Rheumatology Advance Access originally published online on February 3, 2006
Rheumatology 2006 45(5):644-645; doi:10.1093/rheumatology/kel031
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LETTER TO THE EDITOR |
Re: Sensory-motor incongruence and reports of ‘pain’, by G. L. Moseley and S. C. Gandevia. Rheumatology 2005;44:1083–1085
The Royal National Hospital for Rheumatic Diseases in conjunction with The School for Health and The Department of Pharmacy and Pharmacology, University of Bath, Bath, 1 Royal Devon and Exeter Hospital (Wonford), Exeter and 2 School of Psychology, University of Cardiff, Cardiff, UK
Correspondence to: C. S. McCabe, The Royal National Hospital for Rheumatic Diseases, Upper Borough Walls, Bath BA1 1RL, UK. E-mail: candy.mccabe{at}rnhrd-tr.swest.nhs.uk
SIR, We were generally pleased with the editorial by Moseley and Gandevia [1] that accompanied our paper which described the novel findings of generating somaesthetic disturbances in healthy individuals [2]. However, their editorial contains a number of inaccuracies that need to be addressed as they could form the basis of important criticisms of our paper if not corrected.
Moseley and Gandevia state that our ‘subjects were related to patients’ who were currently undergoing therapeutic mirror visual feedback and that this may have biased our findings. This was simply not the case. Some subjects were recruited from the hospital and may have been related to patients with a musculoskeletal disease attending the hospital, but none were related to the patients with complex regional pain syndrome (CRPS) who received mirror visual feedback therapy.
The authors also state that ‘the time course of the sensations ... was not described in detail’, and that without this it is difficult to confirm the relationship ‘between apparent incongruence and symptoms’. They also query why a longer time period was not selected as this may have established whether habituation to the task would reduce evoked symptoms. These aspects were clearly described in the Methods, Results and Discussion sections of our paper. There was a ‘timed 20-second’ experimental condition and immediate resolution of abnormal sensations once normal visual input was restored. Consequently, it is not unreasonable to assume a direct relationship between the experimental condition and symptom generation. A longer time period was considered but discounted as we were concerned that muscle fatigue would distort our findings, although we recognize that, if technically possible, this would be a useful extension to our work.
Our finding that nearly 40% of the study population perceived sensory anomalies during congruent mirror visual feedback is considered surprising by Moseley and Gandevia, given the fact that a similar protocol was used for analgesic benefit and functional return in those with chronic pain [3, 4]. This provides us with an opportunity to clarify our position. In patients with CRPS and phantom limb pain, we hypothesize that a pre-existing gross sensory–motor incongruence is responsible for the generation of some of their symptoms. The almost immediate pain relief in some, with corrective sensory feedback, would appear to confirm this hypothesis. Therefore, the smaller variations between movement and limb that appear to occur in some healthy individuals during congruent mirror feedback are unlikely to be perceived as problematic in those with a significant pre-existing sensorimotor conflict. Conversely, the incoming sensory input of a now healthy-looking limb corrects an already disturbed system rather than alerting a normal one.
We agree with Moseley and Gandevia that cortical reorganization alone can not be solely responsible for the generation of pain in all such cases. The example they provide of enlarged representation of digits in Braille readers clearly demonstrates this point [5]. The somatosensory cortical map, however, is only one of a number of reference points for the motor control system that informs the predicted sensory feedback, or efference copy. Other factors, including joint position sense, the environment, previous experience and emotional factors, inevitably influence the efference copy. If any one of these becomes altered or distorted, the individual is rendered more vulnerable to an inaccurate or distorted sensory prediction. It is possible that some of these factors are differentially weighted and hence it is not surprising that the level of pain experienced has been shown to be significantly correlated with the level of cortical reorganization in some patients with phantom limb and in CRPS populations [6, 7]. In addition, functional MRI data show a dramatic increase in activity in the somatosensory cortex, as well as other cortical areas involved in cognitive and motor processing, when pin-prick hyperalgesia is induced in the CRPS-affected limb compared with the unaffected limb [8].
The authors have declared no conflicts of interest.
References
- Moseley GL, Gandevia SC. Sensory-motor incongruence and reports of ‘pain’. Rheumatology 2005;44:1083–5.
[Free Full Text] - McCabe CS, Haigh RC, Halligan PW, Blake DR. Simulating sensory-motor incongruence in healthy volunteers: implications for a cortical model of pain. Rheumatology 2005;44:509–16.
[Abstract/Free Full Text] - McCabe CS, Haigh RC, Ring EFR, Halligan PW, Wall PD, Blake DR. A controlled pilot study of the utility of mirror visual feedback in the treatment of complex regional pain syndrome (Type 1). Rheumatology 2003;42:97–101.
[Abstract/Free Full Text] - Ramachandran VS, Rogers-Ramachandran D. Synaesthesia in phantom limb induced by mirrors. Proc R Soc B 1996;263:377–86.[Medline]
- Sterr A, Muller MM, Elbert T, Rockstroh B, Pantev C, Taub E. Perceptual correlates of changes in cortical representation of fingers in blind multifinger Braille readers. J Neurosci 1998;18:4417–23.
[Abstract/Free Full Text] - Flor H, Elbert T, Knecht S et al. Phantom-limb pain as a perceptual correlate of cortical reorganisation following arm amputation. Nature 1995;375:482–4.[CrossRef][Medline]
- Maihöfner C, Handwerker HO, Neundörfer B, Birklein F. Cortical reorganization during recovery from complex regional pain syndrome. Neurology 2004;63:693–701.
[Abstract/Free Full Text] - Maihöfner C, Forster C, Birklein F, Neundörfer B, Handwerker HO. Brain processing during mechanical hyperalgesia in complex regional pain syndrome: a functional MRI study. Pain 2005;114:93–103.[Medline]
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