Cancer pain. Anatomy, physiology, and pharmacology.

Cancer pain can be divided into three classes: somatic, visceral, and deafferentation. Somatic and visceral pain result from activation of nociceptors by tumor infiltration of tissues and from secondary inflammatory changes with release of algesic chemicals that act to sensitize nociceptors. Pain may be experienced locally (somatic and visceral) or referred to remote cutaneous sites (visceral). Deafferentation pain results from injury to the nervous system due to tumor infiltration or cancer therapy and may persist even after the cause of the injury has been removed. Somatic, visceral, and deafferentation pain may be complicated by sympathetically maintained pain, in which efferent sympathetic activity promotes persistent pain, hyperpathia, and vasomotor and sudomotor changes after tissue injury from cancer or its therapy. The neurobiology of cancer pain is complex and incompletely understood. This article summarizes current knowledge in this area and briefly discusses approaches to cancer pain management that are based on this knowledge.