Investigators at the Institute of Neurology, University College London, UK, and centers in Slovenia and Spain compared electrophysiological features of primary and secondary dystonia, using transcranial magnetic stimulation of motor cortex and eye blink conditioning. Eleven patients with hemidystonia secondary to basal ganglia or thalamic lesions were tested over both hemispheres, corresponding to the affected and non-affected sides, and compared with 10 patients with primary segmental dystonia with arm involvement and 10 healthy controls. All subjects were tested as adults. The average age at onset of secondary dystonia was 13.6 years (range 1–55; 5 were 1-2 years). The causes of secondary dystonia were perinatal HII in 5, ischemic stroke in 5, and encephalitis in 1.
No differences in motor thresholds were detected between patients with secondary and primary dystonia or controls. In secondary dystonia, short interval intracortical inhibition was reduced on the affected side, but normal on the non-affected side; cortical plasticity and eye blink classical conditioning were normal. In contrast, patients with primary dystonia showed increased cortical plasticity and reduced eye blink classical conditioning. Dystonia is a motor symptom that reflects different pathophysiological mechanisms. 
COMMENT. Adams and Victor’s Principles of Neurology  lists 4 main groups of diseases characterized by dystonia: 1) Hereditary and Degenerative dystonias (dystonia musculorum deformans, Huntington chorea, Juvenile dystonia – Parkinson syndrome [L-dopa responsive]), 2) Drug-induced (phenothiazine, haloperidol), 3) Symptomatic (secondary) (Wilson disease, double athetosis cerebral palsy due to cerebral hypoxia, kernicterus), and 4) Idiopathic focal dystonias (spasmodic torticollis, blepharospasm, hemifacial spasm, oromandibular, spasmodic dysphonia, writer’s cramp).