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Pain is a distressing feeling often caused by intense or damaging stimuli, such as stubbing a toe, burning a finger, or putting alcohol on a cut.[1] Because it is a complex, subjective phenomenon, defining pain has been a challenge. The International Association for the Study of Pain's widely used definition states: "Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage."[2] In medical diagnosis, pain is regarded as a symptom of an underlying condition.

Pain motivates the individual to withdraw from damaging situations, to protect a damaged body part while it heals, and to avoid similar experiences in the future.[3] Most pain resolves once the noxious stimulus is removed and the body has healed, but it may persist despite removal of the stimulus and apparent healing of the body. Sometimes pain arises in the absence of any detectable stimulus, damage or disease.[4]

Pain is the most common reason for physician consultation in most developed countries.[5][6] It is a major symptom in many medical conditions, and can interfere with a person's quality of life and general functioning.[7] Simple pain medications are useful in 20% to 70% of cases.[8]Psychological factors such as social support, hypnotic suggestion, excitement, or distraction can significantly affect pain's intensity or unpleasantness.[9][10] In some arguments put forth in physician-assisted suicide or euthanasia debates, pain has been used as an argument to permit people who are terminally ill to end their lives.[11]

One judgment on the value of pain is given by German philosopher, Friedrich Nietzsche, who wrote: "Only great pain is the ultimate liberator of the spirit….I doubt that such pain makes us ‘better’; but I know that it makes us more profound”.[12] Nietzsche and philosophers influenced by him thus oppose the entirely negative valuation of pain, instead holding that 'What does not destroy me, makes me stronger."[12][13]

ClassificationEdit
In 1994, responding to the need for a more useful system for describing chronic pain, theInternational Association for the Study of Pain(IASP) classified pain according to specific characteristics: However, this system has been criticized byClifford J. Woolf and others as inadequate for guiding research and treatment.[15] Woolf suggests three classes of pain:
 * 1) region of the body involved (e.g. abdomen, lower limbs),
 * 2) system whose dysfunction may be causing the pain (e.g., nervous, gastrointestinal),
 * 3) duration and pattern of occurrence,
 * 4) intensity and time since onset, and
 * 5) cause[14]
 * 1) nociceptive pain,
 * 2) inflammatory pain which is associated with tissue damage and the infiltration of immune cells, and
 * 3) pathological pain which is a disease state caused by damage to the nervous system or by its abnormal function (e.g. fibromyalgia,peripheral neuropathy, tension type headache, etc.).[16]

DurationEdit
Main article: Chronic pain

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 andidiopathic pain, may persist for years. Pain that lasts a long time is called chronic or persistent, 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,[17] though some theorists and researchers have placed the transition from acute to chronic pain at 12 months.[18]:93Others 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.[19] A popular alternative definition of chronic pain, involving no arbitrarily fixed durations, is "pain that extends beyond the expected period of healing".[17] Chronic pain may be classified ascancer pain or else as benign.[19]

NociceptiveEdit
Main article: Nociception

Nociceptive pain is caused by stimulation ofsensory nerve fibers that respond to stimuli approaching or exceeding harmful intensity (nociceptors), and may be classified according to the mode of noxious stimulation. The most common categories are "thermal" (e.g. heat or cold), "mechanical" (e.g. crushing, tearing, shearing, etc.) and "chemical" (e.g. iodine in a cut or chemicals released during inflammation). Some nociceptors respond to more than one of these modalities and are consequently designated polymodal.

Nociceptive pain may also be divided into "visceral", "deep somatic" and "superficial somatic" pain. Visceral structures are highly sensitive to stretch, ischemia andinflammation, but relatively insensitive to other stimuli that normally evoke pain in other structures, such as burning and cutting.Visceral pain is diffuse, difficult to locate and often referred to a distant, usually superficial, structure. It may be accompanied by nausea and vomiting and may be described as sickening, deep, squeezing, and dull.[20] 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 broken bones. Superficialpain is initiated by activation of nociceptors in the skin or other superficial tissue, and is sharp, well-defined and clearly located. Examples of injuries that produce superficial somatic pain include minor wounds and minor (first degree) burns.[18]

NeuropathicEdit
Main article: Neuropathic pain

Neuropathic pain is caused by damage or disease affecting any part of the nervous system involved in bodily feelings (thesomatosensory system).[21] Peripheralneuropathic pain is often described as "burning", "tingling", "electrical", "stabbing", or "pins and needles".[22] Bumping the "funny bone" elicits acute peripheral neuropathic pain.

