Mapping finger movement representation in focal hand dystonia

Introduction: Representational abnormalities in the motor as well as in the somatosensory cortex may be related to the pathophysiology of focal dystonia such as writer's cramp (WC). Previously, we have shown that finger movements evoked by transcranial magnetic stimulation (TMS) are organized somatotopically and exhibit modular properties (Gentner and Classen 2006). The central nervous system may control finger movements by combining a limited number of such neuro-motor modules. We investigated motor representation (somatotopy and modular properties) of the hand muscles of patients with WC.

Methods: TMS-evoked finger movements of the right hand were recorded (5DT-Glove, 5DT inc., Pretoria, South Africa) from 10 patients with WC (6 females, 4 males, age 48±10 years) and 10 healthy age- and sex-matched controls. TMS was used to evoke contralateral finger movements (1,3 to 1,4* RMT) of the dominant hand from 30 different scalp sides (15 stimuli per site) arranged as a 5×6 grid overlying the primary motor cortex. To examine the somatotopical order of finger movement representation, the centres of gravity (CoGs) for movements of each finger and muscle were computed. The somatotopical order of finger representations was indexed by the latero-medial distance of the thumb and the little finger CoGs ("D1-D5-distance"). Principal component analysis was used to extract from TMS-evoked finger movements modules (principal components, PCs) that can be linearly combined to reconstruct the TMS-evoked movements. The number of PCs explaining at least 90% of data variance (#PC90%) and the similarity of PCs between subjects was calculated. The similarity of the first #PC90% PCs between subjects in each group served as a marker of finger movement variability.

Results: The variability of finger movements was reduced in WC patients compared to healthy controls (p=0.042). D1-D5-distance was similar between patients (2.4±3.3mm) and controls (2.7±1.7mm; p=0.791). #PC90% did not differ between both groups (p=0.216).

Conclusions: Loss of variability of finger movements may correspond to the impairment of fine motor skills in WC. Normal topographic finger movement representation was an unexpected finding in focal hand dystonia and may hint to a stronger involvement of the somatosensory cortex than believed previously.