The International Association for the Study of Pain (IASP) classification system describes 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. Chronic pain may be classified as"cancer" or "benign".

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,Turk, DC; Okifuji, A (2001). "Pain terms and taxonomies of pain". In Loeser, JD; Bonica, JJ. Bonica's management of pain (third ed.) . Philadelphia: Lippincott Williams & Wilkins. ISBN 0683304623. 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."

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, such as myofascial pain (emanating from skeletal muscles or the fibrous sheath surrounding them), rheumatic pain (emanating from the joints and surrounding tissue), 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 divided 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 peripheral nerve fibers that respond only to stimuli approaching or exceeding harmful intensity ( 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 divided into "visceral," "deep somatic" and "superficial somatic" pain. Visceral pain originates in the viscera (organs) and often is extremely difficult to locate, and nociception from some visceral regions produces "referred" pain, where the sensation is located in an area distant from the site of injury or pathology. 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. Superficial 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.

Neuropathic

Neuropathic pain is caused by damage to or malfunction of the nervous system, and is divided into "peripheral" (originating in the peripheral nervous system) and " central " (originating in the brain or spinal cord). 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. Cleveland Clinic, Health information

^ "Psychogenic pain - definition from Biology-Online.org" Biology-online.org. Retrieved 2008-11-05. Headache, back pain, and stomach pain are sometimes diagnosed as psychogenic.

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 infallibility... 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 brain 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 percent of patients had phantom limb pain, and six months later, 65 percent reported it. 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, or phantom limb pain may accompany urination or defecation.Melzack, R; Wall, PD (1996). The challenge of pain (2 ed.). London: Penguin. pp. 61???69. ISBN 4780140256703.

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 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.

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. 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

Although unpleasantness is an essential part of the IASP definition of pain, it is possible to induce a state described as intense pain devoid of unpleasantness in some patients, with morphine injection or psychosurgery. 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.

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 individuals at constant risk from the consequences of unrecognized injury or illness. Children with this condition incur carelessly repeated damage to their tongue, eyes, joints, skin, and muscles. They may attain adulthood, but have a reduced life expectancy.

Experimental subjects challenged by acute pain and patients in chronic pain experience impairments in attention control, working memory, mental flexibility, problem solving, and information processing speed.

Specificity

In his 1664 Treatise of Man, Ren?? Descartes traced a pain pathway. "Particles of heat" (A) activate a spot of skin (B) attached by a fine thread (cc) to a valve in the brain (de) where this activity opens the valve, allowing the animal spirits to flow from a cavity  (F) into the muscles that then flinch from the stimulus, turn the head and eyes toward the affected body part, and move the hand and turn the body protectively. The underlying premise of this model - that pain is the direct product of a noxious stimulus activating a dedicated pain pathway, from a receptor in the skin, along a thread or chain of nerve fibers to the pain center in the brain, to a mechanical behavioral response - remained the dominant perspective on pain until the mid-nineteen sixties.

Pattern

Specificity theory (dedicated pain receptor and pathway) has been challenged by the theory, proposed initially in 1874 by Wilhelm Erb , that a pain signal can be generated by stimulation of any sensory receptor, provided the stimulation is intense enough: the pattern of stimulation (intensity over time and area), not the receptor type, determines whether nociception occurs. Alfred Goldscheider (1894) proposed that over time, activity from many sensory fibers might accumulate in the dorsal horns of the spinal cord and begin to signal pain once a certain threshold of accumulated stimulation has been crossed. In 1953, Willem Noordenbos observed that a signal carried from the area of injury along large diameter "touch, pressure or vibration" fibers may inhibit the signal carried by the thinner "pain" fibers - the ratio of large fiber signal to thin fiber signal determining pain intensity; hence, we rub a smack. This was taken as a demonstration that pattern of stimulation (of large and thin fibers in this instance) modulates pain intensity.

Gate Control

Melzack and Wall introduced their "gate control" theory of pain in the 1965 Science article "Pain Mechanisms: A New Theory". The authors proposed that thin ("pain") and large diameter ("touch, pressure, vibration") nerve fibers carry information from the site of injury to two destinations in the dorsal horn of the spinal cord: the "inhibitory" cells and the "transmission" cells. Signals from both thin and large diameter fibers excite the transmission cells, and when the output of the transmission cells exceeds a critical level, pain begins. The job of the inhibitory cells is to inhibit activation of the transmission cells. The transmission cells are the gate on pain, and inhibitory cells can shut the gate. When thin (pain) and large (touch, etc.) fibers, activated by a noxious event, excite a spinal cord transmission cell, they also act on its inhibitory cells. The thin fibers impede the inhibitory cells (tending to leave the gate open) while the large diameter fibers excite the inhibitory cells (tending to close the gate). So, the more large fiber activity relative to thin fiber activity coming from the inhibitory cell's receptive field, the less pain is felt. The authors had conceived a neural "circuit diagram" to explain why we rub a smack.

