Dystonias

Dystonia is described as a motor syndrome characterized by involuntary prolonged muscle contractions causing sustained twisting movements and abnormal postures of affected body parts. The main characteristics of dystonia in surface EMG investigations are cocontractions of antagonistic muscles and overflow activities in distant muscles. Voluntary ballistic movements of affected body parts are slowed and the three-burst pattern in agonist and antagonist muscles is prolonged. A number of brainstem and spinal cord reflex investigations demonstrated an abnormal decrease of inhibitory interneuronal activity. The decrease of inhibitory action may contribute to the genesis of dystonia and the disturbance of voluntary movements but cannot be the main cause of dystonia because a decrease of inhibitory action can also be present in unaffected extremities of dystonic patients. Abnormalities of cortical function have been described in functional imaging and neurophysiological studies in patients with various forms of dystonia. Magnetic brain stimulation of the motor cortex revealed changes in motor cortical excitability and activity of intracortical inhibitory interneuronal activity. A decrease in surround inhibition is a modern hypothesis. Most probably, dystonia results from a functional disturbance within the basal ganglia either induced by lesions of basal ganglia or the thalamus including connection between them (secondary dystonias) or by genetically determined abnormalities of proteins with resulting abnormal neuronal activities without degeneration of the affected cells. Overuse dystonia may have a different physiological origin such as exaggerated use-dependent plastic changes within the sensorimotor cortex.