SY2.4. Sensori-motor organisation in focal hand dystonia

Abstract

Focal Hand Dystonia is a task specific dystonia with co-contraction of agonists and antagonistic muscles. This disorder is attributed to a plausible dysfunction of the basal ganglia, but the exact role and sites are not yet well characterized. At our institution, we did experiments’ to explore the ontogeny of muscle contraction, sensory abnormalities and precision grip studies to explore dysfunction. The first study looked at the utility of 16 Channel surface EMG using customized software to objectively assess FHD during different phases of a writing task used to study timing, activation patterns and spread of muscle contractions in FHD when compared to normal controls. SEMG of FHD subjects showed ‘early onset’ during motor imagery, rapid proximal recruitment, co-contraction of agonist-antagonist proximal muscle groups, delayed offset after stopping to write, mirror activity in contralateral limb when compared to controls. The second experiment aimed to look at the degree of sensory dysfunction in subjects with Writer’s cramp using tasks for spatial discrimination and Aristotle illusory phenomenon and to look for Neural correlates using f MRI. There were abnormalities of sensory processing in patients with FHD in both spatial and illusory phenomenon. Spatial discriminatory abnormality was see in both hands indicating abnormalities in bilateral somatosensory cortex leading to abnormal sensory motor integration and resultant maladaptive plasticity and f MRI showed lack of good separation of activity for index and little finger stimulation with JVP dome. The third experiment looked at precision grip (both qualitative and quantitative measures) to discriminate FHD and PD from normal subjects. Healthy controls and patients (PD/FHD) gripped and lifted objects of 2 different weights;1.3 N &1.7 N with Dry Skin Condition and Very Wet Skin Condition. After practice trials, subjects were asked to reach, lift and hold stationary an object for 5 seconds and release the object on command. The force, rate and time parameters are measured and a ROC curve was measured and this study could qualitatively differentiate FHD and PD. These studies highlight that in FHD there is motor and sensory processing abnormalities demonstrating abnormal plasticity.