Continuous Vagal Intraoperative Monitoring Prevents Recurrent Laryngeal Nerve Paralysis by Revealing Initial EMG Changes of Impending Neuropraxic Injury

Abstract

The larynx is the phonating mechanism, specifically designed for voice production.  Through movement of the cartilages, the larynx varies the opening between the vocal cords and thereby varies the pitch of sounds produced by the passage of air through them.  The human larynx receives innervations from the vagus nerve via the superior laryngeal nerve (SLN) and the recurrent laryngeal nerve (RLN).  The reported prevalence of recurrent laryngeal nerve injuries after thyroid surgery varies widely- the average temporary vocal cord palsy (VCP) rate is approximately 9.8%. The rate of permanent VCP ranges from 0%- 18.6%.  Due to the RLN vulnerability and variable anatomic course, the chief controversy in thyroid surgery for years revolved around the issue of RLN preservation through visual identification during surgery.  Several single centre and multicentre studies have confirmed that RLN integrity is preserved significantly more often with routine visual identification than without it.  Various intraoperative medical devices have been developed over the past two decades or so to help identify the RLN. Essentially these devices convert laryngeal muscle activity into audible plus or minus visual electromyographic (EMG) signals  Recent studies have shown the intraoperative nerve monitoring (IONM) can help improve RLN identification and may also help to recognize impending nerve injury Studies consistently show a high negative predictive value of 92-100% for IONM.- Thus patients with a negative IONM (intact nerve function) after thyroid surgery generally will not have a RLN palsy.  The risk of RLN dysfunction is substantial with a loss of IONM signal.  Injury to the RLN usually results from severing, crushing, suturing, stretching or tearing the main trunk or its branches.  Current IONM devices only allow the surgeon to intermittently stimulate and assess RLN function which allows the nerve to be theoretically at risk for damage in-between stimulations.  The main advantage of a continuous nerve monitor device is that it has the potential to identify injury to the RLN earlier and thus the nerve trauma maybe reversible  The Medtronic Automatic Periodic Stimulation (APS) electrode provides periodic, low-level stimulation of the vagus nerve and is designed to potentially allow detection of recurrent nerve injury via early warning of changes in nerve amplitude and latency waveforms.  To identify clinically relevant significant adverse EMG signal changes we categorized EMG signals based on vagal evoked signal amplitude and latency waveform characteristics. Single events were defined as EMG changes affecting either amplitude or latency.  Combined events (CE) were defined as concordant changes in both signal amplitude and latency and are stratified below into mild (mCE) and severe CEs (sCE).  We hypothesized that such combined events may more reliably track with impending neuropraxia  VCP is the outcome of neural injury which can be identified by an intraoperative progression of increasing severe adverse evoked EMG events. Our study shows CIONM allows identification of the EMG changes (sCE) heralding imminent RLN injury.