Continuous Vagus nerve monitoring using the LAR is a recently developed technique for the intra-operative monitoring of recurrent laryngeal nerve during thyroid surgeries.

LAR is a vagus-vagal reflex necessary for airway protection that is organized by the brainstem. It consists of an early R1 and late R2 responses that prevent aspiration by causing thyroarytenoid and lateral cricoarytenoid muscles contraction and thus vocal fold closure. The afferent sensation from the glottis and supraglottis mucosa travels through the internal branch of the superior laryngeal nerve (a branch of the vagus nerve) to the nucleus of the solitary tract. The motor neurons contributing to the efferent arc, through the vagus nerve, lie in the nucleus ambiguous.

Using the electrodes on an EMG endotracheal tube, vocal fold mucosa is electrically stimulated. Contralateral R1 and R2 responses resulting in vocal fold adduction are recorded using the endotracheal tube electrodes contralateral to the stimulation site.

We have successfully monitored a large series of patients using this methodology during neck endocrine procedures. Monitorability of the LAR in the First 100 patients in series was 85%. After the implementation of technical improvements described here in detail, the monitorability of the LAR in the following group of patients increased to almost 100%. Mean amplitude of LAR at opening was 398 μV (SD 191). A decrement of the LAR amplitude > 60% correlated with post-operative weakness of the vocal folds. 

The LAR is a comprehensive tube-based methodology for evaluating the integrity of the Vagus nerve and laryngeal brainstem reflex. Advantages over current monitoring techniques include simplicity, ability to continuously monitor neural function without placement of additional neural probes and the ability to assess the integrity of both sensory and motor pathways.