Integration of sensory force feedback is disturbed in CRPS-related dystonia

Complex regional pain syndrome (CRPS) is characterized by pain and disturbed blood flow, temperature regulation and motor control. Approximately 25% of cases develop fixed dystonia, a movement disorder of sustained muscle contraction and abnormal postures. The origin of fixed dystonia is poorly understood, yet recent insights involve disturbed force feedback. Assessment of sensorimotor integration may provide insight into the pathophysiology of fixed dystonia. Sensory weighting is the process of integrating and weighting sensory feedback channels in the central nervous system. Position and force are physically related, allowing translation from one modality into the other. When stiffness is known, combining the sensory feedback of position and force (sensory integration) provides increased accuracy of the estimate of either modality. It was hypothesized that patients with CRPS-related dystonia bias sensory weighting of force and position toward position due to the unreliability of force feedback.

CRPS-patients with dystonia (n=10) and age and gender-matched healthy subjects blindly reproduced a trained force against a linear spring which on occasion was covertly replaced by a non-linear spring, revealing the sensory weighting between force and position feedback (Mugge et al. 2009). The current study provides experimental evidence for dysfunctional sensory integration in fixed dystonia, showing that CRPS-patients with fixed dystonia do not reweight force and position feedback as controls do. The study shows that patients always favor position feedback, making it the first to demonstrate disturbed integration of force feedback in fixed dystonia, an important step towards understanding the pathophysiology of fixed dystonia.

With a modeling study we investigated the effect of the sensorimotor interaction on the phase-frequency relationship of the CMC. The model includes two systems, representing the efferent and afferent pathways modeled as gains and realistic neural time delays. We found that the closed-loop formed by the sensorimotor interaction has a huge effect on the phase-frequency relationship, depending on the relative strength of the afferent pathways. Within the beta band the slope of the phase-frequency relation is reduced (i.e. is less negative than expected for a pure efferent pathway) and has a nonzero intersect. If the afferent pathways are stronger than the efferent pathways a phase advance will result, i.e. the slope can become positive. In conclusion, the phase-frequency relation emerges from the interaction in the sensorimotor loop and here we demonstrated how the relation depends on a complex interaction between the afferent and efferent pathways.