Pain

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Pain, in the sense of physical pain,^[1] is a typical sensory experience that may be described as the unpleasant awareness of a noxious stimulus or bodily harm. Individuals experience pain by various daily hurts and aches, and occasionally through more serious injuries or illnesses. For scientific and clinical purposes, pain is defined by the International Association for the Study of Pain (IASP) as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage”.^[2][3]

Pain is highly subjective to the individual experiencing it. A definition that is widely used in nursing was first given as early as 1968 by Margo McCaffery: “‘Pain is whatever the experiencing person says it is, existing whenever he says it does”.^[4][5]

Pain of any type is the most frequent reason for physician consultation in the United States, prompting half of all Americans to seek medical care annually.^[6] It is a major symptom in many medical conditions, significantly interfering with a person’s quality of life and general functioning. Diagnosis is based on characterizing pain in various ways, according to duration, intensity, type (dull, burning or stabbing), source, or location in body. Usually pain stops without treatment or responds to simple measures such as resting or taking an analgesic, and it is then called ‘acute’ pain. But it may also become intractable and develop into a condition called chronic pain, in which pain is no longer considered a symptom but an illness by itself. The study of pain has in recent years attracted many different fields such as pharmacology, neurobiology, nursing sciences, dentistry, physiotherapy, and psychology. Pain medicine is a separate subspecialty^[7] figuring under some medical specialties like anesthesiology, physiatry, neurology, psychiatry.

Pain is part of the body’s defense system, triggering a reflex reaction to retract from a painful stimulus, and helps adjust behaviour to increase avoidance of that particular harmful situation in the future. Given its significance, physical pain is also linked to various cultural, religious, philosophical, or social issues.

“Pain (n.) 1297, “punishment,” especially for a crime; also (c.1300) “condition one feels when hurt, opposite of pleasure,” from O.Fr. peine, from L. poena “punishment, penalty” (in L.L. also “torment, hardship, suffering”), from Gk. poine “punishment,” from PIE *kwei- “to pay, atone, compensate” (…).” ^[8]

Stimulation of a nociceptor, due to a chemical, thermal, or mechanical event that has the potential to damage body tissue, may cause nociceptivepain.

X-rays produce pictures of the body’s structures, such as bones and joints

Imaging, especially magnetic resonance imaging or MRI, provides physicians with pictures of the body’s structures and tissues. MRI uses magnetic fields and radio waves to differentiate between healthy and diseased tissue.

Electrodiagnostic procedures include electromyography (EMG), nerve conduction studies, and evoked potential (EP) studies. Information from EMG can help physicians tell precisely which muscles or nerves are affected by weakness or pain. Thin needles are inserted in muscles and a physician can see or listen to electrical signals displayed on an EMG machine. With nerve conduction studies the doctor uses two sets of electrodes (similar to those used during an electrocardiogram) that are placed on the skin over the muscles. The first set gives the patient a mild shock that stimulates the nerve that runs to that muscle. The second set of electrodes is used to make a recording of the nerve’s electrical signals, and from this information the doctor can determine if there is nerve damage. EP tests also involve two sets of electrodes-one set for stimulating a nerve (these electrodes are attached to a limb) and another set on the scalp for recording the speed of nerve signal transmission to the brain.

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

“Pain (n.) 1297, “punishment,” especially for a crime; also (c.1300) “condition one feels when hurt, opposite of pleasure,” from O.Fr. peine, from L. poena “punishment, penalty” (in L.L. also “torment, hardship, suffering”), from Gk. poine “punishment,” from PIE *kwei- “to pay, atone, compensate” (…).” ^[1]

Ancient civilizations recorded on stone tablets accounts of pain and the treatments used: pressure, heat, water, and sun. Early humans related pain to evil, magic, and demons. Relief of pain was the responsibility of sorcerers, shamans, priests, and priestesses, who used herbs, rites, and ceremonies as their treatments.

