Chronic pain is pain that has lasted for a long time. In medicine, the distinction between acute and chronic pain has traditionally been determined by an arbitrary interval of time since onset; the two most commonly used markers being 3 months and 6 months since onset, though some theorists and researchers have placed the transition from acute to chronic pain at 12 months. 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. A popular alternative definition of chronic pain, involving no arbitrarily fixed durations is "pain that extends beyond the expected period of healing."
Nociceptive pain may be divided into "superficial" and "deep", and deep pain into "deep somatic" and "visceral". Superficial pain is initiated by activation of nociceptors in the skin or superficial tissues. 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. Visceral pain originates in the viscera (organs). Visceral pain may be well-localized, but often it is extremely difficult to locate, and several visceral regions produce "referred" pain when damaged or inflamed, where the sensation is located in an area distant from the site of pathology or injury.
Neuropathic pain is divided into "peripheral" (originating in the peripheral nervous system) and "central" (originating in the brain or spinal cord). Peripheral neuropathic pain is often described as “burning,” “tingling,” “electrical,” “stabbing,” or “pins and needles.”.
Under persistent activation nociceptive transmission to the dorsal horn may induce a wind up phenomenon. This induces pathological changes that lower the threshold for pain signals to be transmitted. In addition it may generate nonnociceptive nerve fibers to respond to pain signals. Nonnociceptive nerve fibers may also be able to generate and transmit pain signals. In chronic pain this process is difficult to reverse or eradicate once established.
Chronic pain of different etiologies has been characterized as a disease affecting brain structure and function. Magnetic resonance imaging studies have shown abnormal anatomical and functional connectivity, even during rest  involving areas related to the processing of pain. Also, persistent pain has been shown to cause grey matter loss, reversible once the pain has resolved.
These structural changes can be explained by the phenomenon known as neuroplasticity. In the case of chronic pain, the somatototic representation of the body is inappropriately reorganized following peripheral and central sensitization. This maladaptative change results in the experience of allodynia and/or hyperalgesia. Brain activity in individuals suffering from chronic pain, measured via electroencephalogram (EEG), has been demonstrated to be altered, suggesting pain-induced neuroplastic changes. More specifically, the relative beta activity (compared to the rest of the brain) is increased, the relative alpha activity is decreased, and the theta activity both absolutely and relatively is diminished.
Complete and sustained remission of many neuropathies and most idiopathic chronic pain (pain that extends beyond the expected period of healing, or chronic pain that has no known underlying pathology) is rarely achieved, but much can be done to reduce suffering and improve quality of life.
Pain management is the branch of medicine employing an interdisciplinary approach to the relief of pain and improvement in the quality of life of those living with pain. The typical pain management team includes medical practitioners, clinical psychologists, physiotherapists, occupational therapists, and nurse practitioners. Acute pain usually resolves with the efforts of one practitioner; however, the management of chronic pain frequently requires the coordinated efforts of the treatment team.
Psychological treatments including cognitive behavioral therapy and acceptance and commitment therapy have been shown effective for improving quality of life in those suffering from chronic pain. Clinical hypnosis, including self-hypnosis, has been shown effective not only for improving quality of life, but for direct improvement of chronic pain symptoms.
The emergence of studies relating chronic pain to neuroplasticity also suggest the utilization of neurofeedback rehabilitation techniques to resolve maladaptive cortical changes and patterns. The proposed goal of neurofeedback intervention is to abolish maladaptive neuroplastic changes made as a result of chronic nociception, as measured by abnormal EEG, and thereby relieve the individual's pain. However, this field of research lacks randomized control trials, and therefore requires further investigation.
