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Author: Karl G. Henriksson, MD, PhD

Affiliation: Associate Professor, Department of Rehabilitation Medicine. Pain Clinic and Neuromuscular Unit, Faculty of Health Sciences, Linköping University, Sweden. Address correspondence to: Karl G. Henriksson, MD, PhD, Neuromuscular Unit, University Hospital, Linköping, S-58185 Sweden [E-mail address: mailto:karl-g@telia.com ].

SUMMARY.

Objective: To review the literature concerning pain mechanisms in fibromyalgia [FMS].

Findings and Conclusions: Thirteen investigations using different methods, comprising 250 patients with FMS, confirm a biological dysfunction of the nociceptive system, especially in the central nervous system in the majority of patients with FMS. The hyperexcitability in the nociceptive nervous system may have different causes in the individual patient. Localized long-standing muscle pain, chronic stress, genetic factors, and hormonal changes may all playa role. Pain generators in the muscle may not be specific for FMS but may be of importance for initiating and maintaining pain and allodynia/hyperalgesia.

KEYWORDS. Fibromyalgia, pain mechanisms, muscular pain, central sensitization

INTRODUCTION

According to the classification criteria proposed by the American College of Rheumatology [ACR] fibromyalgia [FMS] comprises one symptom - chronic multifocal pain - and one sign - generalized allodynia/hyperalgesia (]). The patient who is diagnosed with FMS, however, is polysymptomatic. Besides pain there is fatigue, sleep disturbance, psychological distress, impaired muscle function, and symptoms that are usually regarded as stress-related.

Fibromyalgia is an illness, a syndrome, that effects three systems that regulate our well being: the nociceptive system, the stress-regulating system, and the immune system. These systems interact with each other, making it difficult to determine which of them is primarily affected in an individual patient. Psychological factors, personality traits, and social circumstances play a role for the total clinical picture. Fibromyalgia is indeed a biopsychosocial syndrome. The biological part concerns mainly pain and allodynia/hyperalgesia as well as biological changes related to continuous physical and emotional stress. This article will deal only with pain and allodynia. Allodynia is pain elicited by normally nonpainful stimuli. Hyperalgesia is increased pain intensity and prolonged pain duration evoked by stimuli that normally are painful.

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CONCLUDING REMARKS

There is strong support for the notion that pain and allodynia/hyperalgesia in FMS have an organic cause. The hyperexcitability in the nociceptive nervous system is mainly due to changes in the CNS. Longstanding excitation of nociceptors and low threshold mechano-receptors in the muscle may initiate and maintain such hyperexcitability. The permanent changes constitute a disease. There are methods for objectively diagnosing this disease.

The psychological and social consequences of chronic pain in FMS are the main determinants for the degree of disability and handicap.

Many causes could initiate and maintain the disease: e.g.,longstanding local or regional musculoskeletal pain, changes in stress-regulating systems, hormonal changes, changes in serotonin metabolisms, and genetic factors.

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