Chronic pain

Classification

The International Association for the study of pain defines chronic pain as pain with no biological value, that persists past normal tissue healing. The DSM-5 recognizes one chronic pain disorder, somatic symptom disorders, a reduction from the three previously recognized pain disorders. The criteria include it lasting for greater than six month.

The suggested ICD-11 chronic pain classification suggests 7 categories for chronic pain.

1. Chronic primary pain: defined by 3 months of persistent pain in one or more anatomical regions that is unexplainable by another pain condition.
2. Chronic cancer pain: defined as cancer or treatment related visceral, musculoskeletal, or bony pain.
3. Chronic posttraumatic pain: pain lasting 3 months post trauma or surgery, excluding infectious or preexisting conditions.
4. Chronic neuropathic pain: pain caused by damage to the somatosensory nervous system damage.
5. Chronic headache and orofacial pain: pain that originates in the head or face, and occurs for 50% or more days over a 3 months period.
6. Chronic visceral pain: pain originating in an internal organ.
7. Chronic musculoskeletal pain: pain originating in the bones, muscles, joints or connective tissue.

Chronic pain may be divided into "nociceptive" (caused by inflamed or damaged tissue activating specialised pain sensors called nociceptors), and "neuropathic" (caused by damage to or malfunction of the nervous system).

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".

Pathophysiology

Under persistent activation nociceptive transmission to the dorsal horn may induce a pain 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. The type of nerve fibers that are believed to propagate the pain signals are the C-fibers, since they have a slow conductivity and give rise to a painful sensation that persists over a long time. In chronic pain this process is difficult to reverse or eradicate once established. In some cases, chronic pain can be caused by genetic factors which interfere with neuronal differentiation, leading to a permanent reduction in the threshold for pain.

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 somatotopic representation of the body is inappropriately reorganized following peripheral and central sensitization. This maladaptive change results in the experience of allodynia or hyperalgesia. Brain activity in individuals with 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.

Dopaminergic dysfunction has been hypothesized to act as a shared mechanism between chronic pain, insomnia and major depressive disorder. Increased tonic dopamine activity and a compensatory decrease in phasic dopamine activity, which is important in inhibiting pain. This is supported by the implication of COMT in fibromyalgia and temporomandibular joint syndrome. Astrocytes, microglia, and Satellite glial cells have been found to be dysfunctional in chronic pain. Increased activity of microglia, alterations of microglial networks as well as increased production of chemokines and cytokines by microglia are proposed to act to potentiate pain. Astrocytes have been observed to lose their ability to regulate the excitability of neurons, increasing spontaneous neural activity in pain circuits.

Management

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 (particularly anesthesiologists), clinical psychologists, physiotherapists, occupational therapists, physician assistants, 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.

Complete and sustained remission of many types of chronic pain is rare, though some can be done to improve quality of life.

Some people with chronic pain benefit from opioid treatment and others do not; some are harmed by the treatment. Possible harms include reduced sex hormone production, hypogonadism, infertility, impaired immune system, falls and fractures in older adults, neonatal abstinence syndrome, heart problems, sleep-disordered breathing, opioid-induced hyperalgesia, physical dependence, addiction, and overdose.

Various nonopioid medicines are also used, depending on whether the pain originates from tissue damage or is neuropathic. Limited evidence suggests that chronic pain from tissue inflammation or damage (as in rheumatoid arthritis and cancer pain) is best treated with opioids, while for neuropathic pain (pain caused by a damaged or dysfunctional nervous system) other drugs may be more effective, such as tricyclic antidepressants, serotonin-norepinephrine reuptake inhibitors, and anticonvulsants. Because of the weakness of the evidence, it is not clear which are the best approaches to treating many types of pain, and doctors must rely on their own clinical experience. Doctors often cannot predict who will use opioids just for pain management and who will go on to develop addiction, and cannot always distinguish between those who are and those who are not seeking opioids due primarily to an existing addiction. Withholding, interrupting or withdrawing opioid treatment in people who benefit from it can cause harm.

Interventional pain management may be appropriate, including techniques such as trigger point injections, neurolytic blocks, and radiotherapy.

Psychological treatments including cognitive behavioral therapy and acceptance and commitment therapy have been shown effective for improving quality of life and reducing pain interference in those with chronic pain.

While exercise has been offered as a method to lessen chronic pain and there is some evidence of benefit, this evidence is tentative. Side effects from exercise are few in this population.

Alternative medicine

Hypnosis, including self-hypnosis, has tentative evidence. Evidence does not support hypnosis for chronic pain due to a spinal cord injury. Medical Marijuana has been used and identified as a treatment for chronic pain

Epidemiology

A systematic literature review of chronic pain found that the prevalence of chronic pain varied studies in various countries from 10.1% to 55.2% of the population, affected women at a higher rate than men, and that chronic pain consumes a large amount of healthcare resources around the globe.

A 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 the United States, the prevalence of chronic pain has been estimated to be approximately 35%, with approximately 50 million Americans experiencing partial or total disability as a consequence. According to the Institute of Medicine, there are about 116 million Americans living with chronic pain, which suggests that approximately half of American adults have some chronic pain condition. 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.

Outcomes

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, often resulting in weight gain. Such comorbid disorders can be very difficult to treat due to the high potential of medication interactions, especially when the conditions are treated by different doctors.

Severe chronic pain is associated with increased 10 year mortality, particularly from heart disease and respiratory disease. Several mechanisms have been proposed for the increased mortality, e.g. abnormal endocrine stress response. Additionally, chronic stress seems to affect cardiovascular risk by acceleration of the atherosclerotic process. However, further research is needed to elucidate the relationship between severe chronic pain, stress and cardiovascular health.

Personality

Two of the most frequent personality profiles found in people with chronic pain 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 people with chronic pain, also shows striking improvement once pain has resolved.

It has been suggested that catastrophizing may play a role in the experience of pain. Pain catastrophizing is the tendency to describe a pain experience in more exaggerated terms than the average person, to think a great deal more about the pain when it occurs, or to feel more helpless about the experience. People who score highly on measures of catastrophization are likely to rate a pain experience as more intense than those who score low on such measures. It is often reasoned that the tendency to catastrophize causes the person to experience the pain as more intense. One suggestion is that catastrophizing influences pain perception through altering attention and anticipation, and heightening emotional responses to pain. However, at least some aspects of catastrophization may be the product of an intense pain experience, rather than its cause. That is, the more intense the pain feels to the person, the more likely they are to have thoughts about it that fit the definition of catastrophization.

Social support

Social support has important consequences for individuals with chronic pain. In particular, pain intensity, pain control, and resiliency to pain has been implicated as outcomes influenced by different levels and types of social support. Much of this research has focused on emotional, instrumental, tangible and informational social support. People with persistent pain conditions tend to rely on their social support as a coping mechanism and therefore have better outcomes when they are a part of larger more supportive social networks. Across a majority of studies investigated, there was a direct significant association between social activities or social support and pain. Higher levels of pain were associated with a decrease in social activities, lower levels of social support, and reduced social functioning.

Effect on cognition

Chronic pain's impact on cognition is an under-researched area, but several tentative conclusions have been published. Most people with chronic pain 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.