Spasmodic torticollis is a form of dystonia in which involuntary spasm of the neck muscles causes the head to assume abnormal positions, sometimes with shaking movements. These movements are frequently painful. There are three different varieties of the condition: tonic, causing sustained turning of the head to one side; clonic, causing shaking movements of the head; and mixed tonic and clonic, involving both kinds of movements. The exact cause of this condition is generally unknown, and it tends to appear in adults. Symptoms usually start in the fourth and fifth decades. There are a number of types of spasmodic torticollis and the genetic basis for these is still being researched.