True dystonia is defined as a movement disorder characterised 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.
Functional dystonia may affect any part of the body or be generalised. Features that suggest a functional etiology include inconsistent dystonia movements over time, dystonia presenting as a fixed posture or a paroxysmal disorder (not due to a known paroxysmal movement disorder), the presence of incongruous dystonic movements and postures (eg, facial grimacing with pulling of the mouth to one side), excessive slowness, marked resistance to passive movements, and multiple somatizations.
While not necessarily pathognomonic, additional features suggesting functional dystonia include fixed foot dystonia in an adult, the presence of other atypical movements or postures such as a bizarre gait (not fitting the gait pattern of other movement disorders, including true dystonia), dystonia occurring when the affected body part is at rest, and pain as a prominent aspect.
Muscle atrophy or joint contractures may be present in longstanding functional dystonia due to disuse or prolonged maintenance of the tonic posture. Muscle hypertrophy may be present due to sustained contracture. Since true dystonia historically has been misdiagnosed as functional dystonia, the examiner should be aware of this diagnostic pitfall.