The term dystonia can be used to describe a phenomenology, a syndrome with several possible etiologies or a specific etiological diagnosis. A dystonic syndrome is characterized by the presence of prolonged muscle contractions involving agonist and antagonist muscles (co-contraction) that cause sustained postures and abnormal movements. Typically, dystonic movements are repeated in the same pattern involving the same muscle groups, which allows them to be differentiated from other involuntary movements such as chorea or tics. The clinical presentation of dystonia is heterogeneous and is therefore often misdiagnosed. Tremor is not uncommon in patients with dystonia. In 25% of patients, postural tremor of the hands identical to essential tremor coexists; in other patients, the tremor is slower and localized in the areas affected by dystonic spasms (dystonic tremor).
Dystonic spasms often appear or are aggravated when the patient performs a movement (action dystonia). Characteristic is the extension of spasms to adjacent regions when performing a motor act (e.g., shoulder spasms during writing in a subject with hand dystonia). Mirror movements are also common (e.g., spasms during writing in the hand contralateral to the dystonic hand). The triggering motor act can sometimes be a highly specific act. Thus, for example, writing or playing the guitar may trigger dystonia in one hand. Another characteristic of dystonia is its improvement when the patient applies certain sensory stimuli, often close to the area of spasm.
An example of these sensory “tricks” (geste antagonistique) is the improvement sometimes observed in torticollis when the patient applies light pressure to the chin, or in blepharospasm, when a light rubbing is applied to the temporal region of the skull. Dystonic spasms typically worsen with stress and fatigue and improve or disappear with relaxation and sleep.
Classification
The clinical features of dystonia are extremely varied in terms of age of onset, body region affected, and degree of generalization and severity. In addition, more environmental and genetic causes are constantly being identified. To cope with this clinical and etiological heterogeneity, several classifications have been proposed. The most recent focuses on clinical features and classifies dystonia as isolated (dystonia is the only manifestation) or combined (if there are also other neurological or systemic signs). Depending on the extension, a distinction is made between focal, segmental, generalized or multifocal dystonia, and according to the etiology it can be hereditary (of known genetic origin), acquired (or symptomatic dystonia) or idiopathic (of unknown cause). Another complementary etiological feature is the existence or not of identifiable pathological changes.
Epidemiology
The true prevalence is unknown. Among the dystonic patients evaluated in the Neurology Department of the Hospital Clínic de Barcelona, 70% suffer from idiopathic dystonia and the most frequent acquired forms are those induced by drugs, perinatal asphyxia, cranioencephalic trauma, cerebral ictus, encephalitis, tumors or vascular malformations.
The prevalence of childhood generalized dystonia has been estimated at 3.6 patients per 100,000 population and adult focal dystonia at 30 cases per 100,000 population. Childhood and adolescent dystonia is three times higher among the Jewish population of Ashkenazi descent. In adults, it is more frequent in women than in men (2:1).