Pain mechanisms in patients with chronic pain.

The mechanisms involved in the development of chronic pain are varied and complex. Pain processes are plastic and unrelieved pain may lead to changes in the neural structure involved in pain generation. Nociceptive pain announces the presence of a potentially damaging stimulus that occurs when noxious stimuli activate primary afferent neurons. Neuropathic pain is initiated or caused by a primary lesion or dysfunction in the nervous system resulting from trauma, infection, ischaemia, cancer or other causes such as chemotherapy. The exact mechanisms involved in the pathophysiology of chronic pain are not well understood, but rapid and long-term changes are thought to occur in parts of the central nervous system that are involved in the transmission and modulation of pain following injury. Peripheral and central sensitization of sensory nerve fibres are the primary reasons for hypersensitivity to pain after injury, and mainly occur in inflammatory and neuropathic pain. During these processes the sensation of pain is enhanced as a result of changes in the environment, the nerve fibres and modifications of the functional properties and the genetic programme of primary and secondary afferent neurons. Non-steroidal anti-inflammatory drugs and opioid analgesics are two of the most common classes of drugs used for the treatment of pain. Response to drug treatment shows significant interindividual variability and can lead to side effects. The neurobiological mechanisms that cause pain may account for the different types of pain observed. Identification of these mechanisms may allow us to move from an empirical therapeutic approach to one that it is specifically targeted at the particular mechanisms of the type of pain experienced by an individual patient.