Pain

Pain is a distressing feeling often caused by intense or damaging stimuli. The International Association for the Study of Pain defines pain as "an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage."^[1]

Pain motivates organisms to withdraw from damaging situations, to protect a damaged body part while it heals, and to avoid similar experiences in the future.^[2] 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.^[3]

Pain is the most common reason for physician consultation in most developed countries.^[4][5] It is a major symptom in many medical conditions, and can interfere with a person's quality of life and general functioning.^[6] People in pain experience impaired concentration, working memory, mental flexibility, problem solving and information processing speed, and are more likely to experience irritability, depression and anxiety.

Simple pain medications are useful in 20% to 70% of cases.^[7] Psychological factors such as social support, cognitive behavioral therapy, excitement, or distraction can affect pain's intensity or unpleasantness.^[8][9]

Etymology

First attested in English in 1297, the word peyn comes from the Old French peine, in turn from Latin poena meaning "punishment, penalty"^[10][11] (also meaning "torment, hardship, suffering" in Late Latin) and that from Greek ποινή (poine), generally meaning "price paid, penalty, punishment".^[12][13]

Classification

The International Association for the Study of Pain recommends using specific features to describe a patient's 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
5. Cause^[14]

Chronic versus acute

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

Allodynia is pain experienced in response to a normally painless stimulus.^[18] It has no biological function and is classified by characteristics of the stimuli as cold, heat, touch, pressure or pinprick.^[18][19]

Phantom

The prevalence of phantom pain in upper limb amputees is nearly 82%, and in lower limb amputees is 54%.^[20] One study found that eight days after amputation, 72% of patients had phantom limb pain, and six months later, 67% reported it.^[21][22] Some amputees experience continuous pain that varies in intensity or quality; others experience several bouts of pain per day, or it may reoccur less often. 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.^{[23]: 61–69}

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.^{[23]: 61–69}

Breakthrough pain is transitory 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, including fentanyl.^[25][26]

Asymbolia and insensitivity

The ability to experience pain is essential for protection from injury, and recognition of the presence of injury. Episodic

Although unpleasantness is an essential part of the IASP definition of pain,^[28] it is possible in some patients to induce a state known as pain asymbolia, described as intense pain devoid of unpleasantness, with morphine injection or psychosurgery.^[29] 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.^[30] 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.^[31]

Insensitivity to pain may also result from abnormalities in the

Experimental subjects challenged by acute pain and patients in chronic pain experience impairments in attention control, working memory capacity, mental flexibility, problem solving, and information processing speed.^[36] Pain is also associated with increased depression, anxiety, fear, and anger.^[37]

If I have matters right, the consequences of pain will include direct physical distress, unemployment, financial difficulties, marital disharmony, and difficulties in concentration and attention…

— Harold Merskey 2000^[38]

On subsequent negative emotion

Although pain is considered to be aversive and unpleasant and is therefore usually avoided, a

In 1644,

Some sensory fibers do not differentiate between noxious and non-noxious stimuli, while others, nociceptors, respond only to noxious, high intensity stimuli. At the peripheral end of the nociceptor, noxious stimuli generate currents that, above a given threshold, send signals along the nerve fiber to the spinal cord. The "specificity" (whether it responds to thermal, chemical or mechanical features of its environment) of a nociceptor is determined by which ion channels it expresses at its peripheral end. Dozens of different types of nociceptor ion channels have so far been identified, and their exact functions are still being determined.^[45]

The pain signal travels from the periphery to the spinal cord along

Spinal cord fibers dedicated to carrying A-delta fiber pain signals, and others that carry both A-delta and C fiber pain signals to the thalamus have been identified. Other spinal cord fibers, known as wide dynamic range neurons, respond to A-delta and C fibers, but also to the much larger, more heavily myelinated A-beta fibers that carry touch, pressure and vibration signals.^[46]

In 1968 Ronald Melzack and Kenneth Casey described chronic 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 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 the sensory-discriminative and affective-motivational dimensions of pain, while suggestion and placebos may modulate only the affective-motivational dimension and leave the sensory-discriminative dimension relatively undisturbed.^[52] (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."^[52] (p. 435)

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.^[53][54] It is an important part of animal life, vital to healthy survival. People with congenital insensitivity to pain have reduced life expectancy.^[31]

In The Greatest Show on Earth: The Evidence for Evolution, biologist Richard Dawkins addresses the question of why pain should have the quality of being painful. He describes the alternative as a mental raising of a "red flag". To argue why that red flag might be insufficient, Dawkins argues that drives must compete with one another within living beings. The most "fit" creature would be the one whose pains are well balanced. Those pains which mean certain death when ignored will become the most powerfully felt. The relative intensities of pain, then, may resemble the relative importance of that risk to our ancestors.^[a] This resemblance will not be perfect, however, because natural selection can be a poor designer. This may have maladaptive results such as supernormal stimuli.^[55]

