498 THE JOURNAL OF BONE AND JOINT SURGERY  UPPER LIMB Evidence that central sensitisation is present in patients with shoulder impingement syndrome and influences the outcome after surgery S. E. Gwilym, H. C. L. Oag, I. Tracey, A. J. Carr From the Nuffield Orthopaedic Centre and Oxford NIHR Musculoskeletal Biomedical Research Unit, Oxford, United Kingdom  S. E. Gwilym, DPhil (Oxon), FRCS(Tr&Orth), Orthopaedic Clinical Lecturer  H. C. L. Oag, MA(Hons), MRCS, Orthopaedic Clinical Research Fellow  A. J. Carr, ChM, FRCS, Professor of Orthopaedic Surgery, Head of Department Department of Orthopaedics, Rheumatology and Musculoskeletal Sciences, Nuffield Orthopaedic Centre, Windmill Road, Headington, Oxford OX3 7LD, UK.  I. Tracey, DPhil (Oxon), Nuffield Professor of Anaesthetic Sciences Oxford Centre for Functional Magnetic Resonance Imaging of the Brain University of Oxford, John Radcliffe Hospital, Headington, Oxford OX3 9DU, UK. Correspondence should be sent to Mr S. E. Gwilym; e-mail: sgwilym@gmail.com ©2011 British Editorial Society of Bone and Joint Surgery doi:10.1302/0301-620X.93B4. 25054 $2.00 J Bone Joint Surg [Br] 2011;93-B:498-502. Received 21 April 2010; Accepted after revision 16 November 2010 Impingement syndrome in the shoulder has generally been considered to be a clinical condition of mechanical origin. However, anomalies exist between the pathology in the subacromial space and the degree of pain experienced. These may be explained by variations in the processing of nociceptive inputs between different patients. We investigated the evidence for augmented pain transmission (central sensitisation) in patients with impingement, and the relationship between pre-operative central sensitisation and the outcomes following arthroscopic subacromial decompression. We recruited 17 patients with unilateral impingement of the shoulder and 17 age- and gender-matched controls, all of whom underwent quantitative sensory testing to detect thresholds for mechanical stimuli, distinctions between sharp and blunt punctate stimuli, and heat pain. Additionally Oxford shoulder scores to assess pain and function, and PainDETECT questionnaires to identify ‘neuropathic’ and referred symptoms were completed. Patients completed these questionnaires pre-operatively and three months post-operatively. A significant proportion of patients awaiting subacromial decompression had referred pain radiating down the arm and had significant hyperalgesia to punctate stimulus of the skin compared with controls (unpaired t-test, p < 0.0001). These are felt to represent peripheral manifestations of augmented central pain processing (central sensitisation). The presence of either hyperalgesia or referred pain pre-operatively resulted in a significantly worse outcome from decompression three months after surgery (unpaired t-test, p = 0.04 and p = 0.005, respectively). These observations confirm the presence of central sensitisation in a proportion of patients with shoulder pain associated with impingement. Also, if patients had relatively high levels of central sensitisation pre-operatively, as indicated by higher levels of punctate hyperalgesia and/or referred pain, the outcome three months after subacromial decompression was significantly worse. The source of subacromial pain is unknown, but there are free nerve endings containing substance P and calcitonin gene-related peptides in the sub- acromial space, 1 and they may be the origin of nociception in impingement syndrome. Pain attributed to impingement syndrome is often referred from other sites. 2 A recent study 3 suggests that over half of patients with impingement have paraesthesiae that radiate into the forearm and which resolve following successful surgery. Similarly, experimental models of induced shoulder pain have shown a high incidence of pain referred into the arm. 4,5 Patients with painful musculoskeletal con- ditions also have increased sensitivity to pain- ful stimuli within the referred pain area. 5-7 Features of referred pain, hypersensitivity to peripheral stimuli and ‘neuropathic’ pain are felt to represent peripheral manifestations of augmented central pain processing. This phe- nomenon is termed central sensitisation, 2,8 which may be induced by long-term bom- bardment from peripheral nociceptors 9 and may be mediated by alterations in the endog- enous facilitatory/inhibitory systems. 10,11 To date, no attempt has been made to iden- tify the sensory disturbances associated with radiating pain secondary to shoulder impingement syndrome. Also, the prevalence of neuropathic-like symptoms has not been reported in this population.