Back pain

Different regions (curvatures) of the vertebral column
ICD-10	M54
ICD-9	724.5
DiseasesDB	15544
MedlinePlus	007425
MeSH	D001416

Back pain (also known as dorsalgia) is pain felt in the back that usually originates from the muscles, nerves, bones, joints or other structures in the spine.

Back pain may have a sudden onset or can be a chronic pain; it can be constant or intermittent, stay in one place or radiate to other areas. It may be a dull ache, or a sharp or piercing or burning sensation. The pain may radiate into the arms and hands as well as the legs or feet, and may include symptoms other than pain, such as weakness, numbness or tingling.

Back pain is one of humanity's most frequent complaints. In the U.S., acute low back pain (also called lumbago) is the fifth most common reason for physician visits. About nine out of ten adults experience back pain at some point in their life, and five out of ten working adults have back pain every year.^[1]

The spine is a complex interconnecting network of nerves, joints, muscles, tendons and ligaments, and all are capable of producing pain. Large nerves that originate in the spine and go to the legs and arms can make pain radiate to the extremities^[2].

Classification[link]

Back pain can be divided anatomically: neck pain, middle back pain, lower back pain or tailbone pain.

By its duration: acute (less than 4 weeks), subacute (4 – 12 weeks), chronic (greater than 12 weeks).

By its cause: nonspecific back pain, back pain with radiculopathy or spinal stenosis, and back pain associated with another specific cause (such as infection or cancer).^[3] Non specific pain indicates that the cause is not known precisely^[4] but is believed to be due from the soft tissues such as muscles, fascia, and ligaments.^[5]

Back pain is classified according to etiology in mechanical or nonspecific back pain and secondary back pain. Approximately 98% of back pain patients are diagnosed with nonspecific acute back pain which has no serious underlying pathology. However, secondary back pain which is caused by an underlying condition accounts for nearly 2% of the cases. Underlying pathology in these cases may include metastatic cancer, spinal osteomyelitis and epidural abscess which account for 1% of the patients. Also, herniated disc is the most common neurologic impairment which is associated with this condition, from which 95% of disc herniations occur at the lowest two lumbar intervertebral levels.^[6]

Associated conditions[link]

Back pain can be a sign of a serious medical problem, although this is not most frequently the underlying cause:

• Typical warning signs of a potentially life-threatening problem are bowel and/or bladder incontinence or progressive weakness in the legs.
• Severe back pain (such as pain that is bad enough to interrupt sleep) that occurs with other signs of severe illness (e.g. fever, unexplained weight loss) may also indicate a serious underlying medical condition.
• Back pain that occurs after a trauma, such as a car accident or fall, may indicate a bone fracture or other injury.
• Back pain in individuals with medical conditions that put them at high risk for a spinal fracture, such as osteoporosis or multiple myeloma, also warrants prompt medical attention.
• Back pain in individuals with a history of cancer (especially cancers known to spread to the spine like breast, lung and prostate cancer) should be evaluated to rule out metastatic disease of the spine.

Back pain does not usually require immediate medical intervention. The vast majority of episodes of back pain are self-limiting and non-progressive. Most back pain syndromes are due to inflammation, especially in the acute phase, which typically lasts for two weeks to three months.

A few observational studies suggest that two conditions to which back pain is often attributed, lumbar disc herniation and degenerative disc disease may not be more prevalent among those in pain than among the general population, and that the mechanisms by which these conditions might cause pain are not known.^{[7][8][9][10]} Other studies suggest that for as many as 85% of cases, no physiological cause can be shown.^[11][12]

A few studies suggest that psychosocial factors such as on-the-job stress and dysfunctional family relationships may correlate more closely with back pain than structural abnormalities revealed in x-rays and other medical imaging scans.^{[13][14][15][16]}

Differential diagnosis[link]

There are several potential sources and causes of back pain.^[17] However, the diagnosis of specific tissues of the spine as the cause of pain presents problems. This is because symptoms arising from different spinal tissues can feel very similar and is difficult to differentiate without the use of invasive diagnostic intervention procedures, such as local anesthetic blocks.

One potential source of back pain is skeletal muscle of the back. Potential causes of pain in muscle tissue include muscle strains (pulled muscles), muscle spasm, and muscle imbalances. However, imaging studies do not support the notion of muscle tissue damage in many back pain cases, and the neurophysiology of muscle spasm and muscle imbalances is not well understood.