The Greeks and Romans were the first to advance a theory of sensation, the idea that the brain and nervous system have a role in producing the perception of pain. But it was not until the Middle Ages and well into the Renaissance-the 1400s and 1500s-that evidence began to accumulate in support of these theories. Leonardo da Vinci and his contemporaries came to believe that the brain was the central organ responsible for sensation. Da Vinci also developed the idea that the spinal cord transmits sensations to the brain.

In the 17th and 18th centuries, the study of the body-and the senses-continued to be a source of wonder for the world’s philosophers. In 1664, the French philosopher René Descartes described what to this day is still called a “pain pathway.” Descartes illustrated how particles of fire, in contact with the foot, travel to the brain and he compared pain sensation to the ringing of a bell.

In the 19th century, pain came to dwell under a new domain-science-paving the way for advances in pain therapy. Physician-scientists discovered that opium, morphine, codeine, and cocaine could be used to treat pain. These drugs led to the development of aspirin, to this day the most commonly used pain reliever. Before long, anesthesia-both general and regional-was refined and applied during surgery.

“It has no future but itself,” wrote the 19th century American poet Emily Dickinson, speaking about pain. As the 21st century unfolds, however, advances in pain research are creating a less grim future than that portrayed in Dickinson’s verse, a future that includes a better understanding of pain, along with greatly improved treatments to keep it in check.

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

The central nervous system (CNS) refers to the brain and spinal cord together. The peripheral nervous system refers to the cervical, thoracic, lumbar, and sacral nerve trunks leading away from the spine to the limbs. Messages related to function (such as movement) or dysfunction (such as pain) travel from the brain to the spinal cord and from there to other regions in the body and back to the brain again. The autonomic nervous system controls involuntary functions in the body, like perspiration, blood pressure, heart rate, or heart beat. It is divided into the sympathetic and parasympathetic nervous systems. The sympathetic and parasympathetic nervous systems have links to important organs and systems in the body; for example, the sympathetic nervous system controls the heart, blood vessels, and respiratory system, while the parasympathetic nervous system controls our ability to sleep, eat, and digest food.

The peripheral nervous system also includes 12 pairs of cranial nerves located on the underside of the brain. Most relay messages of a sensory nature. They include the olfactory (I), optic (II), oculomotor (III), trochlear (IV), trigeminal (V), abducens (VI), facial (VII), vestibulocochlear (VIII), glossopharyngeal (IX), vagus (X), accessory (XI), and hypoglossal (XII) nerves. Neuralgia, as in trigeminal neuralgia, is a term that refers to pain that arises from abnormal activity of a nerve trunk or its branches. The type and severity of pain associated with neuralgia vary widely.

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

What is pain? The International Association for the Study of Pain defines it as: An unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage.

It is useful to distinguish between two basic types of pain, acute and chronic, and they differ greatly.

• Acute pain, for the most part, results from disease, inflammation, or injury to tissues. This type of pain generally comes on suddenly, for example, after trauma or surgery, and may be accompanied by anxiety or emotional distress. The cause of acute pain can usually be diagnosed and treated, and the pain is self-limiting, that is, it is confined to a given period of time and severity. In some rare instances, it can become chronic.
• Chronic pain is widely believed to represent disease itself. It can be made much worse by environmental and psychological factors. Chronic pain persists over a longer period of time than acute pain and is resistant to most medical treatments. It can—and often does—cause severe problems for patients.

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Stimulation of a nociceptor, due to a chemical, thermal, or mechanical event that has the potential to damage body tissue, may cause nociceptive pain.

Damage to the nervous system itself, due to disease or trauma, may cause neuropathic (or neurogenic) pain.^[1] Neuropathic pain may refer to peripheral neuropathic pain, which is caused by damage to nerves, or to central neuropathic pain, which is caused by damage to the brain, brainstem, or spinal cord.

