Unearthing a consistent bilateral R1 component of the laryngeal adductor reflex in awake humans

The laryngeal adductor reflex (LAR) is an essential tracheobronchial protective mechanism resulting in vocal fold adduction to laryngeal stimulation. It was thought to consist of an early ipsilateral R1 component and a later, bilateral but highly centrally modulated R2 component. We recently demonstrated that bilateral R1 responses are robustly present in humans under general anesthesia. We herein give evidence that the R1 response is also bilateral in awake humans and is likely the primary component responsible for initiating the LAR.

Seven volunteers were included (3 males, 4 females). The reflex was elicited by direct percutaneous monopolar needle stimulation of the internal superior laryngeal nerve. Electromyography traces from bilateral lateral cricoarytenoid muscles were recorded using hookwire electrodes. Reflex responses to variations in stimulus intensity and duration were evaluated.

Bilateral R1 responses were recorded in all patients, even during deep inspiration when the vocal folds were maximally abducted. R1, but not R2, responses increased linearly in amplitude, with sequential increases in both stimulation intensity (1-8 mA) and duration (100-500 µsec) (Pearson correlation 0.94).

Contradicting over 40 years of research, we demonstrate that the R1 LAR component is consistently bilateral in awake humans. It increases linearly with stimulus intensity and is unaffected by conscious state suggesting minimal central control. These findings may provide a means to objectively stratify patients for risk of laryngeal aspiration, even in unconscious states, and its potentially cardinal role in disease states such as laryngospasm and sudden infant death needs to be reevaluated.

4. Laryngoscope, 2581-2587, 2018.