Is tremor in dystonia a phenotypic feature of dystonia?

To understand better the features and mechanisms distinguishing tremor in dystonia, we reviewed the epidemiologic, clinical, and neurophysiologic data in patients with dystonia and tremor. Clinical studies suggest that tremor starts at or after dystonia onset in body parts affected or unaffected by dystonia. Tremor in dystonia manifests during posture or voluntary movements even though some dystonic patients may have tremor at rest. Prevalence rates for tremor in dystonia are higher in patients with adult-onset dystonia and cervical dystonia than in other dystonias and highest in patients in whom dystonia spreads. Neurophysiologic investigations in patients with dystonia and tremor show reduced reciprocal inhibition between agonist and antagonist upper limb muscles, a lack of brainstem interneuronal inhibition, and abnormal sensory integration. The neurophysiologic abnormalities in patients with dystonia and tremor resemble those in dystonia but differ from those described in essential tremor. Tremor is a phenotypic motor feature in dystonia.