Background Muscles present different responses to muscle relaxants, a mechanism of importance in surgeries requiring facial nerve evoked electromyography under general anaesthesia. The non-depolarizing muscle relaxants have multiple reaction formats in the neuromuscular junction, in which pre-synaptic quantal release of acetylcholine was one of the important mechanisms. This study was to compare the pre-synaptic quantal release of acetylcholine from the neuromuscular junctions innervated by normal/damaged facial nerves and somatic nerve under the effect of rocuronium in rats in vitro. Methods Acute right-sided facial nerve injury was induced by nerve crush axotomies. Both sided facial nerve connected orbicularis oris strips and tibial nerve connected gastrocnemius strips were isolated to measure endplate potentials （EPP） and miniature endplate potentials （MEPP） using an intracellular microelectrode gauge under different rocuronium concentrations. Then, the pre-synaptic quantal releases of acetylcholine were calculated by the ratios of the EPPs and the MEPPs, and compared among the damaged or normal facial nerve innervated orbicularis oris and tibial nerve innervated gastrocnemius. Results The EPP/MEPP ratios of the three neuromuscular junctions decreased in a dose dependent manner with the increase of the rocuronium concentration. With the concentrations of rocuronium being 5 pg/ml, 7.5 IJg/ml and 10 pg/ml, the decrease of the EPP/MEPP ratio in the damaged facial nerve group was greater than that in the normal facial nerve group. The decrease in the somatic nerve group was the biggest, with significant differences. Conclusions Rocuronium presented different levels of inhibition on the pre-synaptic quantal release of acetylcholine in the three groups of neuromuscular junctions. The levels of the inhibition showed the following sequence： somatic nerve 〉 damaged facial nerve 〉 normal facial nerve. The difference may be one of the reasons causing the different sensitivities to rocuronium among the muscl
Background The evoked electromyography （EMG） is frequently used to identify facial nerve in order to prevent its damage during surgeries. Partial neuromuscular blockade （NMB） has been suggested to favor EMG activity and insure patients＇ safety. The aim of this study was to determine an adequate level of NMB correspondent to sensible facial nerve identification by evaluating the relationship between facial EMG responses and peripheral NMB levels during the middle ear surgeries. Methods Facial nerve evoked EMG and NMB monitoring were performed simultaneously in 40 patients who underwent tympanoplasty. Facial electromyographic responses were recorded by insertion of needle electrodes into the orbicularis otis and orbicularis oculi muscles after electrical stimulation on facial nerve. The NMB was observed objectively with the hypothenar muscle＇s twitching after electrical stimulation of ulnar nerve, and the intensity of blockade was adjusted at levels of 0, 25%, 50%, 75%, 90%, and 100% respectively with increased intravenous infusion of Rocuronium （muscle relaxant）. Results All of the patients had detectable EMG responses at the levels of NMB 〈50%. Four out of forty patients had no EMG response at the levels of NMB 〉75%. A significant linear positive correlation was present between stimulation thresholds and NMB levels while a linear negative correlation was present between EMG amplitudes and NMB levels. Conclusions The facial nerve monitoring via facial electromyographic responses can be obtained when an intraoperative partial neuromuscular blockade is induced to provide an adequate immobilization of the patient. The 50% NMB should be considered as the choice of anesthetic management for facial nerve monitoring in otologic microsurgery based on the relationship of correlation.
