Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage. It is the feeling common to such experiences as stubbing a toe, burning a finger, putting iodine on a cut, and bumping the "funny bone".

 

Pain motivates us to withdraw from damaging or potentially damaging situations, protect the damaged body part while it heals, and avoid those situations in the future. It is initiated by stimulation of nociceptors in the peripheral nervous system, or by damage to or malfunction of the peripheral or central nervous systems.

Most pain resolves promptly once the painful stimulus is removed and the body has healed, but sometimes pain persists despite removal of the stimulus and apparent healing of the body; and sometimes pain arises in the absence of any detectable stimulus, damage or pathology.

Pain is the most common reason for physician consultation in the United States. It is a major symptom in many medical conditions, and can significantly interfere with a person's quality of life and general functioning. Social support, hypnotic suggestion, excitement in sport or war, distraction, and appraisal can all significantly modulate pain's intensity or unpleasantness.

Etymology : "Pain (n.) 1297, "punishment," especially for a crime; also (c.1300) "condition one feels when hurt, opposite of pleasure," from Old French peine, in turn from Latin poena, "punishment, penalty" (in L.L. also "torment, hardship, suffering") and that from Greek "p????" (poine), generally "price paid", "penalty", "punishment", from PIE *kwei- "to pay, atone, compensate" (...)."

Classification

The International Association for the Study of Pain (IASP) classification system recommends describing pain according to five categories: duration and severity, anatomical location, body system involved, cause, and temporal characteristics (intermittent, constant, etc.). This system has been criticized by Woolf and others as inadequate for guiding research and treatment, and an additional category based on neurochemical mechanism has been proposed.

Duration

Pain is usually transitory, lasting only until the noxious stimulus is removed or the underlying damage or pathology has healed, but some painful conditions, such as rheumatoid arthritis, peripheral neuropathy, cancer and idiopathic pain, may persist for years. Pain that lasts a long time is called chronic, and pain that resolves quickly is called acute. Traditionally, the distinction between acute and chronic pain has relied upon an arbitrary interval of time from onset; the two most commonly used markers being 3 months and 6 months since the onset of pain, though some theorists and researchers have placed the transition from acute to chronic pain at 12 months. Others apply acute to pain that lasts less than 30 days, chronic to pain of more than six months duration, and subacute to pain that lasts from one to six months. A popular alternative definition of chronic pain, involving no arbitrarily fixed durations is "pain that extends beyond the expected period of healing." Chronic pain may be ided into "cancer" and "benign".

Region and system

Pain can be classed according to its location in the body, as in headache, low back pain and pelvic pain; or according to the body system involved, i.e., myofascial pain (emanating from skeletal muscles or the fipous sheath surrounding them), rheumatic (emanating from the joints and surrounding tissue), causalgia ("burning" pain in the skin of the arms or, sometimes, legs; thought to be the product of peripheral nerve damage), neuropathic pain (caused by damage to or malfunction of any part of the nervous system), or vascular (pain from blood vessels).

Cause

The crudest example of classification by cause simply distinguishes "somatogenic" pain (arising from a perturbation of the body) from "psychogenic" pain (arising from a perturbation of the mind: when a thorough physical exam, imaging, and laboratory tests fail to detect the cause of pain, it is assumed to be the product of psychic conflict or psychopathology). Somatogenic pain is ided into "nociceptive" (caused by activation of nociceptors) and "neuropathic" (caused by damage to or malfunction of the nervous system).

Nociceptive

Nociceptive pain is initiated by stimulation of nociceptors, and may be classified according to the mode of noxious stimulation; the most common categories being "thermal" (heat or cold), "mechanical" (crushing, tearing, etc.) and "chemical" (iodine in a cut, chili powder in the eyes).

Nociceptive pain may also be ided into "superficial somatic" and "deep", and deep pain into "deep somatic" and "visceral". Superficial somatic pain is initiated by activation of nociceptors in the skin or superficial tissues, and is sharp, well-defined and clearly located. Examples of injuries that produce superficial somatic pain include minor wounds and minor (first degree) burns. Deep somatic pain is initiated by stimulation of nociceptors in ligaments, tendons, bones, blood vessels, fasciae and muscles, and is dull, aching, poorly-localized pain; examples include sprains and poken bones. Visceral pain originates in the viscera (organs) and often is extremely difficult to locate, and several visceral regions produce "referred" pain when injured, where the sensation is located in an area distant from the site of injury or pathology.

