The purpose is to determine the neurophysiological bases of phonatory control in voice disorders. The primary emphasis has been on spasmodic dysphonia, an idiopathic laryngeal focal dystonia. Following a previous study of respiratory control during phonation in spasmodic dysphonia, a second study examined whether respiratory control was also affected during whisper, a nonphonatory speech task. Both in phonation and whisper, patients were slower, uncoordinated, and had abnormal respiratory movements in comparison with normal. These results suggest that this is a motor control disorder often involving other areas in addition to the larynx, and is not task specific. Neurophysiological studies of the intrinsic laryngeal muscles demonstrated increased tone both at rest (during respiration) and on phonation in spasmodic dysphonia. In the normal controls, diazepam reduced tone in laryngeal muscles in both respiration and phonation. No effects were found in the patients. An experimental temporary nerve block in patients altered the muscle activation levels on the opposite side of the larynx, suggesting that muscle spindle feedback is important in the pathophysiology of this disorder. Quantitative and objective procedures have been developed for evaluating treatments with this disorder. Patients are highly variable over time and multiple baseline measures are required.
