Chronic pain has several different meanings in medicine. 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 initiation of pain, 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 somatic" and "deep", and deep pain into "deep somatic" and "visceral". Superficial somatic 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 injured, 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" (originiting in the brain or spinal cord). Peripheral neuropathic pain is often described as “burning,” “tingling,” “electrical,” “stabbing,” or “pins and needles.”  Bumping the "funny bone" elicits peripheral neuropathic pain.
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.
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 (also called pain medicine) is that 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.
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.
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. Substance abuse is highly prevalent in some segments of the chronic pain population such as those with chronic headache. 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.
- Conditions related to pain
- Back pain
- Chronic Fatigue Syndrome
- Clinical depression
- Complex Regional Pain Syndrome
- Irritable Bowel Syndrome
- Myofascial Pain Syndrome
- Post-Vasectomy Pain Syndrome
- Restless Leg Syndrome
- Lumbar spinal stenosis and cervical spinal stenosis
- Local anesthetics
- Acetaminophen (Paracetamol)
- Medical cannabis
- Other agents:
- ^ a b Turk, D.C.; Okifuji, A. (2001). "Pain terms and taxonomies". In Loeser, D.; Butler, S. H.; Chapman, J.J. et al.. Bonica's management of pain (3 ed.). Lippincott Williams & Wilkins. pp. 18–25. ISBN 0683304623. http://books.google.com.au/books?id=TyNEicOiJqQC&pg=RA1-PA18&dq=Pain+terms+and+taxonomies&cd=1#v=onepage&q=Pain%20terms%20and%20taxonomies&f=false.
- ^ Main, C.J.; Spanswick, C.C. (2001). Pain management: an interdisciplinary approach. Elsevier. p. 93. ISBN 0-443-05683-8. http://books.google.com/?id=wcEQPzTOEAoC&pg=PA93&dq=chronic+acute+subacute+pain&cd=48#v=onepage&q=chronic%20acute%20subacute%20pain.
- ^ Thienhaus, O.; Cole, B.E. (2002). "Classification of pain". In Weiner, R.S.. Pain management: A practical guide for clinicians (6 ed.). American Academy of Pain Management. ISBN 0-8493-0926-3. http://books.google.com.au/books?id=lg7sIgP9D3kC&pg=PA28&dq=chronic+acute+subacute+pain+idiopathic&lr=&client=firefox-a&cd=1#v=onepage&q=chronic%20acute%20subacute%20pain%20idiopathic&f=true.
- ^ Keay, KA; Clement, CI; Bandler, R (2000). "The neuroanatomy of cardiac nociceptive pathways". In Horst, GJT. The nervous system and the heart. Totowa, New Jersey: Humana Press. p. 304. ISBN 089603. http://books.google.com.au/books?id=IeElgT1clUcC&pg=PA303&lpg=PA303&dq=%22definitions+of+pain+and+its+central+representation%22&source=bl&ots=mKVpvW_2gA&sig=gs3Z5lQMiMHg2tT7Fc1D2RSzbHI&hl=en&ei=DlpAS83FE8uHkQWOnon1Bg&sa=X&oi=book_result&ct=result&resnum=1&ved=0CAoQ6AEwAA#v=onepage&q=%22definitions%20of%20pain%20and%20its%20central%20representation%22&f=true.
- ^ Coda, BA; Bonica, JJ (2001). "General considerations of acute pain". In Loeser, D; Bonica, JJ. Bonica's management of pain (3 ed.). Philadelphia: Lippincott Williams & Wilkins. ISBN 0443056838.
- ^ Bogduk, N; Merskey, H (1994). Classification of chronic pain: descriptions of chronic pain syndromes and definitions of pain terms (second ed.). Seattle: IASP Press. p. 212. ISBN 0931092051.
- ^ Paice, JA (Jul-Aug 2003). "Mechanisms and management of neuropathic pain in cancer". Journal of supportive oncology 1 (2): 107–20. PMID 15352654. http://www.supportiveoncology.net/journal/articles/0102107.pdf.
- ^ Vadivelu N, Sinatra R (2005). "Recent advances in elucidating pain mechanisms". Current opinion in anaesthesiology 18 (5): 540–7. doi:10.1097/01.aco.0000183109.27297.75. PMID 16534290.
- ^ Geha PY, Baliki MN, Harden RN, Bauer WR, Parrish TB, Apkarian AV (2008). "The brain in chronic CRPS pain: abnormal gray-white matter interactions in emotional and autonomic regions". Neuron 60 (4): 570–581. doi:10.1016/j.neuron.2008.08.022. PMC 2637446. PMID 19038215. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2637446.