Another potential source of low back pain is the synovial joints of the spine (e.g. zygapophysial joints/facet joints. These have been identified as the primary source of the pain in approximately one third of people with chronic low back pain, and in most people with neck pain following whiplash.^[17] However, the cause of zygapophysial joint pain is not fully understood. Capsule tissue damage has been proposed in people with neck pain following whiplash. In people with spinal pain stemming from zygapophysial joints, one theory is that intra-articular tissue such as invaginations of their synovial membranes and fibro-adipose meniscoids (that usually act as a cushion to help the bones move over each other smoothly) may become displaced, pinched or trapped, and consequently give rise to nociception (pain).

There are several common other potential sources and causes of back pain: these include spinal disc herniation and degenerative disc disease or isthmic spondylolisthesis, osteoarthritis (degenerative joint disease) and lumbar spinal stenosis, trauma, cancer, infection, fractures, and inflammatory disease.^[18]

Radicular pain (sciatica) is distinguished from 'non-specific' back pain, and may be diagnosed without invasive diagnostic tests.

New attention has been focused on non-discogenic back pain, where patients have normal or near-normal MRI and CT scans. One of the newer investigations looks into the role of the dorsal ramus in patients that have no radiographic abnormalities. See Posterior Rami Syndrome.

Management[link]

The management goals when treating back pain are to achieve maximal reduction in pain intensity as rapidly as possible; to restore the individual's ability to function in everyday activities; to help the patient cope with residual pain; to assess for side-effects of therapy; and to facilitate the patient's passage through the legal and socioeconomic impediments to recovery. For many, the goal is to keep the pain to a manageable level to progress with rehabilitation, which then can lead to long term pain relief. Also, for some people the goal is to use non-surgical therapies to manage the pain and avoid major surgery, while for others surgery may be the quickest way to feel better.

Not all treatments work for all conditions or for all individuals with the same condition, and many find that they need to try several treatment options to determine what works best for them. The present stage of the condition (acute or chronic) is also a determining factor in the choice of treatment. Only a minority of back pain patients (most estimates are 1% - 10%) require surgery.

Pain[link]

• Heat therapy is useful for back spasms or other conditions. A meta-analysis of studies by the Cochrane Collaboration concluded that heat therapy can reduce symptoms of acute and sub-acute low-back pain.^[19] Some patients find that moist heat works best (e.g. a hot bath or whirlpool) or continuous low-level heat (e.g. a heat wrap that stays warm for 4 to 6 hours). Cold compression therapy (e.g. ice or cold pack application) may be effective at relieving back pain in some cases.

• Use of medications in chronic back pain is controversial. The short term use of muscle relaxants is effective in the relief of acute back pain.^[20] Opioids have not been shown to be better than placebo for chronic back pain when the risks and benefits are considered.^[21] Non-steroidal anti-inflammatory drugs (NSAIDs/NSAIAs)^[22] have been shown to be more effective than placebo, and are usually more effective than paracetamol (acetaminophen). A meta-analysis of randomized controlled trials by the Cochrane Collaboration found that there is insufficient clinical trials to determine if injection therapy, usually with corticosteroids, helps in cases of low back pain^[23] A study of intramuscular corticosteroids found no benefit.^[24]

• Back schools appear to improve pain as compared to standard treatments.^[25]

• Massage therapy, especially from an experienced therapist, can provide short term relief.^[26] Acupressure or pressure point massage may be more beneficial than classic (Swedish) massage.^[27]

Depending on the particular cause of the condition, posture training courses and physical exercises might help with relieving the pain.^[28]

• Exercises can be an effective approach to reducing pain, but should be done under supervision of a licensed health professional. Generally, some form of consistent stretching and exercise is believed to be an essential component of most back treatment programs. However, one study found that exercise is also effective for chronic back pain, but not for acute pain.^[29] Another study found that back-mobilizing exercises in acute settings are less effective than continuation of ordinary activities as tolerated.^[30]

• Studies of manipulation suggest that this approach has a benefit similar to other therapies and superior to placebo.^[31][32][33]
• Acupuncture has some proven benefit for back pain;^[34] however, a recent randomized controlled trial suggested insignificant difference between real and sham acupuncture.^[35]
• Education, and attitude adjustment to focus on psychological or emotional causes^[36] - respondent-cognitive therapy and progressive relaxation therapy can reduce chronic pain.^[37]

Surgery[link]

Surgery may sometimes be appropriate for patients with:

• Lumbar disc herniation or degenerative disc disease
• Lumbar spinal stenosis from lumbar disc herniation, degenerative joint disease, or spondylolisthesis
• Scoliosis
• Compression fracture

Minimally invasive surgical procedures are often a solution for many symptoms and causes of back pain. These types of procedures offer many benefits over traditional spine surgery, such as more accurate diagnoses and shorter recovery times.^[38]

