The neurophysiology of dystonia.

Any model for the physiology of dystonia must be able to explain how dystonia can be produced in various circumstances. Brain lesions can cause dystonia; responsible sites include the basal ganglia, brainstem, and thalamus, but the most common site is the putamen. Dystonia can be hereditary, and genetic linkage has been found for both generalized and focal dystonia. The only genetic dystonia for which the gene product is known is Segawa disease, a hereditary progressive dystonia with marked diurnal fluctuation. The defect is in guanosine triphosphate cyclohydrolase I, a gene that makes a cofactor for the synthesis of dopamine, which explains why this form of dystonia should be amenable to treatment with levodopa. Another example of dystonia in which a disorder of dopamine pharmacology appears responsible is the dystonia occurring in Parkinson disease, either spontaneously or as a result of treatment. Curiously, the dystonia occurs at both peak and trough dopamine levels.