Influence of muscle relaxation on neuromonitoring of the recurrent laryngeal nerve during thyroid surgery

Background. The influence of muscle relaxation on the intra-operative neuromonitoring of the recurrent laryngeal nerve during thyroid surgery is unclear. Methods. In a prospective study involving 200 patients undergoing elective thyroid surgery, the influence of muscle relaxation on neuromonitoring of the recurrent laryngeal nerve was investigated. The patients received balanced anaesthesia with oxygen-nitrous oxide-isoflurane, and rocuronium bromide was used as the non-depolarizing neuromuscular blocking agent. The degree of relaxation was monitored continuously by accelerometry [twitch (% TW)]. Summed action potentials (SAcP) obtained from the vocalis muscle were characterized by the area under the electromyographic curve expressed in millivolt seconds. Results. Evoked potentials were obtainable in all patients and at all time points. With decreasing neuromuscular blockade a significant increase in the potentials evoked at the vocalis muscle was observed. At 0% TW SAcP was 1.27 (sd 1.02) mV s. An increase in TW to 10% was accompanied by an increase in SAcP to 2.68 (2.01) mV s (P < 0.01). At a TW of 25%, mean SAcPs of 5.08 mV s were recorded. Conclusions. There was a significant difference in the degree of relaxation of the adductor pollicis muscle and the vocalis muscle. The laryngeal muscles exhibited a shorter response time than the adductor pollicis and recovered more quickly. These results confirm the feasibility of intra-operative neuromonitoring of the recurrent laryngeal nerve during neuromuscularblockade.