Prognostic value of subthreshold biphasic electromyographic (EMG) waveforms in Type I loss of signal during thyroid intraoperative neuromonitoring

Background: During intraoperative neuromonitoring (IONM), an evoked amplitude below 100 µV is typically regarded as loss of signal (LOS). However, after adjusting the event threshold, previously undetectable electromyographic (EMG) waveforms may become visible. This study aimed to evaluate the diagnostic and clinical significance of such subthreshold EMG waveforms. Methods: A total of 231 patients (335 nerves at risk, NARs) who underwent thyroidectomy with IONM using either EMG endotracheal tube electrodes or adhesive skin electrodes were retrospectively reviewed. Evoked EMG parameters, including mean amplitude and latency after stimulation of the recurrent laryngeal nerve (RLN) and vagus nerve (VN), were analyzed. Preoperative and postoperative laryngoscopic examinations were performed to assess vocal fold mobility. Results: Among the 335 NARs, twelve NARs (3.5%) demonstrated evoked EMG amplitudes below 100 µV and were confirmed as true LOS, all of which were classified as Type I (segmental) LOS. After stepwise lowering the event threshold from 100 µV to 80 and subsequently to 50 µV, biphasic EMG waveforms became detectable in 10 of these 12 cases. Although all 12 nerves exhibited immediate postoperative vocal fold paralysis, the 10 nerves with detectable low-amplitude waveforms showed complete recovery of vocal fold mobility within eight months postoperatively. Conclusions: In cases of Type I LOS during thyroid surgery, the presence of subthreshold biphasic EMG waveforms may reflect residual neural conduction and is strongly associated with postoperative vocal fold recovery. These findings suggest that optimization of threshold settings can improve the sensitivity of IONM interpretation and assist in intraoperative prognostic assessment.