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Developing Concepts in Allodynic Pain

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Developing Concepts in Allodynic Pain I CONFERENCE REPORTS Developing concepts in allodynic pain NG Shenker, HC Cohen and DR Blake Allodynia is the perception of pain in response to Phenomena (eg pain) instruct explanatory theories. NG Shenker1 stimuli that are not normally painful. The manage- These theories change the observed expression of the PhD MRCP, ment of patients with chronic pain and allodynia is original phenomena. Pain can be classified in a similar Consultant suboptimal, partly because of social stigmatisation. manner to visual experiences and this will permit a Rheumatologist Patients are made to feel that they are malingering or new understanding of its expression. HC Cohen2,3 looking for secondary gain. Unfortunately, the legal What immediate implication does this framework MRCP, arc Research profession often propagates this myth and the adver- have? Acute pain drives the subject to respond imme- Fellow and sarial nature of the courts contributes negatively to diately to an external object. Chronic pain is associ- Honorary the patient’s situation. Remarkable recent under- ated with cognitive deficits such as short-term Consultant standings of profound brain changes occurring in memory loss and poor concentration. Chronic pain DR Blake2,3 chronic pain patients will challenge the way that such also affects behaviour without the patient’s aware- FRCP, Professor of medicolegal cases are decided. This ground-breaking ness. The pain matrix intimately involves structures Locomotor conference related expertise and experiences from in the brain related to emotions and these are Medicine patients, clinicians and leading scientists in this field intrinsic to the experience. All of these examples 1Addenbrookes and was the fourth conference in an interdisciplinary are of brain reception and perception rather than Hospital, series centred around pain and suffering organised conception (explicit understanding). Cambridge by Professor David Blake. 2Royal National Neuropathic pain and altered perceptions Hospital for Rheumatic The brainy mind Ms Cath Taylor and Dr Candy McCabe described the Diseases, Bath 3University of Bath The Watson Smith lecture underpinned the day and under-recognised symptoms and signs that affect was delivered by Professor Richard Gregory CBE. patients with complex regional pain syndrome This conference Gregory argued that pain is perceived by the mind. (CRPS) from a bravely personal and objective view. was held at the The undisputed scientific view is that the brain gen- Allodynic pain has a superficial and a deep Royal College of erates the mind. There are many aspects of mind, pain, usually described thermally, that is ill-defined Physicians on from emotions to reasoning, but this fascinating and rated ‘severe to extreme’ by most patients. 1 February 2007 exposition focused on perceptions. Clin Med Visual illusions demonstrated a speculative 2008;8:79–82 working model of perceptions as predictive hypo- Fig 1. Speculative schema of how perceptions are 1 generated. Reproduced with kind permission of the theses of the external world (Fig 1). Different neural Royal Society.1 influences, such as ‘bottom-up’ sensory data or ‘top- down’ processing using past experiences (both con- Conceptual knowledge ceptual and perceptual), were dominant in different illusions. The external world is therefore perceived (hypothesised) using Bayesian rules to generate Feedback qualia, for example the colour ‘grey’. In Figure 2a the Perceptual knowledge from colour ‘grey’ is influenced by both ‘bottom-up’ sen- experience Top-down sory modifications and ‘top-down’ past experiences. Squares A and B are actually the same shade of grey but this is only appreciated when they are dislocated SidewaysHYPOTHESIS Output and placed side by side (Fig 2b). The brain is con- Rules GENERATOR stantly seeking the best meaningful fit of reality rather than an accurate portrayal of the external Objects explored world. Bottom-up In understanding perception it is helpful to consider the brain’s evolution. Three levels of brain response Signal processing can be identified: reception, perception and concep- tion. Reflexes and tropisms are examples of receptions. Perceptions are predictive hypotheses of the external Reality? world. Conception is explicit understanding. Clinical Medicine Vol 8 No 1 February 2008 79 NG Shenker, HC Cohen and DR Blake (a) (b) (a) Fig 3. Evidence or altered A processing of sensorimotor information involving the right parietal lobe. (a) Patient 1, an A accountant, had been asked to B draw a clock face. (b) Patient 2 A had been asked to write down B some items in the kitchen (top) and copy down some spoken B phrases (bottom). The text opposite shows what the patient (b) thought he