Surgery is usually the last resort in the treatment of back pain. It is normally recommended only if all other treatment options have been tried or if the situation is an emergency. A 2009 systematic review of back surgery studies found that, for certain diagnoses, surgery is moderately better than other common treatments, but the benefits of surgery often decline in the long term.^[39]

There are different types of surgical procedures that are used in treating various conditions causing back pain. All of them can be classified into nerve decompression, fusion of body segments and deformity correction surgeries. The first type of surgery is primarily performed in older patients who suffer from conditions causing nerve irritation or nerve damage. Fusion of bony segments is also referred to as a spinal fusion, and it is a procedure used to fuse together two or more bony fragments with the help of metalwork. The latter type of surgery is normally performed to correct congenital deformities or those that were caused by a traumatic fracture. In some cases, correction of deformities involves removing bony fragments or providing stability provision for the spine.

The main procedures used in back pain surgery are discetomies, spinal fusions, laminectomies, removal of tumors, and vertebroplasties.

A discetomy is performed when the intervertebral disc have herniated or torn. It involves removing the protruding disc, either a portion of it or all of it, that is placing pressure on the nerve root.^[40] The disc material which is putting pressure on the nerve is removed through a small incision that is made over that particular disc. This is one of the most popular types of back surgeries and which also has a high rate of success^[41]. The recovery period after this procedure does not last longer than 6 weeks. The type of procedure in which the bony fragments are removed through an endoscope is called percutaneous disc removal.

Microdiscetomies may be performed as a variation of standard discetomies in which a magnifier is used to provide the advantage of a smaller incision, thus a shorter recovery process.

Spinal fusions are performed in cases in which the patient has had the entire disc removed or when another condition has caused the vertebrae to become unstable. The procedure consists in uniting two or more vertebrae by using bone grafts and metalwork to provide more strength for the healing bone. Recovery after spinal fusion may take up to one year, depending greatly on the age of the patient, the reason why surgery has been performed and how many bony segments needed to be fused.

In cases of spinal stenosis or disc herniation, laminectomies can be performed to relieve the pressure on the nerves. During such a procedure, the surgeon enlarges the spinal canal by removing or trimming away the lamina which will provide more space for the nerves. The severity of the condition as well as the general health status of the patient are key factors in establishing the recovery time, which may be range from 8 weeks to 6 months.

Back surgery can be performed to prevent the growth of benign and malignant tumors. In the first case, surgery has the goal of relieving the pressure from the nerves which is caused by a benign growth, whereas in the latter the procedure is aimed to prevent the spread of cancer to other areas of the body. Recovery depends on the type of tumor that is being removed, the health status of the patient and the size of the tumor.

Of doubtful benefit[link]

• Cold compression therapy is advocated for a strained back or chronic back pain and is postulated to reduce pain and inflammation, especially after strenuous exercise such as golf, gardening, or lifting. However, a meta-analysis of randomized controlled trials by the Cochrane Collaboration concluded "The evidence for the application of cold treatment to low-back pain is even more limited, with only three poor quality studies located. No conclusions can be drawn about the use of cold for low-back pain"^[19]

• Bed rest is rarely recommended as it can exacerbate symptoms,^[42][43] and when necessary is usually limited to one or two days. Prolonged bed rest or inactivity is actually counterproductive, as the resulting stiffness leads to more pain.

• Electrotherapy, such as a transcutaneous electrical nerve stimulation (TENS) has been proposed. Two randomized controlled trials found conflicting results.^[44][45] This has led the Cochrane Collaboration to conclude that there is inconsistent evidence to support use of TENS.^[46] In addition, spinal cord stimulation, where an electrical device is used to interrupt the pain signals being sent to the brain and has been studied for various underlying causes of back pain.^{[clarification needed]}

• Inversion therapy is useful for temporary back relief due to the traction method or spreading of the vertebrae through (in this case) gravity. The patient hangs in an upside down position for a period of time from ankles or knees until this separation occurs. The effect can be achieved without a complete vertical hang (90 degree) and noticeable benefits can be observed at angles as low as 10 to 45 degrees.^{[citation needed]}

• Ultrasound has been shown not to be beneficial and has fallen out of favor.^[47]

Pregnancy[link]

About 50% of women experience low back pain during pregnancy.^[48] Back pain in pregnancy may be severe enough to cause significant pain and disability and pre-dispose patients to back pain in a following pregnancy. No significant increased risk of back pain with pregnancy has been found with respect to maternal weight gain, exercise, work satisfaction, or pregnancy outcome factors such as birth weight, birth length, and Apgar scores.

