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Literature review current through: Nov 2015. | This topic last updated: Jan 09, 2015.
INTRODUCTION — Pain is one of the most common and debilitating patient complaints, affecting individual patients, their friends and families, the work force, and society in general.
Research efforts in understanding pain range from the molecular biology of nociceptive pathways to the psychosocial aspects that influence the experience of pain. Although such studies have resulted in significant strides in pain management and quality of life for patients with persistent pain, the evaluation and treatment of pain remains suboptimal.
An overview of the definition, classification, and pathogenesis of chronic pain is presented here. The evaluation of patients presenting with chronic pain, the general therapeutic principles of chronic pain, and specific pain syndromes are discussed separately. (See "Evaluation of chronic pain in adults".)
EPIDEMIOLOGY — Over 100 million Americans suffer chronic pain [1,2] and roughly 63 percent of pain sufferers seek help from their primary care clinicians [3]. Pain accounts for 20 percent of outpatient visits and 12 percent of all prescriptions [4]. Patients with symptoms of chronic pain are seen by clinicians in multiple clinical settings. Most patients who present with pain complaints rate their symptoms as moderate to severe [5,6]. One survey from 2010 estimated that 19 percent of adults in the United States report constant or frequent pain persisting for at least three months [7].
Persistent pain often causes functional impairment and disability, psychological distress (anxiety, depression), and sleep deprivation [8]. Almost 80 percent of chronic pain patients report that pain disrupts their activities of daily living, and two-thirds indicate that pain has negatively impacted personal relationships [9].
Pain is the most common cause of long-term disability, with lost work days in the United States estimated at more than 50 million days per year [10]. The annual cost of untreated or undertreated pain to taxpayers and employers has been calculated at over $100 billion per year, in direct and indirect expenses [4]. The use and misuse of opioids for management of chronic pain is a major concern, with problems arising from their multiple adverse side effects including drug-dependency, from drug diversion, and from under-treatment of chronic pain symptoms for fear of narcotic abuse. Chronic pain is thus a major medical and social issue [11].
PAIN-RELATED DEFINITIONS — Acute pain is a vital, protective mechanism that permits us to live in an environment fraught with potential dangers [12]. Certain stimuli are associated with danger that should be avoided, even at some cost, to prevent tissue damage.
Pain can be considered in two broad categories: adaptive and maladaptive. Adaptive pain contributes to survival by protecting the organism from injury and/or promoting healing when injury has occurred. Maladaptive or chronic pain is pain as disease and represents pathologic functioning of the nervous system.
Pain definition — Pain has been variously defined. A commonly used definition describes pain as an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage [13]. A revised definition identifies pain as "a somatic perception containing: (1) a bodily sensation with qualities like those reported during tissue-damaging stimulation, (2) an experienced threat associated with this sensation, and (3) a feeling of unpleasantness or other negative emotion based on this experienced threat" [14].
Chronic pain — Chronic pain has been defined as pain which lasts beyond the ordinary duration of time that an insult or injury to the body needs to heal. An argument has been made that the term "persistent pain" should be used in lieu of "chronic pain" [15].
Alternative definitions are based on a specified duration of pain, although there is discrepancy on what that duration should be.
●The International Association for the Study of Pain (IASP) definition addresses both duration and appropriateness, defining chronic pain as pain without apparent biologic value that has persisted beyond the normal tissue healing time (usually taken to be three months) [16].
●The American College of Rheumatology (ACR) defines chronic pain as widespread or regional pain for at least three months [17]. ACR criteria for chronic widespread pain include all of the following: pain present for at least three months, pain in the left and right sides of the body, pain above and below the waist, and the presence of axial skeletal pain (cervical spine anterior chest, thoracic spine, or low back).
●The Diagnostic and Statistical Manual of Mental Disorders, 4th edition (DSM-IV) defines chronic pain as persistent pain for six months [18].
●The American Society of Anesthesiologists defines chronic pain as pain of any etiology not directly related to neoplastic involvement, extending in duration beyond the expected temporal boundary of tissue injury and normal healing and adversely affecting the function or well-being of the individual [19].
Pain duration shorter than six months but present beyond the "expected healing period" has been termed "subacute pain" by some clinicians.
