Vagus nerve/recurrent laryngeal nerve ratio: proposal of a new parameter predicting left vocal cord palsy using intraoperative nerve monitoring during esophagectomy

Aim Intraoperative nerve monitoring (IONM) during esophageal cancer surgery can help to identify and preserve the recurrent laryngeal nerve (RLN). To devise a useful parameter for prediction of left vocal cord palsy (VCP), we measured the electromyographic (EMG) amplitude of the left RLN and vagus nerve (VN) using intermittent IONM. Methods We studied 35 consecutive patients who underwent esophagectomy with lymph node dissection around the left RLN. After lymph node dissection, the left RLN and left VN were stimulated, and the EMG amplitude was measured using IONM. The VN/RLN ratio (V/R ratio) was calculated, and the presence of left VCP, diagnosed by laryngoscopy on the first postoperative day, was compared among the patients. Results Ten of the 35 patients (28.6%) had left VCP. In the VCP and non-VCP groups, the left VN amplitude was 190.0 (0-1111) mu V and 520.0 (120-1200) mu V (P = 0.006), and the VR ratio was 0.26 (0-0.75) and 0.71 (0.24-1.0) (P < 0.001), respectively. Receiver operating characteristic curve analysis using the left VN amplitude and V/R ratio showed an area under the curve (AUC) of 0.80 with a cutoff of 354 <mu>V, and an AUC 0.90 with a cutoff of 0.50, respectively(P = 0.05). When left VN amplitudes of < 100 mu V, < 354 mu V, and a V/R ratio of <= 0.50 were defined as left VCP, the accuracy was 80.0%, 74.2%, and 88.6%, respectively. Conclusions Using intermittent IONM, the V/R ratio with a cutoff value of 0.50 has the potential to be a more useful parameter for prediction of VCP after esophagectomy than EMG amplitude during VN stimulation.