Coexisting Irritable Bowel Syndrome and Overactive Bladder Are Associated with Increased Symptom Severity in Women with Fibromyalgia

Background/Objectives: Fibromyalgia (FM) is increasingly recognized as a multisystem disorder characterized by central sensitization and visceral symptom involvement. Irritable bowel syndrome (IBS) and overactive bladder (OAB), which share overlapping pathophysiological mechanisms related to altered visceral processing, are both commonly reported in FM. However, whether gastrointestinal and urinary symptoms cluster within the same patients and whether such clustering is associated with greater FM severity remain insufficiently explored. This study aimed to evaluate the overlap between IBS and OAB in women with FM and to determine whether their coexistence is associated with differences in FM severity. Methods: This cross-sectional observational study included 74 women with FM evaluated at the Physical Medicine and Rehabilitation outpatient clinic of Memorial Ataşehir Hospital between August and December 2025. FM severity was assessed using the Widespread Pain Index (WPI), Symptom Severity Scale (SSS), and FM Severity Score (GSS). OAB symptoms were evaluated using the Overactive Bladder Symptom Score (OABSS), and IBS status was determined based on prior clinical diagnosis. For subgroup severity analyses, patients were categorized as IBS + OAB, OAB only, or neither condition; the two patients with IBS alone were excluded from these comparisons because of the small cell size (n = 72). Results: IBS was present in 13 patients (17.6%) and OAB in 46 patients (62.2%). A substantial overlap between bowel and bladder symptoms was observed: 84.6% of IBS-positive patients had OAB, and IBS prevalence was higher in OAB-positive than OAB-negative patients (23.9% vs. 7.1%; OR = 4.56, 95% CI 0.93–22.24), although this association did not reach statistical significance (p = 0.063). Significant overall group differences were observed for SSS (p < 0.001) and GSS (p = 0.001), whereas WPI did not differ significantly across groups (p = 0.228). Patients with concomitant IBS and OAB had significantly higher GSS scores than both the OAB-only (adjusted p = 0.003) and neither-condition groups (adjusted p = 0.001). In multivariable analysis adjusted for age, concomitant IBS and OAB remained significantly associated with higher GSS (B = 3.77, 95% CI 1.59–5.95; p = 0.001). The regression model explained 18.2% of the variance in GSS (R2 = 0.182; p = 0.003). Conclusions: Concomitant IBS and OAB in women with FM were associated with greater overall FM symptom burden, particularly higher SSS and GSS scores, whereas widespread pain severity did not differ significantly across groups. The presence of either IBS or OAB may warrant evaluation for the other condition, as bowel–bladder symptom clustering may characterize a subgroup with greater overall symptom burden and support consideration of a more integrated clinical assessment.