Spasmodic dysphonia (SD) is a rare dystonia of the larynx. A dystonia is a neuromuscular disorder that causes involuntary contraction of a muscle or a group of muscles. Dystonias may be focal (as in SD, which only affects the larynx), or generalised. SD is characterised by spasms of the intrinsic muscles of the larynx that result in voice breaks in running speech. Two types of spasmodic dysphonia are recognised:
Adductor spasmodic dysphonia, which is characterised by spasm of the adductor muscles of the larynx (principally the thyroarytenoid and lateral cricoarytenoid muscles). This type of SD results in a staccato-sounding voice, or sometimes a “pressed” quality to the voice.
Abductor spasmodic dysphonia, which results from spasm of the vocal fold abductors (posterior cricoarytenoid muscles), causes a quality of voice that is characterised by breathy breaks in the fluency of speech.