Deterioration of pelvic floor muscle function and prolonged urethral dysfunction in a rat model of simulated birth trauma.

Background: Vaginal delivery is a major stress urinary incontinence (SUI) risk factor; simulated birth trauma in rats causes subacute pelvic floor muscle (PFM) and urethral changes, but mid- to long-term effects remain unclear. We aimed to investigate the mid- to long-term effects of simulated birth trauma on PFM composition and urethral function in a rat model of SUI after simulated birth trauma. Methods: Eighteen female Sprague-Dawley rats were divided into sham, 4 weeks (4W), and 12 weeks (12W) post-vaginal distention (VD) groups. Urethral function was evaluated by measuring the amplitude of urethral response to electrical stimulation (A-URE). Muscle wet weights of the iliococcygeus (Icm) and pubococcygeus (Pcm) muscles were measured, and muscle fibers were classified as types I, IIa, and IIb. Cross-sectional area (CSA) and occupancy of each fiber type were determined. To assess the chronological progression of injury, newly acquired 12W data were compared with the sham and 4W data from our previous study. Results: A-URE was significantly lower in the 4W group than in the sham group. Pcm muscle weight was significantly lower in both 4W and 12W groups compared to the sham. Type I fiber ratios in Pcm and Icm were significantly lower in the 4W and 12W groups than in sham, while CSA of Pcm type I fibers was significantly higher in the 12W than in the 4W group. Conclusions: Our results suggest that VD is associated with mid- to long-term changes in PFM composition and decreased muscle endurance, which may contribute to prolonged urethral dysfunction.