Dynamic aspects of phonatory control in spasmodic dysphonia.

To understand the voluntary laryngeal movement disorder in spasmodic dysphonia (SD), SD patients were compared with normal controls on speech tasks with different laryngeal motor-control demands. Nine patients with idiopathic chronic SD and no other speech, otolaryngologic, neurologic, or psychiatric disorders were compared with 15 control subjects who were free of such disorders. Speech production tasks required different degrees of dynamic and precise control of vocal fold movement. Phonatory off times were increased in the SD patients, while maximum phonation time, phonatory on time, frequency and intensity control, and reaction times for CV syllables were not affected. On a reaction-time task, the onset of laryngeal movement was not delayed in the SD patients, however, the time between the onset of laryngeal movement and phonatory onset was significantly increased in the SD patients in comparison with the controls. Therefore, SD patients had no difficulty with the onset of laryngeal movement but were slow to achieve phonation, indicating a movement-control disorder affecting vocal fold adduction for phonation onset.