Abnormal dorsal premotor-motor inhibition in writer's cramp.
The authors hypothesized that a deficient premotor-motor inhibitory network contributes to the unwanted involuntary movements in dystonia. The authors studied nine controls and nine patients with writer's cramp (WC). Dorsal premotor-motor cortical inhibition (dPMI) was tested by applying conditioning transcranial magnetic stimulation (TMS) to the dorsal premotor cortex and then a test pulse to the ipsilateral motor cortex at an interval of 6 ms. The authors used an H-reflex in flexor carpi radialis paired with TMS over the premotor cortex to assess for spinal cord excitability change. Finally, the authors interrupted a choice reaction time task with TMS over dorsal premotor cortex to assess performance in a nondystonic task. The results showed that WC patients exhibited dPMI at rest (88.5%, the ratio of conditioned to unconditioned test pulse), in contrast to controls, who did not show dPMI (109.6%) (P = 0.0198). This difference between patients and controls persisted during contraction (100% vs. 112%) and pen-holding (95.6% vs. 111%). The H-reflex in the arm was not modulated by the premotor cortex stimulation. The WC patients made more errors, and the error rate improved with TMS over the premotor cortex. These results suggest that abnormal premotor-motor interactions may play a role in the pathophysiology of focal dystonia. The dPMI was not modulated by task in either group, but was constantly greater in the patients. The significance of the increased inhibition is likely to be compensatory. It appears to be a robust finding and, in combination with other features, could be further explored as a biomarker.
Department of Neurology, University of New Mexico Health Sciences Center, Albuquerque, New Mexico, USA; Human Motor Control Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland, USA.
Sarah
S
Pirio Richardson
Sandra
S
Beck
Barbara
B
Bliem
Mark
M
Hallett
D000704:Analysis of Variance; D016022:Case-Control Studies; D002755:Choice Behavior; D020821:Dystonic Disorders / Q000473:pathology*; D004576:Electromyography; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D007839:Functional Laterality; D006181:H-Reflex / Q000502:physiology; D006801:Humans; D007266:Inhibition, Psychological; D008297:Male; D009044:Motor Cortex / Q000503:physiopathology*; D009433:Neural Inhibition / Q000502:physiology*; D011930:Reaction Time; D013116:Spinal Cord / Q000503:physiopathology; D050781:Transcranial Magnetic Stimulation
TMS; dystonia; motor cortex