CMCT Central motor conduction time DBS Deep brain stimulation MEP Motor-evoked potential PVL Periventricular leukomalacia AIM Dystonia in childhood has many causes. Imaging may suggest corticospinal tract dysfunction with or without coexistent basal ganglia damage. There are very few published neurophysiological studies on children with dystonia; one previous study has focused on primary dystonia. We investigated central motor conduction in 62 children (34 males, 28 females; age range 3-19y, mean age 10y 8mo, SD 4y 8mo) with severe dystonia to evaluate corticospinal tract integrity before consideration for deep brain stimulation.METHOD Distal motor and F-wave latencies were measured in the ulnar and ⁄ or posterior tibial nerves. Transcranial magnetic stimulation was applied over the motor cortex and motor-evoked potentials were recorded in the activated abductor digiti minimi and ⁄ or abductor hallucis muscles. Central motor conduction time (CMCT) was calculated using the F-wave method.RESULTS CMCT was normal in 50 out of 62 patients; 12 patients showed prolonged CMCT to upper and ⁄ or lower limbs. Most children with severe primary and secondary dystonia had normal CMCT, indicating corticospinal tract integrity despite abnormal imaging in 42 out of 50 patients. Abnormal CMCT was found in two out of 12 patients with normal imaging.INTERPRETATION This study provides new CMCT data for children with severe primary and secondary dystonia. Over 50% of children with evidence of periventricular white-matter damage from magnetic resonance imaging had normal CMCT, challenging traditional pathophysiological models. This is consistent with recent diffusion tensor imaging in children with periventricular white-matter damage, showing disruption of sensory connections rather than corticospinal tract damage. CMCT helps refine our understanding of imaging changes in complex motor disorders of childhood.Dystonia is defined as 'a neurological syndrome characterized by involuntary, sustained, patterned, and often repetitive muscle contraction of opposite muscles, causing twisting movements or abnormal postures during motor tasks'. 1 The underlying pathophysiology of dystonia is not fully understood. Some primary dystonias in adults are characterized by reduced cortical inhibition and increased cortical plasticity using paired associated stimulation studies. 2 However, there are very few neurophysiological studies in children with dystonia, or in patients of any age with secondary dystonia. Deep brain stimulation (DBS) of the globus pallidus internus is now a well established management tool for both adults and children with dystonia. [3][4][5][6] Best DBS results are obtained in patients with primary generalized dystonias, in whom dramatic results have been observed, with up to 88% improvement in some children with this condition, as assessed by the Burke-Fahn-Marsden dystonia rating scale. 3,6,7 Reported levels of improvement with DBS in secondary dystonia vary widely between individual patients from 0 to 71%, 4,7,8 averaging arou...