Pain, however, does not only wave a "red flag" within living beings but may also act as a warning sign and a call for help to other living beings. Especially in humans who readily helped each other in case of sickness or injury throughout their evolutionary history, pain might be shaped by natural selection to be a credible and convincing signal of need for relief, help, and care.^[56]

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 some

In pain science, thresholds are measured by gradually increasing the intensity of a stimulus in a procedure called quantitative sensory testing which involves such stimuli as electric current, thermal (heat or cold), mechanical (pressure, touch, vibration), ischemic, or chemical stimuli applied to the subject to evoke a response.^[58] The "pain perception threshold" is the point at which the subject begins to feel pain, and the "pain threshold intensity" is the stimulus intensity at which the stimulus begins to hurt. The "pain tolerance threshold" is reached when the subject acts to stop the pain.^[58]

Assessment

A person's self-report is the most reliable measure of pain.[59]^[60][61] Some health care professionals may underestimate pain severity.^[62] 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".^[63] 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.^[6]

Visual analogue scale

The visual analogue scale is a common, reproducible tool in the assessment of pain and pain relief.^[64] The scale is a continuous line anchored by verbal descriptors, one for each extreme of pain where a higher score indicates greater pain intensity. It is usually 10 cm in length with no intermediate descriptors as to avoid marking of scores around a preferred numeric value. When applied as a pain descriptor, these anchors are often 'no pain' and 'worst imaginable pain". Cut-offs for pain classification have been recommended as no pain (0–4mm), mild pain (5–44mm), moderate pain (45–74mm) and severe pain (75–100mm).^{[65][check quotation syntax]}

Multidimensional pain inventory

The Multidimensional Pain Inventory (MPI) is a questionnaire designed to assess the psychosocial state of a person with chronic pain. Combining the MPI characterization of the person with their IASP five-category pain profile is recommended for deriving the most useful case description.^[15]

Assessment in non-verbal people

Non-verbal people cannot use words to tell others that they are experiencing pain. However, they may be able to communicate through other means, such as blinking, pointing, or nodding.^[66]

With a non-communicative person, observation becomes critical, and specific behaviors can be monitored as pain indicators. Behaviors such as facial grimacing and guarding (trying to protect part of the body from being bumped or touched) 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 or agitation may signal that discomfort exists, and further assessment is necessary. Changes in behavior may be noticed by caregivers who are familiar with the person's normal behavior.^[66]

Infants do feel pain, but lack the language needed to report it, and so communicate distress by crying. A non-verbal pain assessment should be conducted involving the parents, who will notice changes in the infant which may not be obvious to the health care provider. Pre-term babies are more sensitive to painful stimuli than those carried to full term.^[67]

Another approach, when pain is suspected, is to give the person treatment for pain, and then watch to see whether the suspected indicators of pain subside.^[66]

Other reporting barriers

The way in which one experiences and responds to pain is related to sociocultural characteristics, such as gender, ethnicity, and age.^[68][69] An aging adult may not respond to pain in the same way that a younger person might. Their ability to recognize pain may be blunted by illness or the use of medication. Depression may also keep older adult from reporting they are in pain. Decline in self-care may also indicate the older adult is experiencing pain. They may be reluctant to report pain because they do not want to be perceived as weak, or may feel it is impolite or shameful to complain, or they may feel the pain is a form of deserved punishment.^[70][71]

Cultural barriers may also affect the likelihood of reporting pain. Patients may feel that certain treatments go against their religious beliefs. 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 potentially 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 immediately to receive immediate relief.^[67]

Gender can also be a perceived factor in reporting pain.

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.^[75][76]

Physiological measurement

Functional magnetic resonance imaging brain scanning has been used to measure pain, and correlates well with self-reported pain.^[77][78][79]

Mechanisms

Nociceptive

Nociceptive pain is caused by stimulation of sensory 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 classed according to the site of origin and divided into "visceral", "deep somatic" and "superficial somatic" pain.

Neuropathic pain is caused by damage or disease affecting any part of the

Some manifestations of neuropathic pain include: traumatic neuropathy,

Nociplastic pain is pain characterized by a changed nociception (but without evidence of real or threatened tissue damage, or without disease or damage in the somatosensory system).^[9]

Psychogenic

Psychogenic pain, also called psychalgia or somatoform pain, is pain caused, increased or prolonged by mental, emotional or behavioral factors.^[84] Headache, back pain and stomach pain are sometimes diagnosed as psychogenic.^[84] Those affected 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.^[29]

People with

Pain can be treated through a variety of methods. The most appropriate method depends upon the situation. Management of

Acute pain is usually managed with medications such as analgesics and anesthetics.^[102] Caffeine when added to pain medications such as ibuprofen, may provide some additional benefit.^[103][104] Ketamine can be used instead of opioids for short-term pain.^[105] Pain medications can cause paradoxical side effects, such as opioid-induced hyperalgesia (severe generalized pain caused by long-term opioid use).^[106][107]

