Part 2 of a two-part series. This article will cover the role of pain processing in fibromyalgia (FM) and potential treatment methods. Part 1 appeared in the October 2009 issue (p. 1) and covered the history of the FM concept, current thinking on FM, and the epidemiology of the disease.
Augmented Pain and Sensory Processing Most Reproducible Feature
Once FM is established, the most consistently detected objective abnormalities involve pain and sensory processing systems. Since FM is defined in part by tenderness, considerable work has been performed exploring the potential reason for this phenomenon. The results of two decades of psychophysical pressure pain testing in FM have been very instructive.1
One of the earliest findings in this regard was that the tenderness in FM is not confined to tender points, but instead extends throughout the entire body.2,3 Theoretically, such diffuse tenderness could be either primarily due to a psychological factor (e.g., hypervigilance, where individuals are too attentive to their surroundings) or neurobiological influence factors (e.g., the plethora of factors that can lead to temporary or permanent amplification of sensory input).
Early studies typically used dolorimetry to assess pressure pain threshold, and concluded that tenderness was largely related to psychological factors, because these measures of pain threshold were correlated with levels of distress.3-5 To minimize the biases associated with “ascending” measures of pressure pain threshold (i.e., the individual knows that the pressure will be predictably increased), Petzke and colleagues performed a series of studies using more sophisticated paradigms using random delivery of pressures.6-8 These studies showed that: 1) the random measures of pressure pain threshold were not influenced by levels of distress of the individual, whereas tender point count and dolorimetry exams were; 2) patients with FM were much more sensitive to pressure even when these more sophisticated paradigms were used; 3) patients with FM were not any more “expectant” or “hypervigilant” than controls; and 4) pressure pain thresholds at any four points in the body are highly correlated with the average tenderness at all 18 tender points and four “control points” (the thumbnail and forehead). In addition to the heightened sensitivity to pressure noted in FM, other types of stimuli applied to the skin are also judged as more painful or noxious by these patients. Patients with FM also display a decreased threshold to heat,8-11 cold,10,12 and electrical stimuli.13
Gerster and colleagues were the first to demonstrate that patients with FM also display a low noxious threshold to auditory tones, suggesting a more global problem in sensory processing in some.14 A recent study by Geisser and colleagues used an identical random staircase paradigm to test FM patients’ threshold to the loudness of auditory tones and to pressure.15 This study found that patients with FM displayed low thresholds to both types of stimuli, and the correlation between the results of auditory and pressure pain threshold testing suggested that some of this was due to shared variance and some unique to one stimulus or the other. The notion that FM and related syndromes might represent biological amplification of all sensory stimuli has significant support from functional imaging studies that suggest that the insula is the most consistently hyperactive region, as discussed below. This region plays a critical role in sensory integration, with the posterior insula serving a purer sensory role, and the anterior insula associated with the emotional processing of sensations.16-18