In a recent large-scale telephone survey of 15 European countries and Israel, 19% of respondents over 18 years of age had suffered pain for more than 6 months, including the last month, and more than twice in the last week, with pain intensity of 5 or more for the last episode, on a scale of 1(no pain) to 10 (worst imaginable). 4839 of these respondents with chronic pain were interviewed in depth. Sixty six percent scored their pain intensity at moderate (5–7), and 34% at severe (8–10); 46% had constant pain, 56% intermittent; 49% had suffered pain for 2–15 years; and 21% had been diagnosed with depression due to the pain. Sixty one percent were unable or less able to work outside the home, 19% had lost a job, and 13% had changed jobs due to their pain. Forty percent had inadequate pain management and less than 2% were seeing a pain management specialist.
In a systematic literature review published by the International Association for the Study of Pain (IASP), 13 chronic pain studies from various countries around the world were analyzed. (Of the 13 studies, there were three in the United Kingdom, two in Australia, one each in France, the Netherlands, Israel, Canada, Scotland, Spain, and Sweden, and a multinational.) The authors found that the prevalence of chronic pain was very high and that chronic pain consumes a large amount of healthcare resources around the globe. Chronic pain afflicted women at a higher rate than men. They determined that the prevalence of chronic pain varied from 10.1% to 55.2% of the population.
In the United States, the prevalence of chronic pain has been estimated to be approximately 30%. According to the Institute of Medicine, there are about 116 million Americans living with chronic pain. The Mayday Fund estimate of 70 million Americans with chronic pain is slightly more conservative. In an internet study, the prevalence of chronic pain in the United States was calculated to be 30.7% of the population: 34.3% for women and 26.7% for men. These estimates are in reasonable agreement and indicate a prevalence of chronic pain in the US that is relatively comparable to that of other countries.
Comorbidities and sequelae 
Chronic pain is associated with higher rates of depression and anxiety. Sleep disturbance, and insomnia due to medication and illness symptoms are often experienced by those with chronic pain. Chronic pain may contribute to decreased physical activity due to fear of exacerbating pain.
Two of the most frequent personality profiles found in chronic pain patients by the Minnesota Multiphasic Personality Inventory (MMPI) are the conversion V and the neurotic triad. The conversion V personality, so called because the higher scores on MMPI scales 1 and 3, relative to scale 2, form a "V" shape on the graph, expresses exaggerated concern over body feelings, develops bodily symptoms in response to stress, and often fails to recognize their own emotional state, including depression. The neurotic triad personality, scoring high on scales 1, 2 and 3, also expresses exaggerated concern over body feelings and develops bodily symptoms in response to stress, but is demanding and complaining.
Some investigators have argued that it is this neuroticism that causes acute pain 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 striking improvement once pain has resolved. 
Effect on cognition 
Chronic pain's impact on cognition is an under-researched area, but several tentative conclusions have been published. Most chronic pain patients complain of cognitive impairment, such as forgetfulness, difficulty with attention, and difficulty completing tasks. Objective testing has found that people in chronic pain tend to experience impairment in attention, memory, mental flexibility, verbal ability, speed of response in a cognitive task, and speed in executing structured tasks. In 2007, Shulamith Kreitler and David Niv advised clinicians to assess cognitive function in chronic pain patients in order to more precisely monitor therapeutic outcomes, and tailor treatment to address this aspect of the pain experience.
See also 
- Conditions related to pain
- Ehlers Danlos Syndrome
- Temporomandibular joint disorder
- Back pain
- Chronic Fatigue Syndrome
- Clinical depression
- Complex Regional Pain Syndrome
- Interstitial Cystitis
- Irritable Bowel Syndrome
- Myofascial Pain Syndrome
- Neuropathic pain
- Pelvic pain
- Post-Vasectomy Pain Syndrome
- Restless Leg Syndrome
- Lumbar spinal stenosis and cervical spinal stenosis
- Local anesthetics
- Acetaminophen (Paracetamol)
- Medical cannabis
- Other agents:
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- Chronic Pain Syndromes at the Open Directory Project
- Chronic Pain Australia
- American Chronic Pain Association
- American Pain Foundation
- International Association for the Study of Pain- IASP
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