Force modulation deficits in complex regional pain syndrome: a potential role for impaired sense of force production.

BACKGROUND Compelling evidence points at both impaired proprioception and disturbed force control in patients with chronic complex regional pain syndrome (CRPS). Because force modulation at least partly relies on proprioception, we evaluated if impaired sense of force production contributes to disturbances of force control in patients with CRPS. METHODS Characteristics of voluntary force modulation were examined in the affected upper extremity in 28 CRPS patients with abnormal postures, in 12 CRPS patients without abnormal postures, and in 32 healthy controls. Isometric grip-force matching was compared between conditions with and without visual feedback to identify potential deficits in the sense of force production in terms of force reproduction errors. RESULTS Voluntary force modulation was impaired in CRPS patients, but more so in patients with abnormal postures. In particular, CRPS patients with abnormal postures were characterized by reduced maximum force, reduced ability to increase force output according to task instructions, higher variability of force output and less adequate correction of deviations from the target force. Although effects of visual feedback removal appeared largely similar for the two patient groups and controls, our findings with respect to force reproduction errors suggested that an impaired sense of force production may contribute to the motor dysfunction in CRPS. CONCLUSIONS CRPS patients, in particular those with abnormal postures, showed impaired voluntary force control and an impaired sense of force production. This suggests that therapeutic strategies aimed at restoration of proprioceptive impairments, possibly using online visual feedback, may promote the recovery of motor function in CRPS.