Autonomic nervous system dysfunction may explain the multisystem features of fibromyalgia

Abstract

T HE MAIN COMPLAINT of patients with fibromyalgia (FM) is widespread pain. This is the reason why patients with this illness are seen by rheumatologists. Nevertheless, very frequently, subjects with FM have diverse nonrheumatic symptoms. According to the American College of Rheumatology Multicenter Criteria Committee, such symptoms are fatigue, sleep disorders, paresthesias, headache, anxiety, sicca symptoms, Raynand's phenomenon, and irritable bowel. All of these complaints are significantly more prevalent in FM patients when compared with patients with other rheumatic diagnoses (1). The main theme of our discussion will be that most (if not all) of these multisystem symptoms of FM could be explained on the basis of autonomic nervous system (ANS) dysfunction. The ANS is an intricate network that works below the level of consciousness to maintain homeostasis. This network regulates the function of different organs and glands through antagonistic sympathetic/parasympathetic stimulation. One peculiarity of the ANS tone is its instantaneous and continuous variability. Thus. any static blood measurement of autonomic neurotransmitters or of their urinary eatabolites does not reflect properly the dynamic behavior of the system. The regulation of the ANS is integrated in the central nervous system with those of higher brain functions. So, the ANTS constitutes the main component of the stress response system in charge of the fight or flight reaction (2). In clinical practice, the function of the ANS has been estimated by means of the useful bedside tests described by Ewing and Clarke (3) that measure heart rate variation to deep breathing, the Valsalva maneuver, standing-upright, or sustained handgrip. Other methods used to assess the ANS are skin electrical conductance that reflects stimulation of sweat glands by cholinergic fibers, or laser Doppler flowmeter to monitor changes in skin microcirculation. Using these latter methods, Qiao et al (4) and Vaeroy et al (5) in seminal studies showed that subjects with fibromyalgia have a decreased sympathetic nervous system response to acoustic stimulation or cold pressure test. In the last decade, a new powerful tool has emerged to evaluate the performance of the ANS, namely, heart rate variability (HRV) analysis. This technique is based on the fact that the heart rate is not fixed, but it varies constantly and at random. The periodic components of such variation of heart rate are modulated by impulses of the sympathetic or parasympathetic branches of the ANS on the sinus node. Spectral analysis of such variability provides a measurement of the influx of these 2 branches of the ANS over the sinus node. Clinical and pharmacological studies have shown that the high-frequency band power reflects parasympa-