Nociceptive pain and neuropathic pain are the two main kinds of pain when the primary mechanism of production is considered. A third kind may be mentioned: see below psychogenic pain.

Nociceptive pain may be classified further in three types that have distinct organic origins and felt qualities.^[2]

1. Superficial somatic pain (or cutaneous pain) is caused by injury to the skin or superficial tissues. Cutaneous nociceptors terminate just below the skin, and due to the high concentration of nerve endings, produce a sharp, well-defined, localized pain of short duration. Examples of injuries that produce cutaneous pain include minor wounds, and minor (first degree) burns.
   
   
2. Deep somatic pain originates from ligaments, tendons, bones, blood vessels, fasciae, and muscles. It is detected with somatic nociceptors. The scarcity of pain receptors in these areas produces a dull, aching, poorly-localized pain of longer duration than cutaneous pain; examples include sprains, broken bones, and myofascial pain.
   
   
3. Visceral pain originates from body’s viscera, or organs. Visceral nociceptors are located within body organs and internal cavities. The even greater scarcity of nociceptors in these areas produces pain that is usually more aching or cramping and of a longer duration than somatic pain. Visceral pain may be well-localized, but often it is extremely difficult to localize, and several injuries to visceral tissue exhibit “referred” pain, where the sensation is localized to an area completely unrelated to the site of injury.

Nociception is the unconscious afferent activity produced in the peripheral and central nervous system by stimuli that have the potential to damage tissue. It should not be confused with pain, which is a conscious experience.It is initiated by nociceptorsthat can detect mechanical, thermal or chemical changes above a certain threshold. All nociceptors are free nerve endings of fast-conducting myelinated A delta fibers or slow-conducting unmyelinated C fibers, respectively responsible for fast, localized, sharp pain and slow, poorly-localized, dull pain. Once stimulated, they transmit signals that travel along the spinal cord and within the brain. Nociception, even in the absence of pain, may trigger withdrawal reflexes and a variety of autonomic responses such as pallor, diaphoresis,bradycardia, hypotension, lightheadedness, nausea and fainting.^[3]

Brain areas that are particularly studied in relation with pain include the somatosensory cortex which mostly accounts for the sensory discriminative dimension of pain, and the limbic system, of which the thalamus and the anterior cingulate cortex are said to be especially involved in the affective dimension.

The gate control theory of pain describes how the perception of pain is not a direct result of activation of nociceptors, but instead is modulated by interaction between different neurons, both pain-transmitting and non-pain-transmitting. In other words, the theory asserts that activation, at the spine level or even by higher cognitive brain processes, of nerves or neurons that do not transmit pain signals can interfere with signals from pain fibers and inhibit or modulate an individual’s experience of pain.

Pain may be experienced differently depending on genotype; as an example individuals with red hair may be more susceptible to pain caused by heat,^[4]but redheads with a non-functional melanocortin 1 receptor (MC1R) gene are less sensitive to pain from electric shock.^[5] Gene Nav1.7 has been identified as a major factor in the development of the pain-perception systems within the body. A rare genetic mutation in this area causes non-functional development of certain sodium channels in the nervous system, which prevents the brain from receiving messages of physical damage, resulting in congenital insensitivity to pain.^[6] The same gene also appears to mediate a form of pain hyper-sensitivity, while other mutations may be the root of paroxysmal extreme pain disorder.^[6][7]

Pain is part of the body’s defense system, triggering mental and physical behavior to end the painful experience. It promotes learning so that repetition of the painful situation will be less likely.

Despite its unpleasantness, pain is an important part of the existence of humans and other animals; in fact, it is vital to healthy survival (see below Insensitivity to pain). Pain encourages an organism to disengage from the noxious stimulus associated with the pain. Preliminary pain can serve to indicate that an injury is imminent, such as the ache from a soon-to-be-broken bone. Pain may also promote the healing process, since most organisms will protect an injured region in order to avoid further pain.