Neuropathic

Neuropathic pain is caused by damage to or malfunction of the nervous system, and is ided into "peripheral" (originating in the peripheral nervous system) and "central" (originating in the pain or spinal cord). Peripheral neuropathic pain is often described as “burning,” “tingling,” “electrical,” “stabbing,” or “pins and needles.” Bumping the "funny bone" elicits peripheral neuropathic pain.Psychogenic

Psychogenic pain, also called psychalgia or somatoform pain, is pain caused, increased, or prolonged by mental, emotional, or behavioral factors. Headache, back pain, and stomach pain are sometimes diagnosed as psychogenic. Sufferers are often stigmatized, because both medical professionals and the general public tend to think that pain from a psychological source is not "real". However, specialists consider that it is no less actual or hurtful than pain from any other source.

People with long term pain frequently display psychological disturbance, with elevated scores on the Minnesota Multiphasic Personality Inventory scales of hysteria, depression and hypochondriasis (the "neurotic triad"). Some investigators have argued that it is this neuroticism that causes acute injuries to turn chronic, but clinical evidence points the other way, to chronic pain causing neuroticism. When long term pain is relieved by therapeutic intervention, scores on the neurotic triad and anxiety fall, often to normal levels. Self-esteem, often low in chronic pain patients, also shows striking improvement once pain has resolved.

“The term 'psychogenic' assumes that medical diagnosis is so perfect that all organic causes of pain can be detected; regrettably, we are far from such infallability... All too often, the diagnosis of neurosis as the cause of pain hides our ignorance of many aspects of pain medicine.” Ronald Melzack, 1996.Phantom pain

Phantom pain is pain from a part of the body that has been lost or from which the pain no longer receives physical signals. It is a type of neuropathic pain. Phantom limb pain is a common experience of amputees. One study found that eight days after amputation, 72 per cent of patients had phantom limb pain, and six months later, 65 percent reported it. Some experience continuous pain that varies in intensity or quality; others experience several bouts a day, or it may occur only once every week or two. It is described as shooting, crushing, burning or cramping. If the pain is continuous for a long period, parts of the intact body may become sensitized, so that touching them evokes pain in the phantom limb, or phantom limb pain may accompany urination or defecation.

Local anesthetic injections into the nerves or sensitive areas of the stump may relieve pain for days, weeks or, sometimes permanently, despite the drug wearing off in a matter of hours; and small injections of hypertonic saline into the soft tissue between vertepae produces local pain that radiates into the phantom limb for ten minutes or so and may be followed by hours, weeks or even longer of partial or total relief from phantom pain. Vigorous vipation or electrical stimulation of the stump, or current from electrodes surgically implanted onto the spinal cord all produce relief in some patients.

Paraplegia, the loss of sensation and voluntary motor control after serious spinal cord damage, may be accompanied by root ("girdle") pain at the level of the spinal cord damage, visceral pain evoked by a filling bladder or bowel, or, in five to ten per cent of paraplegics, phantom body pain in areas of complete sensory loss. Phantom body pain is initially described as burning or tingling but may evolve into severe crushing or pinching pain, fire running down the legs, or a knife twisting in the flesh. Onset may be immediate or may not occur until years after the disabling injury. Surgical treatment rarely provides lasting relief.

Pain asymbolia

Pain science acknowledges, in a puzzling challenge to IASP definition, that pain may be experienced as a sensation devoid of any unpleasantness: this happens in a syndrome called pain asymbolia or pain dissociation, caused by conditions like lobotomy, cingulotomy or morphine analgesia. Typically, such patients report that they have pain but are not bothered by it, they recognize the sensation of pain but are mostly or completely immune to suffering from it.

Insensitivity to pain

The ability to experience pain is essential for protection from injury, and recognition of the presence of injury. Episodic analgesia may occur under special circumstances, such as in the excitement of sport or war: a soldier on the battlefield may feel no pain for many hours from a traumatic amputation or other severe injury. However, insensitivity to pain may also be acquired following conditions such as spinal cord injury, diabetes mellitus, or more rarely leprosy. A small number of people suffer from congenital analgesia ("congenital insensitivity to pain"), a genetic defect that puts these iniduals at constant risk from the consequences of unrecognized injury or illness. Children with this condition suffer carelessly repeated damage to their tongue, eyes, joints, skin, and muscles. They may attain adulthood, but have a shortened life expectancy.

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