Laryngeal reflex responses continued to be investigated in an effort to understand the mechanisms underlying cough and the modulation of afferent reflex responses during respiration and speech. Bilateral long latency thyroarytenoid responses at 60 ms occur in response to superior laryngeal nerve stimulation and result in vocal fold adduction in the awake human. This response (R2) seems neurophysiologically independent of an earlier ipsilateral response at 17 ms, R1. The inhibition of the early R1 and late bilateral R2 responses was studied in normal volunteers using repeated electrical stimulation of the superior laryngeal nerve. R1 responses were increasingly reduced in amplitude with repeated stimulation at inter- stimulus intervals below 1 ms while R2 responses were reduced in frequency of occurrence primarily at inter-stimulus intervals of 500ms. The same reflex responses were studied in patients with adductor spasmodic dysphonia and differed from normal: R1 responses were slower and reduced in amplitude; and R2 responses were increased in frequency. Inhibitory mechanisms controlling R1 and R2 were affected in the patients. R1 responses were not reduced with repeated stimulation and in some patients increased with repeated stimulation below 1 ms while R2 responses were not reduced in frequency at 500 ms. Investigations are continuing to determine if laryngeal reflex disinhibition is specific to adductor spasmodic dysphonia and could be used as an objective criterion for identification of patients with this disorder. Currently patient identification is based on voice symptoms alone.
