Dystonia redefined as central non-paretic loss of control of muscle action: a concept including inability to activate muscles required for a specific movement, or 'negative dystonia'.

Dystonia is defined as a syndrome of sustained muscle contractions, frequently causing twisting and repetitive movements, or abnormal postures. Although this definition comprises an essential feature of dystonia, the clinical observation indicates that there is an additional aspect of dystonia; failure to adequately activate muscles required for specific movement, exemplified by the lack of contractions of the levator palpebrae superioris muscles in apraxia of lid opening, as well as by inability to activate appropriate muscles in cervical dystonia or in the paretic form of writer's cramp, and possibly by dropped head syndrome or camptocormia seen in parkinsonian patients without apparent truncal dystonia or rigidity. Taking this "negative dystonia" into consideration, the author proposes a revised definition of dystonia as a symptom characterized by the central non-paretic loss of voluntary control of muscle activities, which may result in either excessive or deficient contractions of muscles, frequently causing twisting and repetitive movements, limitation of movements, or abnormal postures.