Resting Functional Connectivity of the Periaqueductal Gray Is Associated With Normal Inhibition and Pathological Facilitation in Conditioned Pain Modulation

• Published in: The journal of pain Vol. 19; no. 6; pp. 635.e1 - 635.e15
• Main Authors: Harper, Daniel E., Ichesco, Eric, Schrepf, Andrew, Hampson, Johnson P., Clauw, Daniel J., Schmidt-Wilcke, Tobias, Harris, Richard E., Harte, Steven E.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01.06.2018
• Subjects: descending analgesia; diffuse noxious inhibitory controls; Endogenous pain modulation; functional magnetic resonance imaging; quantitative sensory testing; descending analgesia; Endogenous pain modulation; diffuse noxious inhibitory controls; functional magnetic resonance imaging; quantitative sensory testing
• ISSN: 1526-5900; 1528-8447
• DOI: 10.1016/j.jpain.2018.01.001

•Smaller periaqueducal gray (PAG) volume was observed in fibromyalgia (FM) patients.•Conditioned pain modulation (CPM) facilitation was observed in FM patients.•PAG resting connectivity with cortical areas was associated with CPM inhibition.•Aberrant PAG-to-brainstem connectivity helps explain CPM facilitation in FM.•The results are consistent with animal models of descending pain facilitation. Conditioned pain modulation (CPM), a psychophysical paradigm that is commonly used to infer the integrity of endogenous pain-altering systems by observation of the effect of one noxious stimulus on another, has previously identified deficient endogenous analgesia in fibromyalgia (FM) and other chronic pain conditions. The mechanisms underlying this deficiency, be they insufficient inhibition and/or active facilitation, are largely unknown. The present cross-sectional study used a combination of behavioral CPM testing, voxel-based morphometry, and resting state functional connectivity to identify neural correlates of CPM in healthy controls (HC; n = 14) and FM patients (n = 15), and to probe for differences that could explain the pain-facilitative CPM that was observed in our patient sample. Voxel-based morphometry identified a cluster encompassing the periaqueductal gray (PAG) that contained significantly less gray matter volume in FM patients. Higher resting connectivity between this cluster and cortical pain processing regions was associated with more efficient inhibitory CPM in both groups, whereas PAG connectivity with the dorsal pons was associated with greater CPM inhibition only in HC. Greater PAG connectivity to the caudal pons/rostral medulla, which was pain-inhibitory in HC, was associated with pain facilitation in FM patients. These findings indicate that variation in the strength of the PAG resting functional connectivity can explain some of the normal variability in CPM. In addition, pain-facilitative CPM observed in FM patients likely involves attenuation of pain inhibitory as well as amplification of pain facilitative processes in the central nervous system.