Patients with unilateral complex regional pain syndrome (CRPS) may have a weaker illusory response to a novel finger illusion compared with healthy controls, indicating disrupted weighting of bilateral hand cortical representations, according to a report published in the Journal of Pain.

Although CRPS has been associated with sensorimotor deficits — particularly on the ipsilateral side — the mechanisms of relative weighting of unimanual and bimanual brain processing remain unclear. Investigators sought to evaluate whether participants with upper limb CRPS have impaired processing of hand proprioception.

A total of participants (mean age, 44.4 years; 85.7% women; n=20 with CRPS, n=20 healthy participants matched for age, gender, and dominant hand). Participants’ hands were placed 12 cm apart vertically and randomly assigned for position and side affected, with the design apparatus hidden behind a screen. 

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Participants were asked to estimate the distance between their index fingers under 2 conditions (with and without contacting an artificial finger) and in 3 positions (left, midline, right). In healthy individuals, the finger illusion usually results an underestimation of the distance between the hands. The primary outcome was participants’ perception of the vertical distance between their index fingers. The participants’ sense of ownership of the artificial finger, measured on a 7-point scale, was also evaluated.

Without contact (“no-grasp” condition), patients with CRPS vs healthy controls perceived their hands as more separated (mean, 8.0 cm vs 6.4 cm; P =.041), indicating a weaker spatial illusion. During contact (“grasp” condition) in the form of a pincer grip on the artificial finger, participants in both groups had comparable perception of the distance between both hands (CRPS, 5.2 cm; controls, 4.9 cm; P =.699). Regarding finger ownership, results were equivocal between patients and controls (mean, 5.4 vs mean, 5.8; P =.177).

In the grasp condition, both groups perceived the expected illusory effect, which was more prominent in control participants vs those with CRPS in the no-grasp condition. No differences were detected between groups in terms of position or hand placement.

Study limitations include the lack of horizontal drift assessment and lack of assessment of the role of attention and memory.

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“Our results suggest that people with unilateral CRPS have reduced weighting of bimanual representations, but the deficit can be quickly overcome by adding cutaneous input that is consistent with bimanual activity,” concluded the authors. They recommended that future trials examine this finger illusion more closely in the interest of deriving clinical/therapeutic utility and explore more fully the spatial processing impact of CRPS.

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Wang AP, Butler AA, Valentine JD, et al. A novel finger illusion reveals reduced weighting of bimanual hand cortical representations in people with complex regional pain syndrome [published online September 13, 2018]. J Pain. doi:10.1016/j.jpain.2018.08.008