Interestingly, the brain itself is devoid of nociceptive tissue, and hence cannot experience pain. Thus, a headache is not due to stimulation of pain fibers in the brain itself. Rather, the membrane surrounding the brain and spinal cord, called the dura mater, is innervated with pain receptors, and stimulation of these dural nociceptors is thought to be involved to some extent in producing headache pain. The vasoconstriction of pain-innervated blood vessels in the head is another common cause. Some evolutionary biologists have speculated that this lack of nociceptive tissue in the brain might be because any injury of sufficient magnitude to cause pain in the brain has a sufficiently high probability of being fatal that development of nociceptive tissue therein would have little to no survival benefit.

Chronic pain, in which the pain becomes pathological rather than beneficial, may be an exception to the idea that pain is helpful to survival, although some specialists believe that psychogenic chronic pain exists as a protective distraction to keep dangerous repressed emotions such as anger or rage unconscious.^[8] It is not clear what the survival benefit of some extreme forms of pain (e.g. toothache) might be; and the intensity of some forms of pain (for example as a result of injury to fingernails or toenails) seem to be out of all proportion to any survival benefits.

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Hundreds of pain syndromes or disorders make up the spectrum of pain. There are the most benign, fleeting sensations of pain, such as a pin prick. There is the pain of childbirth, the pain of a heart attack, and the pain that sometimes follows amputation of a limb. There is also pain accompanying cancer and the pain that follows severe trauma, such as that associated with head and spinal cord injuries. A sampling of common pain syndromes follows, listed alphabetically.

• Arachnoiditis
• Arthritis
• Back pain
• Burn
• Cancer
• Headache
• Migraines
• Muscle pain
• Myofascial pain syndrome
• Neuropathic pain
• Reflex sympathetic dystrophy syndrome
• Repetitive stress injury
• Sciatica
• Shingles
• Sports injuries
• Surgery
• Trauma
• Vascular disease

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

The word pain used without a modifier usually refers to physical pain, but it may also refer to pain in the broad sense, i.e.suffering. The latter includes physical pain and mental pain, or any unpleasant feeling, sensation, and emotion. It may be described as a private feeling of unpleasantness and aversion associated with harm or threat of harm in an individual. Care should be taken to make the appropriate distinction when required between the two meanings. For instance, philosophy of pain is essentially about physical pain, while a philosophical outlook on pain is rather about pain in the broad sense. Or, as another quite different instance,nausea or itch are not ‘physical pains’, but they are unpleasant sensory or bodily experience, and a person ‘suffering’ from severe or prolonged nausea or itch may be said ‘in pain’.

Nociception, the unconscious activity induced by a harmful stimulus in sense receptors, peripheral nerves, spinal column and brain, should not be confused with physical pain, which is a conscious experience. Nociception or noxious stimuli usually cause pain, but not always, and sometimes pain occurs without them.^[1]

Qualifiers, such as mental, emotional, psychological, and spiritual, are often used for referring to more specific types of pain or suffering. In particular, ‘mental pain’ may be used in relationship with ‘physical pain’ for distinguishing between two wide categories of pain. A first caveat concerning such a distinction is that it uses ‘physical pain’ in a sense that normally includes not only the ‘typical sensory experience’ of ‘physical pain’ but also other unpleasant bodily experience such as itch or nausea. A second caveat is that the termsphysical or mental should not be taken too literally: physical pain, as a matter of fact, happens through conscious minds and involves emotional aspects, while mental pain happens through physical brains and, being an emotion, it involves important bodily physiological aspects.

The term unpleasant or unpleasantness commonly means painful or painfulness in a broad sense. It is also used in (physical) pain science for referring to the affective dimension of pain, usually in contrast with the sensory dimension. For instance: “Pain-unpleasantness is often, though not always, closely linked to both the intensity and unique qualities of the painful sensation.”^[2] Pain science acknowledges, in a puzzling challenge to IASP definition, that pain may be experienced as a sensation devoid of any unpleasantness:

Suffering is sometimes used in the specific narrow sense of physical pain, but more often it refers to mental pain, or more often yet to pain in the broad sense. Suffering is described as an individual’s basic affective experience of unpleasantness and aversion associated with harm or threat of harm.

