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

To evaluate if hyperreflexia (exaggerated reflexes) due to disinhibition is associated with dystonia in Complex Regional Pain Syndrome (CRPS).

Stretch 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 (T(MVC)) 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% T(MVC) to attain similar torques as patients who have reduced T(MVC). 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.

Compared to controls, patients had a substantially reduced T(MVC). Ramp velocity had a significant effect on M1 onset time and magnitude.

Since M1 magnitude decreased with flexion torque, no significant difference was found between patients and controls at 5% T(MVC), while comparison at similar absolute torques (controls at 1% T(MVC)) resulted in significantly smaller M1 magnitudes for patients with dystonia.

This study suggests that CRPS-patients with dystonia are not hyperreflexive.