[Relaxation and the electromyographic identification of the recurrent laryngeal nerve].

Introduction: Electromyography for the identification of the recurrent laryngeal nerve is gaining ever more acceptance in thyroid surgery. Relaxation of the patient, e.g., to improve intubation conditions for anesthesia, carries a potential risk for error.

Method: After definite identification of the recurrent laryngeal nerve and the vagus nerve (Neurosign 100, Magstim Co., Wales), ten consecutive patients were relaxed with mivacurium and atracurium besylate at a weight-dependent ED95 dosage. After peripheral relaxation was achieved (TOF-Watch, Organon Teknika Corp., at the adductor muscle of the thumb), the signal derived via the vocal muscle was assessed acoustically in 3-min intervals by the surgeon and graphically recorded by computer (EWACS, Inomed Co.).

Results: Complete peripheral relaxation was attained with mivacurium after 3-7 min and with atracurium after 7-11 min. A decrease in amplitude of the vocal muscle signal of >60% was recognized by the surgeon as a weakened signal and could be confirmed during an average duration of 13.3 min with mivacurium (maximum: 37 min) and 17.7 min with atracurium besylate (maximum: 23 min), respectively. Complete obliteration of the acoustic signal (<20% of the initial signal) occurred in three of six patients treated with mivacurium and in four of four patients treated with atracurium.

Conclusions: The accuracy of electromyography of the recurrent laryngeal nerve can be substantially impaired by the administration of relaxants. If this medication cannot be dispensed with, the surgeon must be aware of the situation. In these cases, a peripheral relaxometer should also be employed to monitor relaxation as it subsides.