Physiology and Pathophysiology of Chronic Pain

Pain Pathogenesis

Pain is an unpleasant sensory and emotional experience that involves complex processes of neuronal signaling in the peripheral nervous system and central nervous system (CNS). Chronic pain is defined as pain lasting >3–6 months and is very common in medical practice. It is reported by 20%–50% of patients seen in primary care ( ), with an estimated annual burden of $90 billion ( ). Pain is classified as either neuropathic or nociceptive, though frequently the types can coexist. According to the International Association for the Study of Pain, neuropathic pain is defined as the type of pain caused by a demonstrable lesion or disease of the somatosensory nerve structure itself. Classic signs of neuropathic pain are hyperalgesia, allodynia, and spontaneous pain without a stimulus. Common examples include diabetic neuropathy, postherpetic neuralgia, lumbar radiculopathy, multiple sclerosis, and complex regional pain syndromes. In contrast, nociceptive pain arises from actual or potential damage to nonneural tissue via the activation of nociceptors. Nociceptive pain can be somatic, such as acute pain after a burn, or visceral, such as the pain felt with abdominal cramps ( ).