Focal dystonias of the hand and upper extremity.

Hand dystonia can cause substantial functional disability and is frequently misdiagnosed. In general, dystonia is a disabling disorder of motor control characterized by excessive muscle contractions that can produce involuntary movements and abnormal postures. Prevalence of dystonia can be as high as 1 in 2500 people. Dystonias can be classified based on etiology, age of onset, anatomical part affected, and their tempo. Etiologically, they are classified as primary when the dystonia is the main sign and the cause is genetic or unknown and secondary when there are other disease manifestations and the cause may be identifiable. Dystonias that start before 27 years of age are called childhood-onset dystonia; they usually start in the lower limbs, trunk, or upper extremities and frequently spread to the rest of the body. Adult-onset dystonias usually begin in the upper half of the body, and the risk of progression to other body parts depends on the anatomic site of onset. Anatomically, dystonias can be focal (1 body part), segmental (2 or more contiguous body parts), multifocal (2 noncontiguous areas), hemidystonia, or generalized. Based on their tempo, dystonias can be constant, intermittent, or situational, the latter including task-specific dystonias. The rest of the review focuses on the clinical manifestations, differential diagnosis, pathophysiology, and treatment of hand dystonia.