Sugar (

Individuals with more social support experience less cancer pain, take less pain medication, report less labor pain and are less likely to use epidural anesthesia during childbirth, or suffer from chest pain after coronary artery bypass surgery.^[8]

Suggestion can significantly affect pain intensity. About 35% of people report marked relief after receiving a saline injection they believed to be morphine. This placebo effect is more pronounced in people who are prone to anxiety, and so anxiety reduction may account for some of the effect, but it does not account for all of it. Placebos are more effective for intense pain than mild pain; and they produce progressively weaker effects with repeated administration.^{[23]: 26–28} It is possible for many with chronic pain to become so absorbed in an activity or entertainment that the pain is no longer felt, or is greatly diminished.^{[23]: 22–23}

A number of meta-analyses have found

An analysis of the 13 highest quality studies of pain treatment with acupuncture, published in January 2009, concluded there was little difference in the effect of real, faked and no acupuncture.^[113] However, more recent reviews have found some benefit.^{[114][115][116]} Additionally, there is tentative evidence for a few herbal medicines.^[117] There has been some interest in the relationship between vitamin D and pain, but the evidence so far from controlled trials for such a relationship, other than in osteomalacia, is inconclusive.^[118]

For chronic (long-term)

A survey of 6,636 children (0–18 years of age) found that, of the 5,424 respondents, 54% had experienced pain in the preceding three months. A quarter reported having experienced recurrent or continuous pain for three months or more, and a third of these reported frequent and intense pain. The intensity of chronic pain was higher for girls, and girls' reports of chronic pain increased markedly between ages 12 and 14.[124]

Society and culture

Physical pain is a universal experience, and a strong motivator of human and animal behavior. As such, physical pain is used politically in relation to various issues such as pain management  policy, drug control,  animal rights or  animal welfare,  torture, and  pain compliance. The deliberate infliction of pain and the medical management of pain are both important aspects of biopower, a concept that encompasses the "set of mechanisms through which the basic biological features of the human species became the object of a political strategy".^[125]

In various contexts, the deliberate infliction of pain in the form of corporal punishment is used as retribution for an offence, for the purpose of disciplining or reforming a wrongdoer, or to deter attitudes or behaviour deemed unacceptable. In Western societies, the intentional infliction of severe pain (torture) was principally used to extract confession prior to its abolition in the latter part of the 19th century. Torture as a means to punish the citizen has been reserved for offences posing severe threat to the social fabric (for example, treason).^[126]

The administration of torture on bodies othered by the cultural narrative, those observed as not 'full members of society' ^{[126]: 101–121[AD1]} met a resurgence in the 20th century, possibly due to the heightened warfare.^{[126]: 101–121 [AD2]}

Many cultures use painful ritual practices as a catalyst for psychological transformation.^[127] The use of pain to transition to a 'cleansed and purified' state is seen in religious self-flagellation practices (particularly those of Christianity and Islam), or personal catharsis in neo-primitive body suspension experiences.^[128]

Beliefs about pain play an important role in sporting cultures. Pain may be viewed positively, exemplified by the 'no pain, no gain' attitude, with pain seen as an essential part of training. Sporting culture tends to normalise experiences of pain and injury and celebrate athletes who 'play hurt'.^[129]

Pain has psychological, social, and physical dimensions, and is greatly influenced by cultural factors.^[130]

Non-humans

René Descartes argued that animals lack consciousness and therefore do not experience pain and suffering in the way that humans do.^[131] Bernard Rollin of Colorado State University, the principal author of two U.S. federal laws regulating pain relief for animals,^[b] wrote that researchers remained unsure into the 1980s as to whether animals experience pain, and that veterinarians trained in the U.S. before 1989 were simply taught to ignore animal pain.^[133][134] The ability of invertebrate species of animals, such as insects, to feel pain and suffering is unclear.^{[135][136][137]}

Specialists believe that all vertebrates can feel pain, and that certain invertebrates, like the octopus, may also.^{[135][138][139]} The presence of pain in animals is unknown, but can be inferred through physical and behavioral reactions,^[140] such as paw withdrawal from various noxious mechanical stimuli in rodents.^[141]

While

• Feeling, a perceptual state of conscious experience.
• Hedonic adaptation
  , the tendency to quickly return to a relatively stable level of happiness despite major positive or negative events
• Pain (philosophy), the branch of philosophy concerned with suffering and physical pain
• Pain and suffering, the legal term for the physical and emotional stress caused from an injury

Explanatory notes

References

• Casey K (2019). Chasing Pain: The Search for a Neurobiological Mechanism. New York: Oxford University Press.
  ISBN 978-0190880231
  .

External links

Wikimedia Commons has media related to Pain.

Wikiquote has quotations related to Pain.

The Wikibook Sensory Systems has a page on the topic of: Physiology of Pain

The Wikibook Internal Medicine has a page on the topic of: Pain

• Pain at
  Curlie
• "Pain", Stanford Encyclopedia of Philosophy