They pictured not only a signal traveling from the site of injury to the inhibitory and transmission cells and up the spinal cord to the brain, but also a signal traveling from the site of injury directly up the cord to the brain (bypassing the inhibitory and transmission cells) where, depending on the state of the brain, it may trigger a signal back down the spinal cord to modulate inhibitory cell activity (and so pain intensity). This was the first theory to offer a physiological explanation for the previously reported effect of psychology on pain perception.

Dimensions

In 1968 Melzack and Casey described pain in terms of its three dimensions: "Sensory-discriminative" (sense of the intensity, location, quality and duration of the pain), "Affective-motivational" (unpleasantness and urge to escape the unpleasantness), and "Cognitive-evaluative" (cognitions such as appraisal, cultural values, distraction and hypnotic suggestion). They theorized that pain intensity (the sensory discriminative dimension) and unpleasantness (the affective-motivational dimension) are not simply determined by the magnitude of the painful stimulus, but ???higher??? cognitive activities (the cognitive-evaluative dimension) can influence perceived intensity and unpleasantness. Cognitive activities "may affect both sensory and affective experience or they may modify primarily the affective-motivational dimension. Thus, excitement in games or war appears to block both dimensions of pain, while suggestion and placebos may modulate the affective-motivational dimension and leave the sensory-discriminative dimension relatively undisturbed." (p. 432) The paper ends with a call to action: "Pain can be treated not only by trying to cut down the sensory input by anesthetic block, surgical intervention and the like, but also by influencing the motivational-affective and cognitive factors as well." (p. 435)

Theory today

Wilhelm Erb's (1874) early pattern theory hypothesis, that a pain signal can be generated by intense enough stimulation of any sensory receptor, has been soundly disproved.

Because the A-delta fiber is thinly sheathed in an electrically insulating material ( myelin), it carries its signal faster (2.5???35  m/s) than the unmyelinated C fiber (0.5???2.0 m/s). Pain evoked by the (faster) A-delta fibers is described as sharp and is felt first. This is followed by a duller pain, often described as burning, carried by the C fibers.

A.D.Craig and colleagues have identified fibers dedicated to carrying A-delta fiber pain signals, and others dedicated to carrying C fiber pain signals up the spinal cord to the thalamus in the brain. Pain-related activity in the thalamus spreads to the insular cortex (thought to embody, among other things, the feeling that distinguishes pain from other homeostatic emotions such as itch and nausea) and anterior cingulate cortex (thought to embody, among other things, the motivational element of pain); and pain that is distinctly located also activates the primary and secondary somatosensory cortices.

Koji Inui and colleagues have recently shown that pain reduction due to non-noxious touch or vibration can result from activity within the cerebral cortex, with minimal contribution at the spinal level. Melzack and Casey's 1968 picture of the dimensions of pain is as influential today as ever, firmly framing theory and guiding research in the functional neuroanatomy and psychology of pain.

Evolutionary and behavioral role

Pain is part of the body's defense system, producing a reflexive retraction from the painful stimulus, and tendencies to protect the affected body part while it heals, and avoid that harmful situation in the future. It is an important part of animal life, vital to healthy survival. People with congenital insensitivity to pain have reduced life expectancy. Idiopathic pain (pain that persists after the trauma or pathology has healed, or that arises without any apparent cause), may be an exception to the idea that pain is helpful to survival, although John Sarno argues that such pain is psychogenic , enlisted as a protective distraction to keep dangerous emotions unconscious. It is not clear what the survival benefit of some extreme forms of pain (e.g. toothache) might be, and the intensity of some forms of pain (for example as a result of injury to fingernails or toenails) seems to be out of all proportion to any survival benefits.

Thresholds

Variations in pain threshold or in pain tolerance occur between individuals for various reasons including cultural background, ethnicity, genetics, and gender. In pain science, thresholds are measured by gradually increasing the intensity of a stimulus such as electric current or heat applied to the body. The " pain perception threshold " is the point at which the stimations to other mental states, slerance threshold" is reached when the subject acts to stop the pain. There is significant variation in pain perception and tolerance thresholds between cultural groups. For example, people of Mediterranean origin report as painful certain radiant heat intensities that northern Europeans describe as warmth, and Italian women tolerate less electric shock than Jewish or Native American women. Some individuals in all cultures have considerably higher than normal pain perception and tolerance thresholds. For instance, patients who experience painless heart attacks have significantly higher pain thresholds for electric shock, heat and arm-muscle cramp than those who experience painful heart attacks.

