Studies of normal laryngeal physiology are aimed at characterizing responses to afferent an efferent stimulation. A long latency response between 55 and 79 ms has been identified in normal controls-in response to stimulation of either supraglottic, glottic or subglottic afferents. The response occurs simultaneously on both sides in the thyroarytenoid and ipsilaterally in the cricothyroid muscles. Work was initiated on biomechanical modeling of the larynx in collaboration with the University of Mar and. Focus has been on the action of the cricoarytenoid joint, Fresh tissue dissection has suggested a single axis of rotation of the arytenoid around the cricoid from a superior and lateral position for abduction, to a medial and inferior position for adduction with very limited sliding action. Motor unit analyses in the laryngeal muscles during quiet respiration has demonstrated both tonic and phasically active units in the cricothyroid and thyroarytenoid muscles. The phasic units were activated only during inspiration in both the cricothyroid and thyroarytenoid muscles while some units were active only during expiration in the thyroarytenoid. Studies of muscle activation abnormalities in abductor spasmodic dysphonia (ABSD), demonstrated a variety of muscle abnormalities in these patients. Some had bursting in the posterior cricoarytenoid, others, the cricothyroid, while others had primarily strap muscle activation abnormalities. Treatment with botulinum toxin injection was evaluated in selected ABSD patients with cricothyroid involvement. Significant speech improvement occurred only in patients who had abnormalities confined to that muscle. A double blind cross-over study of bilateral thyroarytenoid injections of botulinum toxin in adductor spasmodic dysphonia demonstrated greater speech improvement following toxin than following saline.