Biomechanical factors of pregnancy that are shown to be associated with low back pain of pregnancy include abdominal sagittal and transverse diameter and the depth of lumbar lordosis. Typical factors aggravating the back pain of pregnancy include standing, sitting, forward bending, lifting, and walking. Back pain in pregnancy may also be characterized by pain radiating into the thigh and buttocks, night-time pain severe enough to wake the patient, pain that is increased during the night-time, or pain that is increased during the day-time. The avoidance of high impact, weight-bearing activities and especially those that asymmetrically load the involved structures such as: extensive twisting with lifting, single-leg stance postures, stair climbing, and repetitive motions at or near the end-ranges of back or hip motion can ease the pain. Direct bending to the ground without bending the knee causes severe impact on the lower back in pregnancy and in normal individuals, which leads to strain, especially in the lumbo-saccral region that in turn strains the multifidus.

Economics[link]

Back pain is regularly cited by national governments as having a major impact on productivity, through loss of workers on sick leave. Some national governments, notably Australia and the United Kingdom, have launched campaigns of public health awareness to help combat the problem, for example the Health and Safety Executive's Better Backs campaign. In the United States lower back pain’s economic impact reveals that it is the number one reason for individuals under the age of 45 to limit their activity, second highest complaint seen in physician’s offices, fifth most common requirement for hospitalization, and the third leading cause for surgery.

References[link]

External links[link]

• Back and spine at the Open Directory Project
• Handout on Health: Back Pain at National Institute of Arthritis and Musculoskeletal and Skin Diseases
• Back pain on MedlinePlus, US National Library of Medicine
• Back pain, American Academy of Orthopaedic Surgeons summary

This article does not cite any references or sources. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (December 2009)

Human back
Illustration of a human back.
Posterior view of the thorax and shoulder
Latin	dorsum humanum
Gray's	subject #281 1303

The human back is the large posterior area of the human body, rising from the top of the buttocks to the back of the neck and the shoulders. It is the surface opposite to the chest, its height being defined by the vertebral column (commonly referred to as the spine or backbone) and its breadth being supported by the ribcage and shoulders. The spinal canal runs through the spine and provides nerves to the rest of the body.

Anatomy of the back[link]

Skeletal structure of the back[link]

The central feature of the human back is the vertebral column, specifically the length from the top of the thoracic vertebrae to the bottom of the lumbar vertebrae, which houses the spinal cord in its spinal canal, and which generally has some curvature that gives shape to the back. The ribcage extends from the spine at the top of the back (with the top of the ribcage corresponding to the T1 vertebra), more than halfway down the length of the back, leaving an area with less protection between the bottom of the ribcage and the hips. The width of the back at the top is defined by the scapula, the broad, flat bones of the shoulders.

Muscles of the back[link]

The spine is bordered by several groups of muscles, including the intertransversarii muscle which facilitate movement between the individual vertebrae, and the multifidus spinae, which facilitate the movement of the spine as a whole.

Other muscles in the back are associated with the movement of the neck and shoulders. The trapezius muscle, which is named from its trapezium-like shape, runs between the neck, the anterior chain, the two shoulders, and the thoracic vertebra, T12. The large latissimus dorsi make a triangle from the shoulder to the hip.

Function of the back[link]

The intricate anatomy of the back provides support for the head and trunk of the body, strength in the trunk of the body, as well as a great deal of flexibility and movement. The upper back has the most structural support, with the ribs attached firmly to each level of the thoracic spine and very limited movement. The lower back (lumbar vertebrae) allows for flexibility and movement in back bending (extension) and forward bending (flexion). It does not permit twisting.

Back pain[link]

The back comprises interconnecting nerves, bones, muscles, ligaments, and tendons, all of which can be a source of pain. Back pain is one of the most common types of pain in adults. By far the most common cause of back pain is muscle strain. The back muscles can usually heal themselves within a couple of weeks, but the pain can be intense and debilitating. Other common sources of back pain include disc problems, such as degenerative disc disease or a lumbar disc herniation, many types of fractures, such as spondylolisthesis or an osteoporotic fracture, or osteoarthritis.

Organs of the back[link]

The lungs are within the ribcage, and extend to the back of the ribcage, making it possible for them to be listened into through the back. The kidneys are situated beneath the muscles in the area below the end of the ribcage, loosely connected to the peritoneum. A strike to the lower back can damage the kidneys of the person being hit.

Surface of the back[link]

The skin of the human back is thicker and has fewer nerve endings than the skin on any other part of the torso. With some notable exceptions (see, e.g. George "The Animal" Steele), it tends to have less hair than the chest on men. The upper-middle back is also the one area of the body which a typical human under normal conditions might be unable to physically touch. When this area is itchy, a backscratcher can be used to ease the discomfort.

Significance in human society[link]

Painting of a woman's back by Edgar Degas.

A substantial area of scar tissue on the back of an oft-whipped slave.

Extensive back tattoo.

