Sole use of the neuromonitoring electrode allowed for identification of the recurrent nerve in 85 % of the trials. CONCLUSIONS: The electrophysiological and pharmacological criteria prove that neuromonitoring reliably measures the electrical response of the inner laryngeal muscles to nerve stimulation. There are, however, many variables (including electrode placement, depth of anaesthesia, extent of neuromuscular blockade) that have to be taken into account. These preclude a quantitative interpretation under surgical conditions. Thus, neuromonitoring only provides qualitative information on nerve function. A review of the literature shows that this conclusion is supported by the statistical analysis of sensitivity, specificity, positive and negative predictive value for postoperative palsy; this may, in part, also be due to the fact that the recording electrode registers the field response which is only a surrogate parameter for vocal cord mobility.