Definition, classification, symptoms and diagnostic
The general term dystonia is used to describe a syndrome characterised by involuntary sustained muscle contractions, frequently causing twisting and repetitive movements, or abnormal postures [1, 2]. If these symptoms are restricted to one body part, the syndrome is termed ‘focal dystonia’. In task specific focal dystonia the most prominent characteristic is the degradation and loss of voluntary control of highly overlearned complex and skilled movement patterns in a specific sensory-motor task.
Dystonia can be classified by age of onset, cause, or by distribution of the body parts affected. Dystonia localised to a single body part such as the hand or neck is referred to as focal. Focal dystonia is by far the most frequent form of dystonia, accounting to about 90% of all dystonia syndromes. Dystonia localised to two contiguous body parts is referred to as segmental dystonia. Dystonia affecting body parts that are not next to each other is referred to as multifocal dystonia. Dystonia affecting one segment and another body part is classified as generalised dystonia. If it affects only one half of the body it is called hemidystonia.
Focal dystonia may be classified according to four criteria: age of onset, aetiology, affected body region and severity of symptoms . According to Fahn, classification by age of onset represents the best prognostic indicator as to whether there will be a spread of dystonic symptoms to other body parts. While it is acknowledged that an age-criterion is rather arbitrary, onset before 28 (median of 9 years) is classified as early-onset primary dystonia and thereafter as late-onset dystonia (median of 45 years).
Second, current classification for aetiology divides focal dystonia into just two major categories, idiopathic or primary (including familial and sporadic forms), and symptomatic or secondary . Secondary focal dystonia can be caused by structural abnormalities of the brain, or by metabolic disorders. For example, focal dystonia may occur as an early sign of Wilson’s disease, a defect of copper metabolism that causes abnormal liver function and central nervous system symptoms such as tremor, and dystonia.