Cortical excitability is abnormal in patients with the "fixed dystonia" syndrome

A form of fixed dystonia (FD) without evidence of basal ganglia lesions or neurodegeneration has been recently characterized (Schrag et al., Brain 2004;127:2360-2372), which may overlap the clinical spectrum of either complex regional pain syndrome or psychogenic dystonia. Transcranial magnetic stimulation studies in typically mobile dystonia revealed abnormal motor cortical excitability and sensori-motor integration. We compared 12 patients with limb FD to 10 patients with primary adult-onset typically mobile dystonia and 11 age-matched healthy volunteers. Measurements at the first digital interosseus representation area on both hemispheres included: short intracortical inhibition (SICI), contralateral silent period (cSP), and short and long afferent inhibition (SAI and LAI). Repeated measure ANOVA and post-hoc t-tests were used for statistical analysis. SICI was significantly reduced in both hemispheres of patients with "typical" and FD, compared to healthy subjects. For both hemispheres, cSP duration was shorter in both fixed and "typical" dystonia patients. SAI and LAI did not significantly differ between the three groups. The abnormal cortical excitability observed in FD might represent an underlying trait predisposing to different clinical forms of dystonia.