The afferent control of glottic closure during swallowing is not well understood. The internal branch of the superior laryngeal nerve (iSLN) mediates sensation of the larynx, with no known motor function in humans (Stockwell et al. 1995). This study was designed to evaluate whether iSLN anaesthesia alters the swallow sequence and the co-ordination of swallowing with breathing.

The institutional ethics committee approved the protocol and the subjects gave written, informed consent. In 16 healthy subjects (20-51 years old), we recorded submental electromyograms (EMG), oronasal airflow, and videofluoroscopic images of the upper airway. Swallowing was initiated spontaneously, was visually cued, or followed a brief (0.5, 0.75, 1 s) infusion (4-5 ml) of liquid barium onto the tongue. The iSLN was anaesthetized bilaterally by transcutaneous injection of bupivacaine in 11 subjects. Five subjects received saline injections. Laryngeal sensation, anatomy and vocal cord movements were evaluated by laryngoscopy. Comparisons of swallowing were made within subjects (injection vs. control, i.e. no injection) and between subjects (anaesthetic vs. saline injection).

In the anaesthetized state (303 swallows), all 11 subjects reported difficulty executing the swallow sequence, but they had no difficulty performing other tests of glottic motor function (e.g. Valsalva, cough). Ten subjects exhibited penetrations (n = 124, Fig. 1) of barium into the larynx typically during the pharyngeal phase of swallowing. Of these penetrations, 76 led to aspiration below the vocal cords in eight subjects without adverse effects. In the unanaesthetized state (saline, 187; control, 437 swallows), there were eight penetrations and no aspirations or symptoms of dysphagia. The increased incidence of penetration or aspiration with iSLN anaesthesia is significant compared with control (P < 0.002, paired Wilcoxon test) and compared with saline (P < 0.02, Mann-Whitney test). The timing of other pharyngeal swallowing events (e.g. duration of bolus transit) and the co-ordination of swallowing with breathing do not appear to be altered by iSLN anaesthesia.Our findings show that iSLN integrity is required for the normal glottic closure reflex during swallowing, and suggest that a laryngeal afferent mechanism is necessary for protection against aspiration.This work was funded by the NIH (HL49848).

Figure 1. Timing of barium entry into the larynx during swallowing and respiration. A, penetration during a single swallow. B, distribution of all penetration times relative to the onset of the pharyngeal phase of deglutition. e.m.g., submental electromyogram; (…) zero airflow; I, inspiration; P, penetration; BT, bolus transit; HMa, anterior movement of hyoid.

Stockwell, M., Lozanoff, S., Lang, S.A. & Nyssen, J. (1995). Clin. Anat. 8, 89-95.