Comparing pain modulation and autonomic responses in fibromyalgia and irritable bowel syndrome patients.

OBJECTIVES: Past studies confirm that patients with fibromyalgia (FM) and irritable bowel syndrome (IBS) show similar pain processing dysfunctions, such as reduced pain inhibition and aberrant autonomic nervous system (ANS) responses. However, patients with FM and IBS have rarely been investigated in the same study. The aim of the present study, therefore, was to compare descending pain inhibition, pain sensitivity, and ANS reactivity to pain in FM, IBS, and healthy controls (HC).
METHODS: Female patients with FM (n=10), IBS (n=13), and HCs (n=10) were exposed to multiple cold water (12°C) immersions to study pain sensitivity and descending pain inhibition. Heart rate variability was also assessed during immersions.
RESULTS: Pain intensity scores were highest in FM, intermediate in IBS, and smallest in HCs. In contrast, pain inhibition was absent in FM, intermediate in IBS, and strongest in HCs. Importantly, controlling for differences in pain inhibition abolished group differences in pain sensitivity. Heart rate variability analyses confirmed that, in response to mild levels of pain, patients with FM showed greater sympathetic activity whereas HCs showed greater parasympathetic activity. Patients with IBS showed intermediate ANS responses. DISCUSSION: Our results confirm the presence of graded levels of somatic hyperalgesia across patients with IBS and FM. A similar pattern of result was observed for pain inhibitory dysfunctions. These pain processing changes were accompanied by abnormal autonomic responses, which maintained patients (principally patients with FM) in a state of sympathetic hyperactivity. Results suggest that patients with IBS and FM may present common, but graded, pain processing and autonomic dysfunctions.