Neuromonitoring in thyroid surgery: prospective evaluation of intraoperative electrophysiological responses for the prediction of recurrent laryngeal nerve injury

Abstract

Objective: We evaluated the ability of neuromonitoring to predict postoperative outcome in patients undergoing thyroid surgery for different indications.

Summary background data: Neuromonitoring has been advocated to reduce the risk of vocal cord palsy and to predict postoperative vocal cord function.

Methods: Three hundred twenty-eight patients (502 nerves at risk) were studied prospectively at a single center. Neuromonitoring was performed with the Neurosign 100 device by transligamental placement of the recording electrode into the vocalis muscles. Cumulative distribution of stimulation thresholds was determined by stepwise decreases in current (1 mA to 0.05 mA) for both the vagus and the recurrent nerve. Patients were grouped according to surgical risk (benign and malignant disease, reoperation for benign and for malignant disease).

Results: If the electrophysiological response was correlated to postoperative vocal cord function, the sensitivity of neuromonitoring was modest (86% in surgery for benign disease) to low (25% in reoperation for malignant disease); the positive predictive value was modest (overall rate 62%) but acceptable (87%) if corrected for technical problems. Specificity and negative predictive values were high (ie, overall >95%). Stimulation thresholds were not augmented in 11 patients, in whom postoperative palsy developed despite normal intraoperative recordings. Similarly, an electrical field response was elicited in 14 of 21 patients with preoperative vocal cord palsy. Electromyographic recordings did not reveal an abnormal amplitude or a decline in nerve conduction velocity.

Conclusions: Neuromonitoring is useful for identifying the recurrent laryngeal nerve, in particular if the anatomic situation is complicated by prior surgery, large tissue masses, aberrant nerve course. However, neuromonitoring does not reliably predict postoperative outcome.

FIGURE 1. Cumulative distribution of stimulation thresholds after stimulation of the vagus nerve and of the recurrent laryngeal nerve. A, The recurrent laryngeal nerves were stimulated with 3-Hz pulses at currents ranging from 3 mA to 0.01 mA before (triangles) and after dissection but prior to wound closure (circles). The stimulation threshold of the vagus nerve was also recorded prior to wound closure (squares). The cumulative distribution was analyzed separately for patients undergoing surgery for benign disease (closed symbols) and for patients who were reoperated for benign disease (open symbols). B, The stimulation threshold was determined before wound closure in patients undergoing surgery for malignant disease (▴, recurrent nerve = recurrent-mal.; ▵, vagus nerve = vagus-mal.) and reoperation for malignant disease (◆, recurrent nerve = recurrent-mal. reop.; ⋄, vagus nerve = vagus-mal. reop.) The control curves for the recurrent laryngeal (•, recurrent-benign) and the vagus nerve (□, vagus-benign) correspond to patients undergoing surgery for benign disease and are the same as those in A. To normalize for the difference in nerves at risk in each group, the data are expressed as percent of total; the distributions were fitted to the logistic equation N=100* [I^P/(I^P + I_0.5^P)], where N is the number of nerves in % of total, I and I_0.5 are the individual threshold current and the median current in mA, respectively, and P is the slope factor.

FIGURE 2. Cumulative distribution of stimulation thresholds in patients who developed postoperative palsy (A) and in patients who presented with preoperative palsy (B). The stimulation threshold was determined before wound closure as outlined in B. Shown are the cumulative distributions of those patients who developed postoperative palsy (▴ and ▵, recurrent and vagus nerve abbreviated as recurrent-postoperative. palsy and vagus-postoperative. palsy, respectively) and those patients who presented with preoperative palsy (◆ and ⋄, recurrent and vagus nerve abbreviated as recurrent-postoperative. palsy and vagus-postoperative. palsy, respectively) and in whom an electrical field response was nevertheless recorded. Control patients (same data as in A, •, recurrent nerve = recurrent-benign; □, vagus nerve = vagus-benign) are included for comparison.

FIGURE 3. Electrical field responses in a patient presenting with preoperative vocal cord palsy (A–C) and a patient with normal pre- and postoperative vocal cord function (D). A and B, Original trace recorded after stimulation of the right vagus nerve with 0.5 mA (A) and 1 mA (B). The interval between the 2 recordings is about 30 minutes. Despite the higher current used in B, the amplitude of the electrical field response is lower than in A because the patient was administered a skeletal muscle relaxant in between (10 mg atracurium). C, Electrical field response to stimulation of the right recurrent laryngeal nerve. Note that the lag between stimulation (time point = 0 milliseconds) and electrical field response is shorter than in B (or A): after vagal stimulation, the delay is about 2 milliseconds longer, and this difference can be used to estimate the nerve conduction velocity (∼15 cm/2 milliseconds ≅ 75 m/s). D, Electrical field responses were elicited by stimulating the right laryngeal nerve of a patient with normal pre- and postoperative vocal cord function. Consecutive original traces are superimposed; the upper 3 traces are 3 consecutive recordings obtained by stimulation at a site above the crossing point of the inferior thyroid artery. The electrode was moved about 3 cm (ie, below the inferior thyroid artery) and 4 consecutive traces were recorded; the 2 vertical lines were drawn to illustrate the difference in time-to-peak of the electrical field responses. This difference (= 0.37 milliseconds) can be used to estimate the nerve conduction velocity (∼3 cm/0.37 milliseconds ≅ 81 m/s).