The curvature of the female back is a frequent theme in paintings, because the sensibilities of many cultures permit the back to be shown nude - implying full nudity without actually displaying it. Indeed, the practice of showing explicitness on the lower back has been performed for centuries. Certain articles of clothing, such as the haltertop and the backless dress, are designed to expose the back in this manner. The lower back is typically exposed frequently by many types of shirts in woman's fashion, and even the more conservative shirts and blouses will reveal the lower back. This happens for a variety of reasons- the lower waist area is a pivot point for the body and lengthens and arches as a person sits or bends. Secondly, woman's fashion typically favors tops that are waist length, allowing the back to be left bare during slight movement, bending or sitting. The back also serves as the largest canvas for body art on the human body. Because of its size and the relative lack of hair, the back presents an ideal canvas on the human body for lower back tattoos, mostly among young women. Indeed, some individuals have tattoos that cover the entirety of the back. Others have smaller tattoos at significant locations, such as the shoulder blade or the bottom of the back.

Many English idioms mention the back, usually highlighting it as an area of vulnerability; one must "watch one's back", or one may end up "with one's back up against the wall"; worse yet, someone may "stab one in the back", but hopefully a friend "has got one's back". The back is also a symbol of strength and hard work, with those seeking physical labor looking for "strong backs", and workers being implored to "put their back into it".

Historically, flagellation of a person across the back with a whip was both a common form of punishment of criminals, and a common means of forcing slaves to work. As well, self-flagellation, as in self punishment, may include the use of whipping oneself. This is one method of mortification, the practice of inflicting physical suffering on oneself with the religious belief that it will serve as penance for one's own sins or those of others. While more moderate forms of mortification are widely practiced—particularly in the Catholic Church—self flagellation is not encouraged by mainstream religions or religious leaders. A well-known instrument used for flagellations is the infamous Cat 'o Nine Tails, a nine-corded whip with one handle enabling a much more effective whipping would be possible with only one lashing at a time.

Wikimedia Commons has media related to: Human backs

v t e Human regional anatomy (TA A01.1) Head Forehead · Ear · Jaw · Face (Cheek, Eye, Nose, Mouth, Chin) · Occiput · Scalp · Temple Neck Throat · Adam's apple Trunk Chest (Breast) · Abdomen (Navel) · Back · Pelvis (Sex organs) Limbs Upper limb Shoulder · Axilla · Arm Elbow · Forearm Wrist · Hand · Finger (Thumb · Index · Middle · Ring · Little) Lower limb/ (see also leg) Hip · Buttocks · Thigh Knee · Calf · Crus Ankle · Heel · Foot · Toe (Hallux · Long  • Third  • Fourth  • Fifth) · Sole General anatomy: systems and organs, regional anatomy, planes and lines, superficial axial anatomy, superficial anatomy of limbs

Pain

A woman grimacing while having an injection
ICD-10	R52
ICD-9	338
DiseasesDB	9503
MedlinePlus	002164
MeSH	D010146

Pain is an unpleasant feeling often caused by intense or damaging stimuli, such as stubbing a toe, burning a finger, putting alcohol on a cut, and bumping the "funny bone."^[1] 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]

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 promptly once the painful stimulus is removed and the body has healed, but sometimes pain persists despite removal of the stimulus and apparent healing of the body; and sometimes pain arises in the absence of any detectable stimulus, damage or disease.^[4]

Pain is the most common reason for physician consultation in the United States.^[5] It is a major symptom in many medical conditions, and can significantly interfere with a person's quality of life and general functioning.^[6] Psychological factors such as social support, hypnotic suggestion, excitement, or distraction can significantly modulate pain's intensity or unpleasantness.^[7][8]

Classification[link]

In 1994, responding to the need for a more useful system for describing chronic pain, the International Association for the Study of Pain (IASP) classified pain according to specific characteristics: (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) etiology.^[9] This system has been criticized by Clifford J. Woolf and others as inadequate for guiding research and treatment.^[10] According to Woolf, there are three classes of pain : nociceptive pain (see hereunder), inflammatory pain which is associated with tissue damage and the infiltration of immune cells, and pathological pain which is a disease state caused by damage to the nervous system (neuropathic pain, see hereunder) or by its abnormal function (dysfunctional pain, like in fibromyalgia, irritable bowel syndrome, tension type headache, etc.).^[11]

Duration[link]

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,^[12] though some theorists and researchers have placed the transition from acute to chronic pain at 12 months.^[13] 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.^[14] A popular alternative definition of chronic pain, involving no arbitrarily fixed durations is "pain that extends beyond the expected period of healing."^[12] Chronic pain may be classified as cancer pain or benign.^[14]

Nociceptive[link]

Nociceptive pain is caused 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 structures are highly sensitive to stretch, ischemia and inflammation, 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.^[15] 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 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.^[13]

Neuropathic[link]

Neuropathic pain is caused by damage or disease affecting any part of the nervous system involved in bodily feelings (the somatosensory system).^[16] Peripheral neuropathic pain is often described as “burning,” “tingling,” “electrical,” “stabbing,” or “pins and needles.”^[17] Bumping the "funny bone" elicits acute peripheral neuropathic pain.

