Hypothesis / aims of study
Multiple sclerosis (MS) is a common cause of lower urinary tract dysfunction (LUTD), reflecting disruption of neural pathways involved in bladder control. Despite this, brainstem contributions during physiologic bladder filling remain poorly understood. The periaqueductal gray (PAG) and pontine centers are key brainstem hubs that integrate bladder afferent and efferent signaling and coordinate bladder storage and voiding.
We hypothesized that, during the empty bladder state, brainstem-cortical functional connectivity engages supraspinal micturition networks but is obscured by inter-subject variability in bladder physiology. We tested whether incorporating bladder capacity (voided volume + post-void residual, PVR) as a covariate would reveal connectivity not detectable in unadjusted analyses.
Study design, materials and methods
Resting-state functional MRI (rs-fMRI) data were acquired in 14 MS patients (mean age 54 years, range 33-74, OAB-AT ≥ 8) using an oral hydration paradigm. Scans were obtained during full bladder (strong desire to void) and post-micturition (empty bladder). Participants exited the scanner to void, enabling uroflow and PVR measurement prior to the second scan.
Preprocessing included motion correction, denoising, co-registration, and normalization. Five brainstem ROIs (three PAG, two pontine) were defined from prior work [1]. Seed-based functional connectivity (FC) was analyzed using CONN and SPM12. Two models were evaluated during the empty bladder state: (i) unadjusted (FC ~ 1) and (ii) covariate-adjusted (FC ~ 1 + BC). Statistical thresholds were voxel-level p < 0.01 (uncorrected) and cluster-level p < 0.05 (FDR-corrected).
Results
In the unadjusted model, no significant connectivity was observed for any brainstem ROI. After adjusting for bladder capacity (mean 296 mL, range 91–615 mL), significant connectivity emerged: (i) PAG 3 with right middle frontal gyrus (70% overlap) and precentral gyrus (15%), and (ii) Pons 2 with right inferior frontal gyrus (opercular division, 79%). No significant connectivity was observed for PAG 1, PAG 2, or Pons 1 outside seed regions.
Interpretation of results
The absence of findings in the unadjusted model, contrasted with their emergence after covariate inclusion, indicates bladder capacity is a key source of variability that can obscure brainstem–cortical relationships.
Connectivity between PAG and the middle frontal and precentral gyri aligns with established supraspinal pathways linking bladder afferent processing to executive and motor control. The middle frontal gyrus is implicated in higher-order control of continence, while the precentral gyrus contributes to pelvic floor and urethral sphincter regulation.
Pontine connectivity with the inferior frontal region supports involvement of inhibitory control networks. The inferior frontal gyrus, particularly in the right hemisphere, is implicated in motor inhibition and may contribute to the suppression of voiding. Its established structural and functional organization, including interactions with the supplementary motor area and insula, supports integration of inhibitory control and interoceptive processing relevant to bladder function.[2] These findings are also consistent with prior resting-state studies in overactive bladder, demonstrating altered connectivity in frontal and interoceptive networks, supporting a central role for inhibitory control dysfunction.[3]