- ^ Baliki MN, Geha PY, Apkarian AV, Chialvo DR (2008). "Beyond feeling: chronic pain hurts the brain, disrupting the default-mode network dynamics". J of Neurosci 28 (6): 1398–1403. doi:10.1523/JNEUROSCI.4123-07.2008. PMID 18256259. http://www.jneurosci.org/cgi/content/full/28/6/1398.
- ^ Tagliazucchi E, Balenzuela P, Fraiman D, Chialvo DR (2010). "Brain resting state is disrupted in chronic back pain patients.". Neurosci Lett 485 (1): 26–31. PMID 20800649. http://www.ncbi.nlm.nih.gov/pubmed/20800649.
- ^ May A (2009). "Chronic pain may change the structure of the brain". Pain 137 (1): 7–15. doi:10.1016/j.pain.2008.02.034. PMID 18410991.
- ^ Chou R, Huffman LH. Ann Intern Med. 2007 Oct 2;147(7):505-14.. http://www.ncbi.nlm.nih.gov/pubmed/17909211.
- ^ Hardy, Paul A. J. (1997). Chronic pain management: the essentials. U.K.: Greenwich Medical Media. ISBN 1 900 151 855. http://books.google.com/?id=EtZ-4eb_aDUC&pg=PA10&lpg=PA10&dq=%22the+reduction+of+suffering+and+enhanced+quality+of+life+%22&q=%22the%20reduction%20of%20suffering%20and%20enhanced%20quality%20of%20life%20%22.
- ^ Main, Chris J.; Spanswick, Chris C. (2000). Pain management: an interdisciplinary approach. Churchill Livingstone. ISBN 0 443 05683 8.
- ^ Thienhaus, Ole; Cole, B. Eliot (2002). "The classification of pain". In Weiner, Richard S,. Pain management: A practical guide for clinicians. CRC Press. p. 29. ISBN 0 8493 0926 3. http://books.google.com/?id=L2CSdeiMZi4C&pg=PA27&dq=%22the+classification+of+pain%22+thienhaus&q=%22chronic%20pain%20frequently%20requires%20the%20coordinated%20efforts%20of%20a%20broadly%20based%20treatment%20team%22.
- ^ Henningsen P, Zipfel S, Herzog W (2007). "Management of functional somatic syndromes". Lancet 369 (9565): 946–55. doi:10.1016/S0140-6736(07)60159-7. PMID 17368156.
- ^ Stanos S, Houle TT (2006). "Multidisciplinary and interdisciplinary management of chronic pain". Physical medicine and rehabilitation clinics of North America 17 (2): 435–50, vii. doi:10.1016/j.pmr.2005.12.004. PMID 16616276.
- ^ Breivik H, Collett B, Ventafridda V, Cohen R, Gallacher D. (2006, May) Survey of chronic pain in Europe: prevalence, impact on daily life, and treatment. Eur J Pain 10(4):287-333. Epub 2005 Aug 10. PMID 16095934.
- ^ a b Pruimboom L, van Dam AC (2007). "Chronic pain: a non-use disease". Med. Hypotheses 68 (3): 506–11. doi:10.1016/j.mehy.2006.08.036. PMID 17071012.
- ^ Ferini-Strambi L (2011). "Sleep disorders in multiple sclerosis". Handb Clin Neurol. 99: 1139–46. PMID 21056246.
- ^ Norton PJ, Asmundson GJ, Norton GR, Craig KD (January 1999). "Growing pain: 10-year research trends in the study of chronic pain and headache". Pain 79 (1): 59–65. PMID 9928777.
- ^ Leo, Raphael (2007). Clinical manual of pain management in psychiatry. Washington, DC: American Psychiatric Publishing. p. 58. ISBN 9781585622757.
- ^ Melzack, R; Wall, PD (1996). The challenge of pain (2 ed.). London: Penguin. pp. 31–32. ISBN 4780140256703.
- ^ Kreitler S; Niv D (2007). "Cognitive impairment in chronic pain" (pdf). Pain: Clinical Updates (International Association for the Study of Pain) XV (4): 1–4. http://www.iasp-pain.org/AM/TemplateRedirect.cfm?template=/CM/ContentDisplay.cfm&ContentID=4132. Retrieved 2008-04-15.
- Chronic Pain Syndromes at the Open Directory Project
- Chronic Pain Australia
- American Chronic Pain Association
- American Pain Foundation
- Patient consumer web page sponsored by the APS
- International Association for the Study of Pain- IASP
- Media related to pain at Wikimedia Commons