Stretch reflex responses in Complex Regional Pain Syndrome-related dystonia are not characterized by hyperreflexia

ObjectiveTo evaluate if hyperreflexia (exaggerated reflexes) due to disinhibition is associated with dystonia in Complex Regional Pain Syndrome (CRPS).MethodsStretch reflexes at the wrist were assessed in healthy controls (n = 10) and CRPS-patients with dystonia (n = 10). Subjects exerted a wrist flexion torque of 5% of maximum voluntary contraction torque (TMVC) to a manipulandum which applied ramp-and-hold stretches to the wrist flexors. Since reflex responses scale with background contraction, controls additionally performed the task at 1% and 3% TMVC to attain similar torques as patients who have reduced TMVC.The M1 onset and the magnitudes of the short latency M1 and long latency M2 were assessed using the electromyographic signals (EMG) of the flexor carpi radialis. EMG of the extensor carpi radialis was recorded to monitor cocontraction.ResultsCompared to controls, patients had a substantially reduced TMVC. Ramp velocity had a significant effect on M1 onset time and magnitude.ConclusionsSince M1 magnitude decreased with flexion torque, no significant difference was found between patients and controls at 5% TMVC, while comparison at similar absolute torques (controls at 1% TMVC) resulted in significantly smaller M1 magnitudes for patients with dystonia.SignificanceThis study suggests that CRPS-patients with dystonia are not hyperreflexive.

► Disinhibition of reflex pathways due to central sensitization may cause dystonia of CRPS.
► No significant differences in stretch reflex responses were found between CRPS-patients with dystonia and controls when compared at equal percentage of maximum voluntary contraction torque.
► Significantly smaller stretch reflex responses were found in CRPS-patients with dystonia in respect to controls when compared at similar absolute torque.