INTRODUCTION
After parkinsonism, dystonia is the most common movement disorder encountered in movement disorder clinics. The term dystonia was coined by Oppenheim in 1911 to indicate that the disorder he was describing manifested hypotonia at one occasion and tonic muscle spasms at another, usually but not exclusively elicited on volitional movements. Although the term dystonia has undergone various definitions since 1911, the most recent definition is: “Dystonia is a movement disorder characterized by sustained or intermittent muscle contractions causing abnormal, often repetitive, movements, postures, or both. Dystonic movements are typically patterned, twisting, and may be tremulous. Dystonia is often initiated or worsened by voluntary action and associated with overflow muscle activation.”
Limb, axial, and cranial voluntary muscles can all be affected by dystonia. The abnormal movements are often exacerbated during voluntary movements, so-called action dystonia.
If the dystonic contractions appear only with a specific action, it is referred to as task-specific dystonia (e.g., writer’s cramp and musician’s cramp). As the dystonic condition progresses, voluntary movements in parts of the body not affected with dystonia can induce dystonic movements of the involved body part, so-called overflow. Talking is a common activity that causes overflow dystonia in other body parts. With further progression, the affected part can develop dystonic movements while at rest, and sustained abnormal postures may be the eventual outcome.
Dystonic movements tend to increase with fatigue, stress, and emotional states; they tend to be suppressed with relaxation, hypnosis, and sleep. Dystonia often disappears during deep sleep, unless the movements are extremely severe. A characteristic and almost unique feature of dystonic movements is that they can be diminished by tactile or proprioceptive “sensory tricks” (gestes antagoniste). For example, patients with cervical dystonia (torticollis) often place a hand on the chin or side of the face to reduce nuchal contractions, and orolingual dystonia is often helped by touching the lips or placing an object in the mouth. Lying down may reduce truncal dystonia; walking backward or running may reduce leg dystonia.
Rapid muscle spasms that occur in a repetitive pattern may be present in torsion dystonia; when rhythmic, the term dystonic tremor is applied. Rarely, some children and adolescents with primary or secondary dystonia may experience a crisis, a sudden increase in the severity of dystonia, which has been called dystonic storm or status dystonicus. This can cause myoglobinuria with a threat of death by renal failure. These patients require treatment in an intensive care unit (ICU) using sedating agents such as propofol and midazolam, intrathecal baclofen, and in some cases, emergency deep brain stimulation of the internal globus pallidus.
EPIDEMIOLOGY
Epidemiologic studies in dystonia typically segregate patients with dystonia into primary (no known cause or no known lesion) and secondary (an environmental or hereditary lesion in the brain) and into focal, segmental, and generalized forms of dystonia. An epidemiologic study of primary dystonia in the population living in Rochester, Minnesota, found the prevalence of generalized dystonia to be 3.4 per 100,000 population and the prevalence of focal dystonia to be 30 per 100,000. The frequency of primary dystonia in the Ashkenazi Jewish population is much higher (between 1/6,000 and 1/2,000) because this population descends from a limited group of founders of the DYT1 mutation. The origin of the mutation was traced to the northern part of the historic Jewish Pale of settlement (Lithuania and Byelorussia) approximately 400 years ago. Focal dystonia is more common than segmental dystonia, and generalized dystonia is very infrequent, about one-tenth as common as focal dystonia. The prevalence rate of focal dystonias varies in different countries, being slightly lower in Japan (between 6 and 14 per 100,000) than in Western Europe (between 11 and 14 per 100,000).
