[Pathophysiology of chronic pain. Classification of three subtypes of pain].

Pain is a physiological sensation which aim is to alert for an upcoming danger that may threaten the individual. This system includes peripheral nociceptors that initiate the nociceptive message. Then, the information is conveyed to the brain through the spinothalamic tract that projects to the thalamus, insula, SII and other areas. In clinical situations, a dysfunction of this nociceptive system explains chronic pain that can be simply classified into 3 subtypes according to pathophysiological mechanisms. Case 1: the nociceptive system is functioning normally, and provides (appropriately) a pain message that informs the brain of a local inflammation, or cancer, or infection... This is the situation of chronic pain because of an excess of nociception. Case 2: there is a (past or present) lesion of the nervous system that induces a dysfunction in the pain system. This is the neuropathic pain. Case 3: between the two former cases, there is no evidence for a lesion in the nociceptive system, and there are no evidences for lesion at the peripheral level, but there are evidences of dysregulation(s). These are the cases of dysfunctional pain.