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Pathophysiological mechanisms in chronic musculoskeletal pain (fibromyalgia): the role of central and peripheral sensitization and pain disinhibition

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Chronic musculoskeletal pain has biological, psychological and social components. This review deals with the biological factors, with emphasis on the fibromyalgia syndrome (FMS). Studies on central sensitization of pain-transmitting neurons, changes in endogenous pain modulation that give rise to pain disinhibition, referred pain, pain-related decrease in muscle strength and endurance, and pain generators in deep tissues are reviewed. In FMS there is strong scientific support for the statement that the biological part of the syndrome is a longstanding or permanent change in the function of the nociceptive nervous system that can be equated with a disease. Further research is necessary in order to determine which methods are best for diagnosis of the pain hypersensitivity in clinical practice. FMS may be the far end of a continuum that starts with chronic localized/regional musculoskeletal pain and ends with widespread chronic disabling pain.

Section snippets

FMS

FMS has biological, psychological and social components. The pain signals from the periphery reach (in the cortex) both somatosensory areas, where pain localization, pain duration and pain intensity are perceived, and other areas, e.g. limbic structures where the emotional responses to pain are perceived and interpreted.3 The psychological factors are always there, as are the social consequences, but these factors are not necessarily the primary causes for the pathological pain

Central sensitization

Sensitization of pain transmission neurons in the CNS, especially the wide-dynamic-range neurons (WDR) in the dorsal horn, is a normal event in acute pain. It becomes pathological if it is longstanding or permanent, as it does in FMS. Permanent central sensitization is considered to be an expression of neuronal plasticity in primary sensory and dorsal-horn neurons, especially the WDR neurons. The WDR neurons change in structure, phenotype, function and biochemistry. A-beta fibres get qualities

The two main subgroups of FMS

There are two main subgroups of FMS. The first subgroup is characterized by widespread pain that is preceded by longstanding localized or regional musculoskeletal pain. Pain generators may be present in muscles or joints or tendons or ligaments. In the other subgroup the primary cause is in the brain. It is clinically important to differentiate between these subgroups. To identify pain generators and to try to eliminate them is part of the treatment in patients where the widespread pain is

In the clinic

In all longlasting or chronic muscle pain conditions examination with respect to the presence and spread of pain hypersensitivity is mandatory. The ideal method should be easy to perform, reliable, not time-consuming, and not expensive. The most commonly used method is to estimate PPT. The pressure at which conversion from painless pressure to painful pressure occurs is the pressure pain threshold. An area with decreased PPT is a tender point (TP). The application of pressure by the tip of the

Predisposing factors

Not all people with chronic localized or regional muscle pain develop FMS. Predisposing factors may be a prerequisite for developing FMS. Hereditary factors might be important. Serotonin- and dopamine-related genes could have a role in the development of FMS.113 People with low pain thresholds caused by acquired or genetic factors may be at greater risk of getting FMS than those with higher pain thresholds. People with localized/regional musculoskeletal pain may be predisposed to developing FMS

Summary: is FMS a disease?

Central sensitization of pain-transmitting neurons and pain disinhibition as described above can be a feature of chronic muscle pain of different origins. When the pain hypersensitivity is longstanding or permanent, there is a change in function of the nociceptive nervous system that could be equated with a disease.115, 116 When pain hypersensitivity is widespread and combined with multifocal muscle pain that is more or less continuous and present at rest, FMS is present according to the ACR

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