The Role of Peripheral Input for Chronic Pain Syndromes like Fibromyalgia Syndrome

Abstract

Objectives: Clinical symptoms of chronic muscle conditions like fibromyalgia syndrome [FMS], include pain, stiffness, subjective weakness, and muscle fatigue. Muscle pain in FMS is usually described as fluctuating and always associated with local or generalized tenderness [hyperalgesia and/or allodynia]. This tenderness related to muscle pain depends on increased peripheral and/or central nervous system responsiveness to peripheral stimuli that can be either noxious [hyperalgesia] or nonnoxious [allodynia]. For example, patients with muscle hyperalgesia will rate painful muscle stimuli higher than normal controls, whereas patients with allodynia may perceive light touch as painful, something that a “normal” individual will never describe as painful. The pathogenesis of such peripheral and/or central nervous system changes in FMS is unclear, but peripheral tissue changes, specifically in muscles have been implicated.

Findings: Indirect evidence from interventions that attenuate tonic peripheral pain impulses in patients with FMS suggest that overall FMS pain is dependent on peripheral input. More important, allodynia and hyperalgesia can be improved or abolished by removal of peripheral impulse input. Another potential mechanism for FMS pain is central disinhibition. However, this pain mechanism also depends on tonic impulse input, even if only inadequately inhibited. Thus a promising approach to understanding FMS pain is to determine whether abnormal activity of receptors in deep tissues is fundamental to the development and maintenance of this chronic pain disorder.

Conclusions: Most FMS patients present with focal tissue abnormalities including myofascial trigger points, ligamentous trigger points, or osteoarthritis of the joints and spine. Though not predictive for the development of FMS, these changes nevertheless represent important pain generators that may initiate or perpetuate chronic pain. Local chemical mediators, including lactic acid, amino acids, and cytokines could play an important role in sensitizing deep tissue nociceptors of FMS patients. Thus the combination of peripheral impulse input and increased central pain sensitivity may be responsible for widespread chronic pain as reported in FMS.

KEYWORDS:

• Nociception
• fibromyalgia syndrome
• chronic pain