Optokinetic stimulation increases limb pain and forehead hyperalgesia in complex regional pain syndrome.

Background: Ambiguous visual stimuli increase limb pain in patients with complex regional pain syndrome (CRPS), possibly due to afferent sensory feedback conflicts. Conflicting sensory stimuli can also generate unpleasant sensations in healthy people such as during motion sickness. We wanted to investigate the mechanisms underlying the link between sensory conflicts and pain in CRPS using optokinetic stimulation (OKS) - a method known to induce motion sickness.

Methods: Twenty-one CRPS patients underwent OKS and rated symptoms of motion sickness. Patients also rated limb pain and pain-related distress before, during and after OKS. In addition, pressure-pain and sharpness sensations were investigated on both sides of the forehead and in the affected and contralateral limb before and after OKS.

Results: Limb pain and forehead hyperalgesia to pressure increased in parallel in response to OKS. In a subgroup of nauseated patients who withdrew early from OKS, hyperalgesia to pressure in the ipsilateral forehead persisted longer than in the remaining participants. Sharpness sensations remained constant at all sites.

Conclusions: Sensory conflicts may facilitate pain in CRPS by activating the mechanisms of general facilitation of nociception and, during more severe sensory conflicts, also a facilitatory mechanism that operates mainly ipsilateral to the affected limb.