Pain

Abstract

Pain is the chief reason why patients attend doctors. To them it signifies harm or damage; to doctors it is a symptom of a pathological process which can be treated only after diagnosis has been made. The International Association for the Study of Pain (Merskey 1986) defined it as “an unpleasant sensory and emotional experience, associated with actual or potential tissue damage, or described in terms of such damage.” This definition recognises pain as a subjective experience, accepting that it often occurs in the absence of any tissue damage (or at least none that doctors can detect). It distinguishes activity in specific cutaneous receptors and their central projections from the experience of pain: a distinction clearly made by Sherrington in 1903. Wall (1985) considered pain as a state akin to hunger or thirst in which action is imperative: behaviour must be changed to prevent further damage (and this includes psychological damage). There are sensory and affective dimensions to pain and these can be disassociated. Rainville and colleagues (1999) modulated the experience of pain by hypnosis in volunteers who were subjected to heat pain stimulus. Unpleasantness (affect) was reduced by suggestion, whereas intensity of the sensation much less so. In one experiment the heart rate was monitored, this correlated with unpleasantness, but not with intensity. Ploner and colleagues (1999) described a 57 year old man who had suffered a right sided post central stroke with infarction of the SI and SII cortices. He lost “sensory-discriminative” pain whereas “motivational – affective” pain was preserved and Ploner et al. commented: “we were able to demonstrate, for the first time in humans, the representation of the sensory- discriminative pain component and first pain sensation in the lateral pain system. In contrast, pain affect and the ability to detect painful stimuli do not, in principle, require integrity of these structures.” Price et al. (2003) restate earlier concepts of exteroceptive and interoceptive pain systems. The exteroceptive system enables the protective behaviour of escape and avoidance whereas the interoceptive system leads to homeostatic behaviour, quiescence with guarding of injured body regions and autonomic responses promoting recuperation and healing. There is an extensive discussion of the pathways involved. Schott (2004) reflects further on these concepts. The difference between these two pain pathways will be apparent to anyone who has experienced, on the one hand, the pain of fracture or dislocation in a limb and on the other, cardiac pain or pain from abdominal viscera.

Rolfe Birch M. Chir, FRCP&S (Glas), FRCS (Edin), FRCS (Eng) by election Professor in Neurological Orthopaedic Surgery, University College, London

Visiting Professor, Department of Academic Neurology, Imperial College, London

Honorary Orthopaedic Consultant, Hospital for Sick Children Great Ormond Street, London

The National Hospital for Nervous Diseases, Queen Square, London

Raigmore Hospital, Inverness

Honorary Orthopaedic Surgeon to the Royal Navy

Consultant in Charge, War Nerve Injuries Clinic at the Defence Medical Rehabilitation Centre, Headley Court, Leatherhead, Surrey