had actually written. Fig 2. (a) Square ‘b’ looks lighter than square ‘a’ because of Cooker the surrounding colours and the concept that it is in a shadow. (b) Squares ‘a’ and ‘b’ are in fact the same shade of grey. Kettle Table 1. Symptoms and signs suggestive of allodynic pain. Spoon Symptoms G Expressing negative feelings towards the affected area Jug G Depersonalising the affected area G Alienation of the affected area G Desire for autotomy The dog sat on the mat Signs G Confrontational pain Shop G Referred sensations Right G Feeling of altered size when visual input is deprived G Alteration of normal body schema Dictate G Neglecting the affected area The dog ran away G Over-attending the affected area G Misperceiving digits (especially fingers) when visual input is The car outside the garage deprived G Difficulty engaging the area for movement The man scratches his head G Right parietal syndrome Confrontational pain is pain behaviour triggered by approach altered perceptions in patients with CRPS. These findings are without touch. Patients with confrontational pain have terms robust and their presence correlated with changes in functional applied to them including somatisation states and, especially in brain imaging that resolve when the clinical state ameliorates. the medicolegal setting, ‘abnormal pain behaviours’. This can These signs include referred sensations, digit misperception, not be appropriate as nearly all patients with allodynic pain phantom swelling, and neglect of the affected area.2–6 Further- exhibit this behaviour. more, motor deficits have been documented in these patients. Certain groups of symptoms indicate allodynic pain states, These include gait, posture, dystonias, tremor and difficulty especially CRPS. Negative feelings directed toward the affected engaging (‘finding’) the affected part in order to move. All of body part, for example, may spill over into the consultation. these signs share their aetiology in the plasticity of the brain to Patients declare that they hate their affected part and that ‘it’ has the external environment, most clearly seen as a reorganisation ruined their life. Feelings of depersonalisation and alienation of Penfield’s somatotopic maps. The same changes are seen in emerge by patients describing their affected area as ‘it’ rather amputees with phantom limb pain, explaining the large overlap than ‘my arm’ or ‘my leg’. They describe their limb as being between the two conditions. ‘stuck on’ or ‘like an attached alien’. Finally patients express a wish to remove (autotomise) the affected area. The psychiatric perspective The ‘neglect’-like phenomena reported by CRPS patients par- allels features displayed by patients with organic right parietal Patients with chronic pain states including CRPS are often lobe pathology. On formal parietal lobe testing clear abnormal- classified using psychiatric definitions. These include F45.0 soma- ities are seen despite no evidence of pathology (Fig 3). tisation disorder, F45.2 hypochondriacal disorder, F45.3 somato- Several groups have described objective signs reflecting form autonomic dysfunction disorder, and F45.4 persistent 80 Clinical Medicine Vol 8 No 1 February 2008 Developing concepts in allodynic pain somatoform pain disorder. For example, somatisation disorder accounts for less than 0.2% of liaison psychiatric consultations Conference programme and is defined by a two-year history, multiple and variable phys- T ical symptoms, refusal to accept reassurance, and impairment of The pain network and perceptual distortions family and social functioning. Professor David Blake, Royal National Hospital for Rheumatic There is growing dissatisfaction with this classification system Diseases, Bath (conference organiser) because it does not reflect aetiology, is not accepted by patients T Attention, distraction and pain and leads to stigmatisation and disengagement from medical Professor Irene Tracey, University of Oxford services (sometimes encouraged by treatment protocols). The T Pain and somatisation system underrecognises and undertreats depression and anxiety, Dr Ben Green, Cheadle Royal Hospital and the University of and falsely reassures physicians who then miss organic disease. Liverpool Patients with pain and depression delay seeking help when they WATSON SMITH LECTURE should be encouraged as a significant proportion will benefit T The brainy mind from treatments. Professor Richard L Gregory, University of Bristol T Pain empathy and survival Perceiving pain Professor Salvatore M Aglioti, University of Rome, Italy T Neuropathic pain and altered perceptions Aberrant processing of painful stimuli occurs in rat models of Dr Candy McCabe, Professor David Blake, Catherine Taylor, neuropathic and inflammatory pain. New insights derive from Reflex Sympathetic Dystrophy UK differential
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