Pain descriptor terminology — The following terms are commonly used descriptors of altered sensation [13]:
●Hyperalgesia — Increased response to a stimulus that normally is painful.
●Hypoalgesia — Diminished response to a normally painful stimulus.
●Analgesia — Absence of pain in response to stimulation that normally is painful.
●Hyperesthesia — Increased sensitivity to stimulation, excluding the special senses.
●Hypesthesia — Diminished sensitivity to stimulation, excluding the special senses.
●Dysesthesia — An unpleasant abnormal sensation, whether spontaneous or evoked.
●Paresthesia — An abnormal sensation, whether spontaneous or evoked.
●Allodynia — Pain resulting from a stimulus (such as light touch) that does not normally elicit pain.
Types of pain — Pain is often categorized as being either nociceptive or neuropathic. Other pain schemes define additional categories (psychogenic or muscle pain) [20,21]. The primary distinction between nociceptive and neuropathic pain has implications for evaluation and treatment decisions.
Nociceptive pain — A nociceptor is a nerve fiber preferentially sensitive to a noxious stimulus or to a stimulus that would become noxious if prolonged. Nociceptive pain is the perception of nociceptive input, usually due to tissue damage (eg, postoperative pain). Nociceptive pain is further subdivided into somatic and visceral pain. Somatic pain arises from injury to body tissues. It is well localized but variable in description and experience. Visceral pain is pain arising from the viscera mediated by stretch receptors. It is poorly localized, deep, dull, and cramping (eg, pain associated with appendicitis, cholecystitis, or pleurisy).
One classification system of pain further subdivides nociceptive pain as musculoskeletal pain, inflammatory pain (eg, inflammatory arthropathies, postoperative pain, tissue injury, infection), or mechanical/compressive pain (eg, low back pain, neck pain, visceral pain from expanding tumor masses) [22].
Neuropathic pain — Neuropathic pain arises from abnormal neural activity secondary to disease, injury, or dysfunction of the nervous system. It commonly persists without ongoing disease (eg, diabetic neuropathy, trigeminal neuralgia, or thalamic pain syndrome). Neuropathic pain is further subdivided as follows:
●Sympathetically mediated pain (SMP) is pain arising from a peripheral nerve lesion and associated with autonomic changes (eg, complex regional pain syndrome I and II, formerly known as reflex sympathetic dystrophy and causalgia) [23,24]. (See "Complex regional pain syndrome in adults: Pathogenesis, clinical manifestations, and diagnosis".)
●Peripheral neuropathic pain is due to damage to a peripheral nerve without autonomic change (eg, postherpetic neuralgia, neuroma formation).
●Central pain arises from abnormal central nervous system (CNS) activity (eg, phantom limb pain, pain from spinal cord injuries, and post-stroke pain).
Neuropathy is also described as mononeuropathy if one nerve is affected, mononeuropathy multiplex if several nerves in different areas of the body are involved, and polyneuropathy if symptoms are diffuse and bilateral.
Pain taxonomy — The International Association for the Study of Pain (IASP) has developed a taxonomy for the classification of pain [13]. This classification system identifies five axes:
●Axis I: Anatomic regions
●Axis II: Organ systems
●Axis III: Temporal characteristics, pattern of occurrence
●Axis IV: Intensity, time since onset of pain
●Axis V: Etiology
The IASP Classification of Chronic Pain is compatible with the International Classification of Diseases (ICD 9 and ICD 10) but provides for more detailed identification of various chronic pain syndromes and major acute pain syndromes.
Simpler schemes often are used in the day to day practice of pain medicine. One scheme identifies eight unidimensional subtypes or classifications for pain [25]:
●Pain location (eg, low back pain, headache)
●Pain duration (eg, acute, subacute, chronic) (see 'Chronic pain' above)
●Pain origin (eg, nociceptive or neuropathic) (see 'Types of pain' above)
●Diagnosis (eg, cancer pain, sickle cell pain, postherpetic neuralgia) (see 'Pathogenesis of pain' below)
●Body system (eg, myofascial, rheumatic, neurologic, vascular) [26]
●Pain severity (eg, mild, moderate, or severe)
●Pain mechanism(s) (eg, peripheral sensitization, disinhibition, or central sensitization) [27] (see 'Mechanisms for persistent pain' below)
●Treatment responsiveness (eg, opioid-responsive pain, op