Anatomy, physiology, and neuropharmacology of cancer pain.
The anatomy, physiology, and pharmacology of nociception and its modification by analgesic drugs have been studied extensively in the past decade. Although the neural mechanisms of nociceptors and the stimuli that activate them are much better understood, it must be emphasized that the perception of pain, as well as the meaning of pain to the individual, is a complex behavioral phenomenon and involves psychologic and emotional processes in addition to activation of nociceptive pathways. Pain related to malignant disease can be classified as somatic, visceral, and deafferentation in type. Somatic pain and visceral pain involve direct activation of nociceptors and are often a complication of tumor infiltration of tissues or injury of tissues as a consequence of cancer therapy. The management of this type of pain is typically accomplished by treating the tumor (with surgery, chemotherapy, and/or radiation therapy) and by using the appropriate non-narcotic, narcotic, and adjuvant analgesic agents. Neuroablative therapies may be helpful in specific circumstances. For example, cordotomy may be helpful for unilateral pain below the waist in patients with somatic and visceral pain. This procedure may also be helpful for early deafferentiation pain (i.e., lumbosacral plexopathy) in which peripheral nerves are compressed but not infiltrated or destroyed by metastatic tumor growth. Deafferentiation pain may be a complication of tumor infiltration of peripheral nerve or of cancer therapy that injures neural tissue. This type of pain is often poorly tolerated and difficult to control, particularly if not treated early and aggressively. Although incompletely understood, the pathophysiology of deafferentation pain appears to be different from that of somatic or visceral pain, and the treatment approaches may be different. Management approaches to deafferentation pain usually emphasize treatment of the pain, because injury to the nervous system may be difficult to reverse, even if one can successfully treat the underlying malignancy, and many deafferentation pain syndromes occur as a complication of cancer therapy. The role of narcotic analgesics in the management of deafferentation pain is not clear, although the published experience suggests that they are less useful than in somatic or visceral pain.
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