Phantom[link]

Phantom pain is pain from 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.

The prevalence of phantom pain in upper limb amputees is nearly 82%, and in lower limb amputees is 54%.^[18] One study found that eight days after amputation, 72 percent of patients had phantom limb pain, and six months later, 65 percent reported it.^[19][20] 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.^[21]

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.^[21]

Work by Vilayanur S. Ramachandran using mirror box therapy allows for illusions of movement and touch in a phantom limb which in turn cause a reduction in pain.^[22]

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, 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.^[21]

Psychogenic[link]

Psychogenic pain, also called psychalgia or somatoform pain, is pain caused, increased, or prolonged by mental, emotional, or behavioral factors.^[23] Headache, back pain, and stomach pain are sometimes diagnosed as psychogenic.^[24] 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.^[25]

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 improvement once pain has resolved.^[26]

“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.^[26]

Breakthrough pain[link]

Breakthrough pain is pain that comes on suddenly for short periods of time and is not alleviated by the patients' normal pain management. It is common in cancer patients who often have a background level of pain controlled by medications, but whose pain periodically "breaks through" the medication. The characteristics of breakthrough cancer pain vary from person to person and according to the cause.

Incident pain[link]

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

Pain asymbolia and insensitivity[link]

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.^[27]

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.^[25] 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.^[28] 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.^[29]

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 this is prevalent.^[30] These individuals are at risk of tissue damage due to undiscovered injury. People with diabetes-related nerve damage, for instance, sustain poorly healing foot ulcers as a result of decreased sensation.^[31]

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".^[29] 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. 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).^[32] These conditions feature decreased sensitivity to pain together with other neurological abnormalties, particularly of the autonomic nervous system.^[29][32] 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 (Na_v1.7) necessary in conducting pain nerve stimuli.^[33][34]

Effect on functioning[link]

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.^[35] Acute and chronic pain are also associated with increased depression, anxiety, fear, and anger.^[36]

"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^[37]

Theory[link]

Historical theories[link]

Before the relatively recent discovery of neurons and their role in pain, various different body functions were proposed to account for pain. There were several competing early theories of pain among the ancient Greeks: Aristotle believed that pain was due to evil spirits entering the body through injury, and Hippocrates believed that it was due to an imbalance in vital fluids.^[38] In the 11th century, Avicenna theorized that there were a number of feeling senses including touch, pain and titillation,^[39] but prior to the scientific Renaissance in Europe pain was not well-understood, and it was thought that pain originated outside the body, perhaps as a punishment from God.^[40]

In 1644, René Descartes theorized that pain was a disturbance that passed down along nerve fibers until the disturbance reached the brain,^[38][41] a development that transformed the perception of pain from a spiritual, mystical experience to a physical, mechanical sensation. Descartes's work, along with Avicenna's, 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".^[42] 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.^[43] By the mid-1890s, specificity was backed mostly by physiologists and physicians, and the intensive theory was mostly backed by psychologists. However, after a series of clinical observations by Henry Head and experiments by Max von Frey, the psychologists migrated to specificity almost en masse, and by century's end, most textbooks on physiology and psychology were presenting pain specificity as fact.^[39][42]

In 1955, DC Sinclair and G Weddell developed "peripheral pattern theory", based on a 1934 suggestion by John Paul Nafe. They proposed that all skin fiber endings (with the exception of those innervating hair cells) are identical, and that pain is produced by intense stimulation of these fibers.^[42] Another 20th-century theory was "gate control" theory, introduced by Ronald Melzack and Patrick Wall in the 1965 Science article "Pain Mechanisms: A New Theory".^[44] The authors proposed that both 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, and that the more large fiber activity relative to thin fiber activity at the inhibitory cell, the less pain is felt.^[41] Both peripheral pattern theory and gate control theory have been superseded by more modern theories of pain.

Three dimensions of pain[link]

In 1968 Ronald Melzack and Kenneth 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).^[8] 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 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[link]

Regions of the cerebral cortex associated with pain.

