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,^[1] though some theorists and researchers have placed the transition from acute to chronic pain at 12 months.^[2] 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.^[3] A popular alternative definition of chronic pain, involving no arbitrarily fixed durations is "pain that extends beyond the expected period of healing."^[1]

Classification

Chronic pain may be divided into "nociceptive" (caused by activation of nociceptors), and "neuropathic" (caused by damage to or malfunction of the nervous system).^[4]

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.^[5]

Neuropathic pain is divided into "peripheral" (originating in the peripheral nervous system) and "central" (originiting in the brain or spinal cord).^[6] Peripheral neuropathic pain is often described as “burning,” “tingling,” “electrical,” “stabbing,” or “pins and needles.” ^[7] Bumping the "funny bone" elicits peripheral neuropathic pain.

Pathophysiology

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.^[8]

Chronic pain of different etiologies has been characterized as a disease affecting brain structure and function. Magnetic Resonance Imaging studies have shown abnormal anatomical^[9] and functional connectivity, even during rest ^{[10] [11]} 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.^[12]

Management

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.^[13] The typical pain management team includes medical practitioners, clinical psychologists, physiotherapists, occupational therapists, and nurse practitioners.^[14] 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.^[15][16][17]

Epidemiology

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. ^[18]

Comorbidities and sequelae

Chronic pain is associated with higher rates of depression and anxiety.^[19] Sleep disturbance, and insomnia due to medication and illness symptoms are often experienced by those with chronic pain^[20]. Substance abuse is highly prevalent in some segments of the chronic pain population such as those with chronic headache.^[21] Chronic pain may contribute to decreased physical activity due to fear of exacerbating pain.^[19]

Psychology

Personality

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.^[22]

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.^[23]

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.^[24]

See also