Pain is a distressing bodily 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 animals 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 over-the-counter pain medications are useful in 20% to 70% of cases of common acute pain, such as post-tooth extraction pain.[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

In many cases, pain fits into one or a combination of three categories:[14]

  • Nociceptive pain (caused by inflamed or damaged tissue that activates pain sensors called nociceptors).[15] Nociceptive pain is divided into "superficial" and "deep" pain. Deep pains are divided into two parts: "deep physical" and "deep visceral" pain.[16]
  • Nociplastic pain is pain that arises despite no clear evidence of tissue or somatosensory system damage causing the pain.[20]


In 1994, the International Association for the Study of Pain recommended using specific features to describe a patient's pain:

  1. Region of the body involved (e.g., abdomen or lower limbs)
  2. System whose dysfunction may be causing the pain (e.g., nervous or gastrointestinal systems)
  3. Duration and pattern of occurrence
  4. Intensity
  5. Cause[21]

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 rheumatoid arthritis, peripheral neuropathy, cancer, and idiopathic pain, may persist for years. Pain that lasts a long time is called "chronic" or "persistent", and pain that resolves quickly is called "acute".

The International Association for the Study of Pain defines "chronic pain" as "pain that persists or recurs for longer than 3 months."[22]

Allodynia

Allodynia is pain experienced in response to an ordinarily painless stimulus.[23] It has no biological function and is classified by characteristics of the stimuli as cold, heat, touch, pressure or a pinprick.[23][24]

Phantom

Phantom pain is pain felt in a part of the body that has been amputated or from which the brain no longer receives signals. It is a type of neuropathic pain.[25]

The prevalence of phantom pain in upper limb amputees is nearly 82%, and in lower limb amputees is 54%.[25] One study found that eight days after amputation, 72% of patients had phantom limb pain, and six months later, 67% reported it.[26][27] 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 an extended period, parts of the intact body may become sensitized, so touching them evokes pain in the phantom limb. Phantom limb pain may accompany urination or defecation.[28]: 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.[28]: 61–69 

Mirror box therapy produces the illusion of movement and touch in a phantom limb, which in turn may cause a reduction in pain.[29]

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 percent of people with paraplegia, 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 or may not occur immediately, and can happen years after the disabling injury. Surgical treatment rarely provides lasting relief.[28]: 61–69 

Breakthrough

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.[30][31]

Asymbolia and insensitivity

A patient and doctor discuss congenital 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.[32]

Although unpleasantness is an essential part of the IASP definition of pain,[33] 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.[34] Such patients report pain but are not bothered by it; they recognize the sensation of pain but suffer little or not at all.[35] 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.[36]

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.[37] 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.[38]

Functional effects

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.[39] Pain is also associated with increased depression, anxiety, fear, and anger,[40] as well as impaired sleep.[41]

On subsequent negative emotion

Although pain is considered to be aversive and unpleasant and is therefore usually avoided, a meta-analysis which summarized and evaluated numerous studies from various psychological disciplines, found a reduction in negative affect. Across studies, participants that were subjected to acute physical pain in the laboratory subsequently reported feeling better than those in non-painful control conditions, a finding which was also reflected in physiological parameters.[43] A potential mechanism to explain this effect is provided by the opponent-process theory.