Stress is a state of disharmony, or threatened homeostasis. A stressor could have a psychological origin or a biological origin. Societies have become more intricate with industrialization, and modern individuals try to adapt to the new defiance by forcing their stress response system. The main component of the stress response network is the autonomic nervous system. The present article reviews current knowledge on autonomic dysfunction in fibromyalgia. Sympathetic hyperactivity has been consistently described by diverse groups of investigators. Fibromyalgia is proposed to be a sympathetically maintained neuropathic pain syndrome, and genomic data support this contention. Autonomic dysfunction may also explain other fibromyalgia features not related to pain.

Autonomic dysfunction is frequent in patients with fibromyalgia (FM). Heart rate variability analyses have demonstrated signs of ongoing sympathetic hyperactivity. Catecholamines are sympathetic neurotransmitters. Catechol-O-methyltransferase (COMT), an enzyme, is the major catecholamine-clearing pathway. There are several single-nucleotide polymorphisms (SNPs) in the COMT gene associated with the different catecholamine-clearing abilities of the COMT enzyme. These SNPs are in linkage disequilibrium and segregate as 'haplotypes'. Healthy females with a particular COMT gene haplotype (ACCG) producing a defective enzyme are more sensitive to painful stimuli. The objective of our study was to define whether women with FM, from two different countries (Mexico and Spain), have the COMT gene haplotypes that have been previously associated with greater sensitivity to pain. All the individuals in the study were female. Fifty-seven Mexican patients and 78 Spanish patients were compared with their respective healthy control groups. All participants filled out the Fibromyalgia Impact Questionnaire (FIQ). Six COMT SNPs (rs2097903, rs6269, rs4633, rs4818, rs4680, and rs165599) were genotyped from peripheral blood DNA. In Spanish patients, there was a significant association between three SNPs (rs6269, rs4818, and rs4680) and the presence of FM when compared with healthy controls. Moreover, in Spanish patients with the 'high pain sensitivity' haplotype (ACCG), the disease, as assessed by the FIQ, was more severe. By contrast, Mexican patients displayed only a weak association between rs6269 and rs165599, and some FIQ subscales. In our group of Spanish patients, there was an association between FM and the COMT haplotype previously associated with high pain sensitivity. This association was not observed in Mexican patients. Studies with a larger sample size are needed in order to verify or amend these preliminary results.

This review demonstrates that sympathetic nervous system predominance is common in fibromyalgia, chronic fatigue syndrome, irritable bowel syndrome, and interstitial cystitis. This concordance raises the possibility that sympathetic dysfunction could be their common underlying pathogenesis that brings on overlapping clinical features. The recognition of sympathetic predominance in these 4 syndromes may have potential clinical implications. It may be worth exploring the use of nonpharmacological measures as well as drug therapies aimed to regain autonomic balance.

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Abnormal activity of the sympathetic nervous system may be involved in the pathogenesis of chronic pain syndromes. This article reviews the animal studies of sympathetically induced pain behavior, the controversy of sympathetically maintained pain in clinical practice, and the dysautonomic nature of fibromyalgia (FM). FM has neuropathic pain features (stimuli-independent pain state accompanied by allodynia and paresthesias). The proposal of FM as a sympathetically maintained pain syndrome is based on the controlled studies showing that patients with FM display signs of relentless sympathetic hyperactivity and that the pain is submissive to sympathetic blockade and is rekindled by norepinephrine injections. Dysautonomia also may explain the multisystem features of FM.

Objective. To determine the accumulated 24-hour cardiovascular autonomic modulation and its circadian variations in patients with fibromyalgia, by means of heart rate variability analysis. Methods. Thirty patients with fibromyalgia and 30 age-and sex-matched controls were studied prospectively. Assessments included a 24-hour ambulatory recording of heart rate variability, time-domain analysis of the accumulated 24-hour R-R interval variations, and power spectral analysis to determine the sympathol vagal balance at different hours (calculated as the power spectral density of the low-frequency rO.04-0.15-Hzl sympathetic band divided by the power of the high-frequency [0.15-0.50-Hz] parasympathetic band). Results. Fibromyalgia patients had diminished accumulated 24-hour heart rate variability, manifested by a decreased standard deviation of all R-R intervals (mean-C SD 126-C 35 ms, versus 150-C 33 ms in controls; P = 0,008) and a decreased ratio of pairs of adjacent R-R intervals differing by >SO ms (mean-C SD 12.0 & 9.0% versus 20.1 f 18.0%; P = 0.031). Patients lost the circadian variations of sympathohagal balance, with nocturnal values significantly higher than those of controls at time 0 (mean f SD 3.5 2 3.2 versus 1.2 f 1.0; P = 0.027) and at 3 hours (3.3 2 3.0 versus 1.6 k 1.4; P = 0.01). Conclusion. Individuals with fibromyalgia have diminished 24-hour heart rate variability due to an increased nocturnal predominance of the low-frequency band oscillations consistent with an exaggerated sym

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