A person's self report is the most reliable measure of pain, with health care professionals tending to underestimate severity. A definition of pain widely employed in nursing, emphasizing its subjective nature and the importance of believing patient reports, was introduced by Margo McCaffery in 1968: "Pain is whatever the experiencing person says it is, existing whenever he says it does". To assess intensity, the patient may be asked to locate their pain on a scale of 0 to 10, with 0 being no pain at all, and 10 the worst pain they have ever felt. Quality can be established by having the patient complete the McGill Pain Questionnaire indicating which words best describe their pain.

Multidimensional pain inventory

The Multidimensional Pain Inventory (MPI) is a questionnaire designed to assess the psychosocial state of a person with chronic pain. Analysis of MPI results by Turk and Rudy (1988) found three classes of chronic pain patient: "(a) dysfunctional, people who perceived the severity of their pain to be high, reported that pain interfered with much of their lives, reported a higher degree of psychological distress caused by pain, and reported low levels of activity; (b) interpersonally distressed, people with a common perception that significant others were not very supportive of their pain problems; and (c) adaptive copers, patients who reported high levels of social support, relatively low levels of pain and perceived interference, and relatively high levels of activity." Combining the MPI characterization of the person with their IASP five-category pain profile is recommended for deriving the most useful case description.

Assessment in nonverbal patients

When a person is non-verbal and cannot self report pain, observation becomes critical, and specific behaviors can be monitored as pain indicators. Behaviors such as facial grimacing and guarding indicate pain, as well as an increase or decrease in vocalizations, changes in routine behavior patterns and mental status changes. Patients experiencing pain may exhibit withdrawn social behavior and possibly experience a decreased appetite and decreased nutritional intake. A change in condition that deviates from baseline such as moaning with movement or when manipulating a body part, and limited range of motion are also potential pain indicators. In patients who possess language but are incapable of expressing themselves effectively, such as those with dementia, an increase in confusion or display of aggressive behaviors, including agitation, may signal that discomfort exists, and further assessment is necessary.

Infants feel pain but they lack the language needed to report it, so communicate distress by crying. A non-verbal pain assessment should be conducted involving the parents, who will notice changes in the infant not obvious to the health care provider. Pre-term babies are more sensitive to painful stimuli than full term babies.

Other barriers to reporting

An aging adult may not respond to pain in the way that a younger person would. Their ability to recognize pain may be blunted by illness or the use of multiple prescription drugs. Depression may also keep the older adult from reporting they are in pain. The older adult may also quit doing activities they love because it hurts too much. Decline in self-care activities (dressing, grooming, walking, etc.) may also be indicators that the older adult is experiencing pain. The older adult may refrain from reporting pain because they are afraid they will have to have surgery or will be put on a drug they become addicted to. They may not want others to see them as weak, or may feel there is something impolite or shameful in complaining about pain, or they may feel the pain is deserved punishment for past transgressions.

Cultural barriers can also keep a person from telling someone they are in pain. Religious beliefs may prevent the individual from seeking help. They may feel certain pain treatment is against their religion. They may not report pain because they feel it is a sign that death is near. Many people fear the stigma of addiction and avoid pain treatment so as not to be prescribed addicting drugs. Many Asians do not want to lose respect in society by admitting they are in pain and need help, believing the pain should be borne in silence, while other cultures feel they should report pain right away and get immediate relief.

As an aid to diagnosis

Pain is a symptom of many medical conditions. Knowing the time of onset, location, intensity, pattern of occurrence (continuous, intermittent, etc.), exacerbating and relieving factors, and quality (burning, sharp, etc.) of the pain will help the examining physician to accurately diagnose the problem. For example, chest pain described as extreme heaviness may indicate myocardial infarction, while chest pain described as tearing may indicate aortic dissection.

Medication

Acute pain is usually managed with medications such as analgesics and anesthetics. Management of chronic pain, however, is much more difficult and may require the coordinated efforts of a pain management team, which typically includes medical practitioners, clinical psychologists, physiotherapists, occupational therapists, and nurse practitioners.

Inadequate treatment of pain is widespread throughout surgical wards, intensive care units, accident and emergency departments, in general practice, in the management of all forms of chronic pain including cancer pain, and in end of life care. This neglect is extended to all ages, from neonates to the frail elderly. African and Hispanic Americans are more likely than others to suffer needlessly in the hands of a physician;

Bonham, VL (2001). "Race, ethnicity, and pain treatment: Striving to understand the causes and solutions to the disparities in pain treatment" . Journal of law, medicine & ethics , 29 : 52???68

^ Green, GR; Anderson, KO; Baker, TA et al; Campbell, Lisa C.; Decker, Sheila; Fillingim, Roger B.; Kaloukalani, Donna A.; Lasch, Kathyrn E. et al. (2003). "The unequal burden of pain: Confronting racial and ethnic disparities in pain". Pain medicine 4 (3): 277???94. doi:10.1046/j.1526-4637.2003.03034.x. PMID 12974827 and women's pain is more likely to be undertreated than men's.