PhantomEdit
Main article: Phantom pain

Phantom pain is pain felt in a part of the body that has been lost or from which the brain no longer receives signals. It is a type of neuropathic pain. Phantom limb pain is a common experience of amputees.[23]

The prevalence of phantom pain in upper limb amputees is nearly 82%, and in lower limb amputees is 54%.[23] One study found that eight days after amputation, 72 percent of patients had phantom limb pain, and six months later, 65 percent reported it.[24][25]Some amputees 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 often 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. Phantom limb pain may accompany urination or defecation.[26]

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 hypertonicsaline into the soft tissue between vertebrae 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 vibration or electrical stimulation of the stump, or current from electrodes surgically implanted onto the spinal cord, all produce relief in some patients.[26]

Mirror box therapy produces the illusion of movement and touch in a phantom limb which in turn may cause a reduction in pain.[27]

Paraplegia, the loss of sensation and voluntary motor control after serious spinal cord damage, may be accompanied by 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. This phantom body pain is initially described as burning or tingling but may evolve into severe crushing or pinching pain, or the sensation of fire running down the legs or of 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.[26]

PsychogenicEdit
Main article: Psychogenic pain

Psychogenic pain, also called psychalgia orsomatoform pain, is pain caused, increased, or prolonged by mental, emotional, or behavioral factors.[28] Headache, back pain, and stomach pain are sometimes diagnosed as psychogenic.[29] 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.[30]

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 pain 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 improvement once pain has resolved.[31]

Breakthrough painEdit
Breakthrough pain is transitory acute pain that comes on suddenly and is not alleviated by the patient's regular pain management. It is common in cancer patients who often have background pain that is generally well-controlled by medications, but who also sometimes experience bouts of severe pain that from time to time "breaks through" the medication. The characteristics of breakthrough cancer pain vary from person to person and according to the cause. Management of breakthrough pain can entail intensive use of opioids, includingfentanyl.[32][33][34]

Incident painEdit
Incident pain is pain that arises as a result of activity, such as movement of an arthriticjoint, stretching a wound, etc.

Pain asymbolia and insensitivityEdit
Main articles: Pain asymbolia and Congenital insensitivity to pain

"Painless" redirects here. For other uses, see Painless (disambiguation).

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.[35]

Although unpleasantness is an essential part of the IASP definition of pain,[2] it is possible to induce a state described as intense pain devoid of unpleasantness in some patients, with morphine injection or psychosurgery.[30]Such patients report that they have pain but are not bothered by it; they recognize the sensation of pain but suffer little, or not at all.[36] Indifference to pain can also rarely be present from birth; these people have normal nerves on medical investigations, and find pain unpleasant, but do not avoid repetition of the pain stimulus.[37]

Insensitivity to pain may also result from abnormalities in the nervous system. This is usually the result of acquired damage to the nerves, such as spinal cord injury, diabetes mellitus (diabetic neuropathy), or leprosy in countries where that disease is prevalent.[38]These individuals are at risk of tissue damage and infection due to undiscovered injuries. People with diabetes-related nerve damage, for instance, sustain poorly-healing foot ulcers as a result of decreased sensation.[39]

A much smaller number of people are insensitive to pain due to an inborn abnormality of the nervous system, known as "congenital insensitivity to pain".[37] Children with this condition incur carelessly-repeated damage to their tongues, eyes, joints, skin, and muscles. Some die before adulthood, and others have a reduced life expectancy.[citation needed] Most people with congenital insensitivity to pain have one of five hereditary sensory and autonomic neuropathies (which includes familial dysautonomia and congenital insensitivity to pain with anhidrosis).[40] These conditions feature decreased sensitivity to pain together with other neurological abnormalities, particularly of the autonomic nervous system.[37][40] A very rare syndrome with isolated congenital insensitivity to pain has been linked with mutations in the SCN9Agene, which codes for a sodium channel (Nav1.7) necessary in conducting pain nerve stimuli.[41]