The terms pain and suffering are often used together in different senses which can become confusing, for example:

• Being used as synonyms;
• Being used in contradistinction to one another: e.g. “pain is inevitable, suffering is optional”, or “pain is physical, suffering is mental”;
• Being used to define each other: e.g. “pain is physical suffering”, or “suffering is severe physical or mental pain”.

To avoid confusion: this article is about physical pain in the narrow sense of a typical sensory experience associated with actual or potential tissue damage. This excludes pain in the broad sense of any unpleasant experience, which is covered in detail by the article Suffering.

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Pain is the number one complaint of older Americans, and one in five older Americans takes a painkiller regularly. In 1998, the American Geriatrics Society (AGS) issued guidelines* for the management of pain in older people. The AGS panel addressed the incorporation of several non-drug approaches in patients’ treatment plans, including exercise. AGS panel members recommend that, whenever possible, patients use alternatives to aspirin, ibuprofen, and other NSAIDs because of the drugs’ side effects, including stomach irritation and gastrointestinal bleeding. For older adults, acetaminophen is the first-line treatment for mild-to-moderate pain, according to the guidelines. More serious chronic pain conditions may require opioid drugs (narcotics), including codeine or morphine, for relief of pain.

Pain in younger patients also requires special attention, particularly because young children are not always able to describe the degree of pain they are experiencing. Although treating pain in pediatric patients poses a special challenge to physicians and parents alike, pediatric patients should never be undertreated. Recently, special tools for measuring pain in children have been developed that, when combined with cues used by parents, help physicians select the most effective treatments.

Nonsteroidal agents, and especially acetaminophen, are most often prescribed for control of pain in children. In the case of severe pain or pain following surgery, acetaminophen may be combined with codeine.

It is now widely believed that pain affects men and women differently. While the sex hormones estrogen and testosterone certainly play a role in this phenomenon, psychology and culture, too, may account at least in part for differences in how men and women receive pain signals. For example, young children may learn to respond to pain based on how they are treated when they experience pain. Some children may be cuddled and comforted, while others may be encouraged to tough it out and to dismiss their pain.

Many investigators are turning their attention to the study of gender differences and pain. Women, many experts now agree, recover more quickly from pain, seek help more quickly for their pain, and are less likely to allow pain to control their lives. They also are more likely to marshal a variety of resources-coping skills, support, and distraction-with which to deal with their pain.

Research in this area is yielding fascinating results. For example, male experimental animals injected with estrogen, a female sex hormone, appear to have a lower tolerance for pain-that is, the addition of estrogen appears to lower the pain threshold. Similarly, the presence of testosterone, a male hormone, appears to elevate tolerance for pain in female mice: the animals are simply able to withstand pain better. Female mice deprived of estrogen during experiments react to stress similarly to male animals. Estrogen, therefore, may act as a sort of pain switch, turning on the ability to recognize pain.

Investigators know that males and females both have strong natural pain-killing systems, but these systems operate differently. For example, a class of painkillers called kappa-opioids is named after one of several opioid receptors to which they bind, the kappa-opioid receptor, and they include the compounds nalbuphine (Nubain®) and butorphanol (Stadol®). Research suggests that kappa-opioids provide better pain relief in women.

Though not prescribed widely, kappa-opioids are currently used for relief of labor pain and in general work best for short-term pain. Investigators are not certain why kappa-opioids work better in women than men. Is it because a woman’s estrogen makes them work, or because a man’s testosterone prevents them from working? Or is there another explanation, such as differences between men and women in their perception of pain? Continued research may result in a better understanding of how pain affects women differently from men, enabling new and better pain medications to be designed with gender in mind.

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