Wilhelm Erb's (1874) "intensive" theory, that a pain signal can be generated by intense enough stimulation of any sensory receptor, has been soundly disproved.^[45]

In humans, the detection of peripheral pain begins at free nerve endings. It is here that the polymodal pain receptors TRPA1, TRPV1, TRPV2, ASICs, and high threshold mechanoreceptors detect noxious stimuli such as strong mechanical forces, H⁺, K⁺, chemicals, and temperature. Some nociceptors do not differentiate noxious from non-noxious stimuli, while others respond only to painfully intense stimuli.^[45]

The sensation of pain then travels from the periphery to the spinal cord along A-delta and C fibers.^[46] Because the A-delta fiber is thicker than the C fiber and is thinly sheathed in an electrically insulating material (myelin), it carries its signal faster (5–30 m/s) than the unmyelinated C fiber (0.5–2 m/s).^[47] 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.^[48] These first order neurons enter the spinal cord via Lissauer's tract.

A-delta and C fibers synapse on second order neurons in substantia gelatinosa (laminae II and III of the dorsal horns). These second order neurons (spinothalamic tract) then decussate, crossing via the anterior white commissure before ascending contralaterally. Before reaching the brain, the spinothalamic tract splits into the lateral neospinothalamic tract and the medial paleospinothalamic tract.^[49]

Second order neospinothalamic tract neurons carry information from A-delta fibers and terminate at the ventral posterolateral nucleus of the thalamus, where they synapse on third order neurons (dendrites of the somatosensory cortex). Paleospinothalamic neurons carry information from C fibers and terminate throughout the brain stem, a tenth of them in the thalamus and the rest in the medulla, pons and periaqueductal grey matter.^[50]

Spinal cord fibers dedicated to carrying A-delta fiber pain signals, and others that carry both A-delta and C fiber pain signals up the spinal cord to the thalamus in the brain have been identified. Other spinal cord fibers, known as wide dynamic range neurons, respond to A-delta and C fibers, but also to the large A-beta fibers that carry touch, pressure and vibration signals.^[47] 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);^[46] and pain that is distinctly located also activates the primary and secondary somatosensory cortices.^[51] 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.

A. D. (Bud) Craig and Derek Denton include pain in a class of feelings they name, respectively, "homeostatic" or "primordial" emotions. These are feelings such as hunger, thirst and fatigue, evoked by internal body states, communicated to the central nervous system by interoceptors, which motivate behavior aimed at maintaining the internal milieu at its ideal state. Craig and Denton distinguish these feelings from the "classical emotions" such as love, fear and anger, which are elicited by environmental stimuli sensed through the nose, eyes and ears.^[52][53]

Evolutionary and behavioral role[link]

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.^[3][54] It is an important part of animal life, vital to healthy survival. People with congenital insensitivity to pain have reduced life expectancy.^[29]

In his book, The Greatest Show on Earth, biologist Richard Dawkins grapples with the question of why pain has to be so very painful. He describes the alternative as a simple, mental raising of a "red flag". To argue why that red flag might be insufficient, Dawkins explains that drives must compete with each other 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 (lack of food, too much cold, or serious injuries are felt as agony, whereas minor damage is felt as mere discomfort). This resemblance will not be perfect, however, because natural selection can be a poor designer. The result is often glitches in animals, including supernormal stimuli. Such glitches help explain pains which are not, or at least no longer directly adaptive (e.g. perhaps some forms of toothache, or injury to fingernails).^[55]

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 psychodynamic psychologists argue that such pain is psychogenic, enlisted as a protective distraction to keep dangerous emotions unconscious.^[56]

Thresholds[link]

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 stimulus begins to hurt, and the pain tolerance threshold is reached when the subject acts to stop the pain.

Differences in pain perception and tolerance thresholds are associated with, among other factors, ethnicity, genetics, and sex. People of Mediterranean origin report as painful some radiant heat intensities that northern Europeans describe as nonpainful, and Italian women tolerate less intense electric shock than Jewish or Native American women. Some individuals in all cultures have significantly higher than normal pain perception and tolerance thresholds. For instance, patients who experience painless heart attacks have higher pain thresholds for electric shock, muscle cramp and heat.^[57] Women have lower pain perception and tolerance thresholds than men, and this sex difference appears to apply to all ages, including newborn infants.^[58]

Assessment[link]

A person's self report is the most reliable measure of pain, with health care professionals tending to underestimate severity.^[59] 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".^[60] 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]

Multidimensional pain inventory[link]

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."^[61] Combining the MPI characterization of the person with their IASP five-category pain profile is recommended for deriving the most useful case description.^[12]

In nonverbal patients[link]

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 or 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.^[62]

Other barriers to reporting[link]

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.^[63]

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 (e.g. Jewish) feel they should report pain right away and get immediate relief.^[62] Gender can also be a factor in reporting pain. Gender differences are usually the result of social and cultural expectations, with women expected to be emotional and show pain and men stoic, keeping pain to themselves.^[62]

