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Bosch E.1, Madden V.2, Bellan V.1, Moseley G.L.1,3, Stanton T.1,3
1The University of South Australia, Adelaide, Australia, 2University of Cape Town, Cape Town, South Africa, 3Neuroscience Research Australia (NeuRA), Sydney, Australia
Background: There is growing evidence to suggest that a complex relationship exists between perception of one's body and pain. Neuroimaging shows differences between those with and without pain in the structure and function of brain areas involved in bodily awareness and perception. Further, distortions of bodily perception occur in people with chronic pain - the painful body part might feel bigger or smaller than it truly is, for example. Intriguingly, preliminary evidence suggests that bodily illusions (changing the way the painful body part looks or feels) may modulate pain. Some illusions, such as mirror therapy, are already used clinically for this purpose. However, to date, the literature on bodily illusions and pain has not been systematically evaluated; this is essential given current clinical application.
Purpose: To summarise and critically appraise the available evidence for the effect of bodily illusions on pain by completing a systematic review and meta-analysis.
Methods: A systematic, sensitive search was performed in six databases and gold standard review methodology (PRISMA statement) was used: two independent reviewers completed study inclusion, risk of bias assessment, and data extraction. Studies were included if they evaluated the effect of a bodily illusion on pain and compared the results with a control group/condition. The definition for what constituted a bodily illusion was determined via consultation with experts in the field. Included studies were grouped according to the type of illusion they used: 1) bodily resizing illusions; 2) mirror therapy; 3) virtual walking illusions; 4) illusions of a new limb; 5) illusions of ownership (rubber hand illusion); 6) illusions of incongruent movement. When two or more studies evaluated a similar illusion and used a comparable control condition, pooling of data using Revman 5.0 was considered. Due to variation in outcome measures, standardised mean differences (SMD) were used for pooling.
Results: Of the 2213 studies identified, 20 studies (consisting of 21 experiments) were included. Risk of bias was high in most studies due to lack of blinding. Consistent evidence of pain reduction was found for illusions of a new limb (functional prosthesis vs cosmetic/no prosthesis; SMD -1.84, 95% CI -2.67 to -1.00) and 4-6 weeks of mirror therapy (SMD -1.11, 95% CI -1.66 to -0.56). Bodily resizing illusions had consistent evidence of pain modulation (in the direction hypothesized). Pooled data found no effect on pain for one session of mirror therapy or for incongruent movement illusions (exception: incongruent movement illusion significantly increased the odds of experiencing pain compared with congruent mirrored movement). Conflicting results were found for virtual walking illusions in both active and inactive control comparisons. Single studies suggest no effect of embodiment illusions, but a significant pain decrease with synchronous mirrored stroking in non-responders to traditional mirror therapy.
Conclusion(s): There is limited evidence to suggest that bodily illusions can alter pain, but some illusions, namely mirror therapy, bodily resizing, and use of functional prostheses in amputees show therapeutic promise.
Implications: Considering body perception in people with pain during clinical assessment may be important given the evidence that some bodily illusions consistently modulate pain.
Funding acknowledgements: EB: UniSA Divisional Honours Scholarship; VB: University of Milano-Bicocca Postgraduate scholarship; GLM: NHMRC PRF (ID1061279); TRS: NHMRC ECF (ID1054041)
Topic: Pain & pain management
Ethics approval: Ethical approval was not required for this project.
All authors, affiliations and abstracts have been published as submitted.
