An exploration into the cortical reorganisation of the healthy hand in upper-limb complex regional pain syndrome.

BACKGROUND AND AIMS Recent evidence demonstrated that complex regional pain syndrome (CRPS) is associated with a larger than normal somatosensory (S1) representation of the healthy hand. The most intuitive mechanism for this apparent enlargement is increased, i.e. compensatory, use of the healthy hand. We investigated whether enlargement of the S1 representation of the healthy hand is associated with compensatory use in response to CRPS. Specifically, we were interested in whether the size of the S1 representation of the healthy hand is associated with the severity of functional impairment of the CRPS-affected hand. We were also interested in whether CRPS duration might be positively associated with the size of the representation of the healthy hand in S1. METHODS Using functional magnetic resonance imaging (fMRI) data from our previous investigation, the size of the S1 representation of the healthy hand in CRPS patients (n=12) was standardised to that of a healthy control sample (n=10), according to hand dominance. Responses to questionnaires on hand function, overall function and self-efficacy were used to gather information on hand use in participants. Multiple regression analyses investigated whether the S1 representation was associated with compensatory use. We inferred compensatory use with the interaction between reported use of the CRPS-affected hand and (a) reported overall function, and (b) self-efficacy. We tested the correlation between pain duration and the size of the S1 representation of the healthy hand with Spearman's rho. RESULTS The relationship between the size of the S1 representation of the healthy hand and the interaction between use of the affected hand and overall function was small and non-significant (β=-5.488×10-5, 95% C.I. -0.001, 0.001). The relationship between the size of the S1 representation of the healthy hand and the interaction between use of the affected hand and self-efficacy was also small and non-significant (β=-6.027×10-6, 95% C.I. -0.001, 0.001). The S1 enlargement of the healthy hand was not associated with pain duration (Spearman's rho=-0.14, p=0.67). CONCLUSION Our exploration did not yield evidence of any relationship between the size of the healthy hand representation in S1 and the severity of functional impairment of the CRPS-affected hand, relative to overall hand use or to self-efficacy. There was also no evidence of an association between the size of the healthy hand representation in S1 and pain duration. The enlarged S1 representation of the healthy hand does not relate to self-reported function and impairment in CRPS. IMPLICATIONS While this study had a hypothesis-generating nature and the sample was small, there were no trends to suggest compensatory use as the mechanism underlying the apparent enlargement of the healthy hand in S1. Further studies are needed to investigate the possibility that inter-hemispheric differences seen in S1 in CRPS may be present prior to the development of the disorder.