As an aid to diagnosis[link]

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.^[64][65]

Management[link]

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.^[66] This neglect is extended to all ages, from neonates to the frail elderly.^[67] African and Hispanic Americans are more likely than others to suffer needlessly in the hands of a physician;^[68] and women's pain is more likely to be undertreated than men's.^[69]

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.^[70] It is a specialty only in China and Australia at this time.^[71] Elsewhere, pain medicine is a subspecialty under disciplines such as anesthesiology, physiatry, neurology, palliative medicine and psychiatry.^[72]

Medication[link]

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, physician assistants, and nurse practitioners.^[73]

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^[74] 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.^[75][76] Sweet oral liquid moderately reduces the incidence and duration of crying caused by immunization injection in children between one and twelve months of age.^[77]

Psychological[link]

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.^[78]

Suggestion can significantly affect pain intensity. About 35% of people report marked relief after receiving a saline injection they believe to have been morphine. This "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.^[79]

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.^[80]

Cognitive behavioral therapy (CBT) is effective in reducing the suffering associated with chronic pain in some patients but the reduction in suffering is quite modest, and the CBT method employed seems to have no effect on outcome.^[81]

Alternative medicine[link]

Pain is the most common reason for people to use complementary and alternative medicine.^[82] An analysis of the 13 highest quality studies of pain treatment with acupuncture, published in January 2009 in the British Medical Journal, concluded there is little difference in the effect of real, sham and no acupuncture.^[83] There is 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 unconvincing.^[84]

A 2007 review of 13 studies found evidence for the efficacy of hypnosis in the reduction of pain in some conditions, though the number of patients enrolled in the studies was low, bringing up issues of power to detect group differences, and most lacked credible controls for placebo and/or expectation. The authors concluded that "although the findings provide support for the general applicability of hypnosis in the treatment of chronic pain, considerably more research will be needed to fully determine the effects of hypnosis for different chronic-pain conditions."^[85]

A 2003 meta-analysis of randomized clinical trials found that spinal manipulation was "more effective than sham therapy but was no more or less effective than general practitioner care, analgesics, physical therapy, exercise, or back school" in the treatment of low back pain.^[86]

Epidemiology[link]

Pain is the main reason for visiting the emergency department in more than 50% of cases^[87] and is present in 30% of family practice visits.^[88] Several epidemiological studies from different countries have reported widely varying prevalence rates for chronic pain, ranging from 12-80% of the population.^[89] It becomes more common as people approach death. A study of 4,703 patients found that 26% had pain in the last two years of life, increasing to 46% in the last month.^[90]

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.^[91]

Society and culture[link]

The okipa ceremony as witnessed by George Catlin, circa 1835.

The nature or meaning of physical pain has been diversely understood by religious or secular traditions from antiquity to modern times.^[92][93]

Physical pain is an important political topic in relation to various issues, including pain management policy, drug control, animal rights or animal welfare, torture, and 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, especially in connection with such views as dualism, identity theory, and functionalism.

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.

In other animals[link]

Portrait of René Descartes by Jan Baptist Weenix 1647-1649

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. René Descartes for example argued that animals lack consciousness and therefore do not experience pain and suffering in the way that humans do.^[94] Bernard Rollin of Colorado State University, the principal author of two U.S. federal laws regulating pain relief for animals,^[95] writes 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.^[96] 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.^[96] Carbone writes that the view that animals feel pain differently is now a minority view. Academic reviews of the topic are more equivocal, noting that although the argument that animals have at least simple conscious thoughts and feelings has strong support,^[97] some critics continue to question how reliably animal mental states can be determined.^[94][98] The ability of invertebrate species of animals, such as insects, to feel pain and suffering is also unclear.^[99][100]

The presence of pain in an animal cannot be known for certain, but it can be inferred through physical and behavioral reactions.^[101] Specialists currently believe that all vertebrates can feel pain, and that certain invertebrates, like the octopus, might too.^[102][103] 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,^[104] except for instance in fruit flies.^[105]

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,^[106] their presence indicates that lobsters may be able to experience pain.^[106][107] Opioids may mediate their pain in the same way as in vertebrates.^[107] Veterinary medicine uses, for actual or potential animal pain, the same analgesics and anesthetics as used in humans.^[108]

Etymology[link]

First attested in English in 1297, the word pain comes from the Old French peine, in turn from Latin poena, "punishment, penalty"^[109] (in L.L. also "torment, hardship, suffering") and that from Greek "ποινή" (poine), generally "price paid", "penalty", "punishment".^[110][111] It also exists in Frisian as "pine" which in turn is related to the English verb "to pine" which means to long for.

References[link]