Mechanism Of Recurrent Laryngeal Nerve Injury And The Application Of Intraoperative Neuromonitoring In Thyroid Surgery

Objective: This study aims to early detect adverse electromyogram (EMG)change and correct the surgical maneuver to prevent progressive nerve damage byIONM during the course of RLN dissection.Background: Recurrent laryngeal nerve (RLN) is at high risk of injury duringnerve dissection in thyroid surgery. Even though during using intraoperativeneuromonitoring (IONM) in thyroid surgery, loss of signal(LOS) often detected aftercomplete RLN dissection and the nerve may have been injured. Therefore, it isessential to monitor the nerve’s function during the phase of RLN dissection.Methods: A total of120cases (which involved208RLNs at risk) thyroidsurgery which performed by the same surgical team were included in this study.208RLNs followed the standardized IONM procedures and the largest EMG signal wascaptured and registered at every step during surgery. During the dissection of RLN，the nerve was repeatedly stimulated at the lower end of exposed RLN(Rp point)with handheld stimulating probe, and watching the quantitative change of EMGamplitude during RLN dissection. Once the EMG amplitude decreased>50%ascompared with R1, we would check whether the device was connected properly atonce to determine whether the real decreased signal happened. if the real decreasedsignal happened, the surgical maneuver would be stopped immediately. Retested thenerve at the lower end of exposed nerve and registered the EMG signal after2,4,6,8and10minutes of waiting. After10minutes or EMG recovered＞70%as comparedwith R1,the operation would be restarted with meticulous dissection and gentletraction. Weak or disrupted point of nerve conduction on the RLN was routinelychecked after complete RLN dissection.Results: The functions of120patients’ vocal cords（VCs） were normal whichwere demonstrated by preoperative laryngoscopies. Of all the208（left：108，right：100） involved nerves, the EMG amplitude which decreased＞50%experienced on19RLNs(9.2%，left：15，right：4).After the EMG recovery or10minutes later, thegradual EMG recovery ranged from51.2%to122.5%(83.6%±19.7%)R1,after thefirst time of EMG decrease in19positive nerves. There were totally15(78.9%of the positive nerves) RLNs’ EMG recovered to the degree of the R1in10minutes or untilthe10minute, the recovery time was2to10minutes(6.53±2.56minutes),the recoveryrate was70.1%-122.5%（89.9%±16.7%）as compared with R1;2(10.5%of the positivenerves) RLNs’ EMG recovered in the extent of60%to70%R1,and their EMG werestill at that degree before closing the wound;2(10.5%of the positive nerves) RLNs’EMG amplitude occurred intractable decrease（50%-60%of R1）,and it hadn’trecovered significantly by the operations completed. None of the19RLNs’ EMGdecrease, could we find LOS（0.0%）.We probed4RLNs existing weak or disrupted point of all19exposed RLNs,3points experienced on the middle section（Equivalent to the level of the middle ofthyroid lobe） of RLNs and1point experienced on RLNs where around the Rppoint(Equivalent to the level of the lower pole of the thyroid) respectively.19patientswith initial EMG decrease, Proximal RLN:3times(15.8%) experienced on RLNswhere inferior to the Rp point (the section inferior to the thyroid lobe);4times (21.1%)experienced on RLNs where around the Rp point. Distal RLN:4times（21.1%）experienced on RLNs where closed to the larynx,8times (42.1%) experienced on themiddle section of RLNs.The supposed mechanism of nerve injury：we consider that1case（5.3%） to bethermal,1cas（e5.3%）to be compression,1cas（e5.3%）to be suture,3case（s15.8%）to be clamping and13cases（68.4%） to be traction respectively.Only2（1.0%） in all the208RLNs of120patients showed impaired cordmovement because of nerve injury by the postoperative laryngoscopy， and vocalfunction recovered approximately60days after the operation. There’s no permanentcord palsy（0.0%）.Conclusion. To compare with other mechanisms of injury, such as thermal,clamping, compression, suture, traction was more common in the thyroid surgery, butthe damage caused by this mechanism was relatively mild and had better prognosis.The left RLN was more easily injured than the right RLN, distal RLN experienced ahigher incidence of EMG decrease when compared with proximal RLN，and theincidence the middle section of RLN was higher than that the RLN where closed tothe larynx. The standardized application of IONM can reduce the incidence of LOS effectively. Continuous monitoring the nerve function during RLN dissection isessential to early detect adverse EMG change.50%decrease of EMG amplitude canbe used as a warning criterion that alerts surgeon to correct the surgical maneuverimmediately to prevent irreversible nerve damage. The decrease of EMG amplitudecaused by the temporary RLN injury is recoverable in a certain time. Once EMGamplitude decreases＞50%as compare with R1by an adverse surgical maneuver,such as traction，clamping etc, stopping this adverse maneuver immediately canprevent the further decrease of EMG amplitude and is conductive to the recovery ofEMG.