title: Impaired interhemispheric inhibition in writer's cramp.
abstract: OBJECTIVE
  Reduced cortical inhibition is a feature of focal hand dystonia and this likely contributes to excessive muscle contractions. Inhibition from the opposite hemisphere, known as interhemispheric inhibition (IHI), was studied bidirectionally in 7 right-handed patients with writer's cramp (WC) and age-matched healthy controls in a cross-sectional physiologic study.
  
  
  METHODS
  IHI was measured with paired transcranial magnetic stimulation with the conditioning stimulus applied to the motor cortex and the test stimulus applied to the contralateral motor cortex. Surface EMG was measured in right and left first dorsal interosseous muscles during rest, and while holding a pen between the thumb and index finger at 20% maximum voluntary contraction with the right dystonia-affected hand. The time course and magnitude of IHI was studied at interstimulus intervals of 6, 8, 10, 12, 30, 40, and 50 msec between the conditioning stimulus and test stimulus.
  
  
  RESULTS
  In WC at rest, IHI was significantly reduced in the dystonia-affected right hand (IHI from right to left motor cortex) at both short (SIHI, 10-12 msec) and long (LIHI, 30-40 msec) intervals compared to the unaffected hand. Compared to controls, SIHI and LIHI were reduced in the dystonia-affected hand only. There was no difference in IHI between controls and WC during the task of holding a pen.
  
  
  CONCLUSIONS
  In WC, both SIHI and LIHI are reduced in the dystonia-affected hand compared to the unaffected hand and to healthy controls. Impaired IHI may contribute to excessive muscle contraction in WC.
authors.affiliation: Division of Neurology and Krembil Neuroscience Centre, Toronto Western Research Institute, University of Toronto, Toronto, Ontario, Canada.
authors.forename: A J
authors.initials: AJ
authors.lastname: Nelson
authors.forename: T
authors.initials: T
authors.lastname: Hoque
authors.forename: C
authors.initials: C
authors.lastname: Gunraj
authors.forename: Z
authors.initials: Z
authors.lastname: Ni
authors.forename: R
authors.initials: R
authors.lastname: Chen
mesh_terms: D000328:Adult; D016022:Case-Control Studies; D020821:Dystonic Disorders / Q000503:physiopathology*; D004576:Electromyography; D019054:Evoked Potentials, Motor; D005260:Female; D007839:Functional Laterality; D006225:Hand / Q000503:physiopathology; D006801:Humans; D008297:Male; D008875:Middle Aged; D009044:Motor Cortex / Q000503:physiopathology*; D009048:Motor Skills / Q000502:physiology; D009119:Muscle Contraction / Q000502:physiology; D009433:Neural Inhibition; D012146:Rest; D013647:Task Performance and Analysis; D013997:Time Factors; D050781:Transcranial Magnetic Stimulation / Q000379:methods