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."
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. Congenital insensitivity to pain may result in reduced life expectancy. 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.
Pain is the most common reason for physician consultation in most developed countries. It is a major symptom in many medical conditions, and can interfere with a person's quality of life and general functioning. 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 acute pain cases. Psychological factors such as social support, cognitive behavioral therapy, excitement, or distraction can affect pain's intensity or unpleasantness.
Etymology
First attested in English in 1297, the word peyn comes from the Old French peine, in turn from Latin poena meaning "punishment, penalty" and that from Greek ποινή, generally meaning "price paid, penalty, punishment".Classification
In many cases, pain fits into one or a combination of three categories:- Nociceptive pain. Nociceptive pain is divided into "superficial" and "deep" pain. Deep pains are divided into two parts: "deep physical" and "deep visceral" pain.
- Neuropathic pain. Neuropathic pain is divided into "peripheral" and "central". Peripheral neuropathy is often described as "burning", "tingling", "electrical", "stabbing", or "pins and needles".
- Nociplastic pain is pain that arises despite no clear evidence of tissue or somatosensory system damage causing the pain.
- Region of the body involved
- System whose dysfunction may be causing the pain
- Duration and pattern of occurrence
- Intensity
- Cause
Chronic versus acute
Allodynia
is pain experienced in response to an ordinarily painless stimulus. It has no biological function and is classified by characteristics of the stimuli as cold, heat, touch, pressure or a pinprick.Phantom
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.The prevalence of phantom pain in upper limb amputees is nearly 82%, and in lower limb amputees is 54%. One study found that eight days after amputation, 72% of patients had phantom limb pain, and six months later, 67% reported it. 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.
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.
Mirror box therapy produces the illusion of movement and touch in a phantom limb, which in turn may cause a reduction in pain.
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 years after the disabling injury. Surgical treatment rarely provides lasting relief.
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.Asymbolia and insensitivity
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.Although unpleasantness is an essential part of the IASP definition of pain, 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. Such patients report pain but are not bothered by it; they recognize the sensation of pain but suffer little or not at all. 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.
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, or leprosy in countries where that disease is prevalent. 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.
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". 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. Most people with congenital insensitivity to pain have one of five hereditary sensory and autonomic neuropathies. These conditions feature decreased sensitivity to pain together with other neurological abnormalities, particularly of the autonomic nervous system. A very rare syndrome with isolated congenital insensitivity to pain has been linked with mutations in the SCN9A gene, which codes for a sodium channel necessary in conducting pain nerve stimuli.
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. Pain is also associated with increased depression, anxiety, fear, and anger.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. A potential mechanism to explain this effect is provided by the opponent-process theory.Theory
Historical
Before the relatively recent discovery of neurons and their role in pain, various body functions were proposed to account for pain. There were several competing early theories of pain among the ancient Greeks: Hippocrates believed that it was due to an imbalance in vital fluids. In the 11th century, Avicenna theorized that there were a number of feeling senses, including touch, pain, and titillation.File:Jan Baptist Weenix - Portrait of René Descartes.jpg|thumb|right|Portrait of René Descartes by Jan Baptist Weenix, 1647–1649
In 1644, René Descartes theorized that pain was a disturbance that passed along nerve fibers until the disturbance reached the brain. The work of Descartes and Avicenna prefigured the 19th-century development of specificity theory. Specificity theory saw pain as "a specific sensation, with its own sensory apparatus independent of touch and other senses". Another theory that came to prominence in the 18th and 19th centuries was intensive theory, which conceived of pain not as a unique sensory modality, but an emotional state produced by stronger than normal stimuli such as intense light, pressure or temperature. By the mid-1890s, specificity was backed primarily by physiologists and physicians, and psychologists mostly backed the intensive theory. However, after a series of clinical observations by Henry Head and experiments by Max von Frey, the psychologists migrated to specificity almost en masse. By the century's end, most physiology and psychology textbooks presented pain specificity as fact.