Dystonia (from Greek, meaning altered muscle tone) refers to a syndrome of involuntary sustained or spasmodic muscle contractions involving co-contraction of the agonist and the antagonist. The movements are usually slow and sustained, and they often occur in a repetitive and patterned manner; however, they can be unpredictable and fluctuate.
Signs and symptoms
The symptoms of dystonia can vary, depending on the type of dystonia and when it develops. In early-onset dystonia, the symptoms begin during childhood or early adulthood. Symptoms usually start in the legs or arms, before spreading to other limbs and sometimes the upper part of the body.
Although dystonic reactions are occasionally dose related, these reactions are more often idiosyncratic and unpredictable. They reportedly arise from a drug-induced alteration of dopaminergic-cholinergic balance in the nigrostriatum (ie, basal ganglia). Most drugs produce dystonic reactions by nigrostriatal dopamine D2 receptor blockade, which leads to an excess of striatal cholinergic output. High-potency D2 receptor antagonists are most likely to produce an acute dystonic reaction.
The incidence of acute dystonic reactions varies according to individual susceptibility, drug identity, dose, and duration of therapy. The actual incidence of dystonic reactions is unknown, owing to misdiagnosis and underreporting.