Spasmodic dysphonia is a neurologic disorder that science does not yet know the cause or exactly how it works. Despite this lack of knowledge, there is a good treatment for the symptoms of the disorder. When a single muscle or small group of muscles contract spontaneously and irregularly without voluntary control, those muscles are dystonic. While there are dystonias where a large number of muscles or a complete region of the body is involved, focal dystonias are limited to a small area or single muscle. Examples would include torticollis where a spasm of a neck muscle causes the head to rotate.
Blepharospasm is when the muscle around the eye spontaneously twitches.
Writers cramp is when the muscles of the hand spasm. Spasms of the muscles in the voice box are a laryngeal dystonia. Early in the twentieth century, a physician thought the disorders were a problem with muscle tone and the name has stuck even though entymologically incorrect. There are several types of laryngeal dystonia. The most common type is when the muscles that bring the vocal folds together for speaking intermittantly spasm.