Association Between Focal Dystonia and Complex Regional Pain Syndrome

OBJECTIVE Dystonia is generally regarded as a motor execution abnormality due to a central nervous system dysfunction in the cortico-striato-thalamo-cortical motor loop. Regional traumas are considered to be risk factors for the development of focal dystonia (FD) in the affected limb. Since focal limb traumas are also associated with complex regional pain syndrome (CRPS), there may be a common underlying pathology in both conditions. In CRPS, many patients have a small fiber neuropathy. The goal of the proposed research is to distinguish (discriminate) among the three disease categories (FD, FD+CRPS, CRPS), to find out whether FD and CRPS share a common physiological substrate and represent two sides of a spectrum, and to see which clinical situations may be secondary to an underlying small fiber neuropathy. Independent studies will be conducted in patients with affected upper limbs and in those with affected lower limbs. STUDY POPULATION We intend to perform two studies on overall 90 subjects. One study will assess the affected upper limb in 15 patients with unilateral focal dystonia, 15 patients with CRPS and 15 patients with focal dystonia and CRPS, while the second study will test the affected lower limb in 15 patients with unilateral focal dystonia, 15 patients with CRPS and 15 patients with focal dystonia and CRPS in the lower limb. DESIGN We propose to explore the central aspects (somatosensory evoked potentials, SEPS, and transcranial magnetic stimulation, TMS) in patients with upper limb disorders, because the cortical representation of hand muscles is easier to study than the lower limb muscle representations. In contrast, the peripheral studies (quantitative EMG and nerve conduction) will be performed in patients with lower limb disorders, due to length, accessibility and separation of pure motor and sensory nerves in the leg. OUTCOME MEASURES The primary outcome measure for the central nervous system features (the upper limb assessment) will be the difference in EEG dipole localization of the SEPs from thumb and index finger between the three groups to assess possible disturbances of cortical representation in the primary sensory cortex. TMS measures will be exploratory. The primary outcome measure of the lower limb testing will be the duration of motor unit action potentials (MUAP) to assess differences and subtle lesions, which were not picked up in clinical routine due to bigger confidence intervals of the single parameter. The unaffected side is the primary target of this study since we are looking for an underlying substrate; however, all clinical tests will be performed bilaterally (if tolerated by the subjects) and the comparison of both sides will be a secondary outcome measure.