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.
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.
The relationship between the size of the S1 representation of the healthyh and 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).
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.
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.
DOI of refers to article: http://dx.doi.org/10.1016/j.sjpain.2016.07.001.
Funding: FDP received support from an Australian Postgraduate Award and from an Early Career Fellowship from the National Health and Medical Research Council (NHMRC) ID 1091415. TRS received support from a fellowship from the Canadian Institutes of Health Research ID 223354 and from an Early Career Fellowship from NHMRC ID 1054041. TRS received travel and accommodation support from Eli Lilly Ltd. for speaking engagements in Western Canada (September 2014); this was unrelated to the present topic. GLM is supported by a Principal Research Fellowship from the NHMRC ID 1061279. GLM consults to Pfizer, Kaiser Permanente, Neuroorthopaedic Institute, and Workers’ Compensation Boards in Australia and North America. He receives payments for lectures and courses on pain and rehabilitation and he receives royalties for several books on pain. ML received support from the Federal Ministry of Education and Research (BMBF) NZL 11/005. JHM is supported by NHMRC Project ID 1047827. This work is supported by NHMRC Project Grant 630431.
Ethical issues: Please see Section 2.2.
Conflicts of interest: The authors report no potential conflicts of interest.
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.sjpain.2016.06.004.
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