The International Association for the Study of Pain advocates that the relief of pain should be recognized as a human right, that chronic pain should be considered a disease in its own right, and that pain medicine should have the full status of a specialty. It is a specialty only in China and Australia at this time. Elsewhere, pain medicine is a subspecialty under disciplines such as anesthesiology, physiatry, neurology, palliative medicine and psychiatry.

Sugar taken orally reduces the total crying time but not the duration of the first cry in newborns undergoing a painful procedure (a single lancing of the heel). It does not moderate the effect of pain on heart rate and a recent single study found that sugar did not significantly affect pain-related electrical activity in the brains of newborns one second after the heel lance procedure but the sample size in that study was small, the researchers used a novel technique to determine pain-related brain activity, and the results will need to be replicated by larger, more rigorous trials before the results can be meaningfully interpreted. Sweet oral liquid moderately reduces the incidence and duration of crying caused by immunization injection in children between one and twelve months of age.

Psychological

Individuals with more social support experience less cancer pain, take less pain medication, are less likely to suffer from chest pain after coronary artery bypass surgery, report less labor pain and are less likely to use epidural anesthesia during childbirth.

Suggestion can significantly affect pain intensity. About 35% of people report marked relief after receiving a saline injection they believe to have been morphine. The placebo effect is more pronounced in people who are prone to anxiety, so anxiety reduction may account for some of the effect, but it does not account for all of the effect. Placebos are more effective in intense pain than mild pain; and they produce progressively weaker effects with repeated administration.

It is possible for many chronic pain sufferers to become so absorbed in an activity or entertainment that the pain is no longer felt, or is greatly diminished.

Alternative medicine

Pain is the most common reason that people use complementary and alternative medicine.

Pain is the main reason for visiting the emergency department in more than 50% of cases and is present in 30% of family practice visits. Several epidemiological studies from different countries have reported widely varying prevalence rates for chronic pain, ranging from 12-80% of the population

It becomes more common as people approach death. In the last two years of life 26% had pain increasing to 46% in the last month.

The nature or meaning of physical pain has been diversely understood by religious or secular traditions from antiquity to modern times.

Physical pain is an important political topic in relation to various issues, including pain management policy, drug control, animal rights, torture, pain compliance. In various contexts, the deliberate infliction of pain in the form of corporal punishment is used as retribution for an offence, or for the purpose of disciplining or reforming a wrongdoer, or to deter attitudes or behaviour deemed unacceptable. In some cultures, extreme practices such as mortification of the flesh or painful rites of passage are highly regarded.

Philosophy of pain is a branch of philosophy of mind that deals essentially with physical pain. Identity theorists assert that the mental state of pain is completely identical with some physiological state. Functionalists consider that pain as a mental state is constituted solely by its functional role, by its causal relations to other mental states, sensory inputs, and behavioral outputs.

More generally, it is often as a part of pain in the broad sense, i.e., suffering, that physical pain is dealt with in culture, religion, philosophy, or society.

The most reliable method for assessing pain in most humans is by asking a question: a person may report pain that cannot be detected by any known physiological measure. However, like infants (Latin infans meaning "unable to speak"), non-human animals cannot answer questions about whether they feel pain; thus the defining criterion for pain in humans cannot be applied to them. Philosophers and scientists have responded to this difficulty in a variety of ways. Ren?? Descartes for example argued that animals lack consciousness and therefore do not experience pain and suffering in the way that humans do. In his interactions with scientists and other veterinarians, he was regularly asked to "prove" that animals are conscious, and to provide "scientifically acceptable" grounds for claiming that they feel pain. The ability of invertebrate species of animals, such as insects, to feel pain and suffering is also unclear.

The presence of pain in an animal cannot be known for certain, but it can be inferred through physical and behavioral reactions. Specialists currently believe that all vertebrates can feel pain, and that certain invertebrates, like the octopus, might too. As for other animals, plants, or other entities, their ability to feel physical pain is at present a question beyond scientific reach, since no mechanism is known by which they could have such a feeling. In particular, there are no known nociceptors in groups such as plants, fungi, and most insects, except for instance in fruit flies .

In vertebrates, endogenous opioids are neurochemicals that moderate pain by interacting with opiate receptors. Opioids and opiate receptors occur naturally in crustaceans and, although at present no certain conclusion can be drawn, Opioids may mediate their pain in the same way as in vertebrates. Veterinary medicine uses, for actual or potential animal pain, the same analgesics and anesthetics as used in humans.

• Pain Journal of the IASP

• Pain Stanford Encyclopedia of Philosophy