Cross-sectional relationships between spinal cord gray matter volume and pain in individuals with fibromyalgia and opioid use.

Brain gray matter volume (GMV) has been extensively examined in chronic pain and opioid therapy, but spinal GMV has not. As a first investigation of spinal cord GMV in chronic pain and opioid therapy, as our primary outcome, we compared averaged C5-C7 GMV of the spinal cord dorsal/ventral horns among 3 female cohorts: controls (CON, n = 30), and individuals with fibromyalgia taking opioids (FMO, n = 27) and not taking opioids (FMN, n = 31). After adjusting for age across the 3 groups (and adjusting for both age and pain duration across the FM patients), we observed differences in dorsal and ventral horn GMVs, with less GMV in FMOs vs. controls. Additionally, we evaluated within-group relationships between GMVs and clinical pain measures. Among FMNs, dorsal (D) and ventral (V) horn GMVs were positively associated with pain duration (D: p = 0.042, V: p = 0.015). Among FMOs, GMV was positively associated with pain severity (D: p = 0.028, V: p = 0.012), pain interference (D: p = 0.010, V: p = 0.004), and cold pain tolerance (D: p = 0.004, V: p < 0.001). Together with distinct within-group pain-related correlations between FMN and FMO cohorts, less spinal cord GMV in the opioid-taking FM patients suggests spinal cord GMV may be influenced by long-term opioid use in FM.