Pain has been defined by International Society for Study of pain as "an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage." Note the last few words, even in the absence of actual/potential tissue damage if a person describe unpleasant sensation in terms of actual/potential tissue damage it is called pain. Take the example of Phantom limb pain. Here patient feel painin the limb which is not there; so question of tissue damage does not arise. Till 1960s doctors thought it was due to psychological problem but now we know the exact pathophysiology of pain, which is a real painand not psychological. Similarly in lumbago, sciatica, CRPS, fibromyalgia etc. there are definite pathophysiological problem in the nervous system which can be reversed with interventional pain management. It is now definitely proved that with a few exception like Rheumatoid arthritis in all chronic pain there are problems in the nervous system and so Chronic pain can be called a disease itself.
Chronic pain has been defined in many ways. American chronic pain association defines it as pain that continues a month or more beyond the usual recovery period. Some has described chronic pain as pain persisting more than 3 months, some by 6 months. In other way all pain is acute pain till it becomes chronic pain. But sensitization is the constant feature in all types of chronic pain.
Sensitization is a phenomenon of inappropriate or disproportionate response to normal stimulus. Sensitization outside central nervous system (CNS) is called Peripheral sensitization and that in the CNS is called Central sensitization.
Acute pain ? AMPA receptors stimulated ? dislodges Mg from NMDA receptors??activation of NMDA receptors. Activation of NMDA receptors leads to following changes :
All these leads to following clinical changes:
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