Repeated birth injuries lead to pelvic floor muscle dysfunction and impairment in regeneration

Objectives Childbirth is a key risk factor for pelvic floor muscle (PFM) injury and dysfunction, and subsequent pelvic floor disorders (PFDs). Multiparity further exacerbates these risks. Using the pre-clinical rat model of simulated birth injury (SBI), we previously identified that an SBI leads to PFM atrophy and fibrosis. We hypothesized that multiple SBIs further overwhelm PFM regenerative capacity, leading to functionally relevant pathological alterations long-term. Study Design Rats underwent SBI and were allowed to recover for 8 weeks to undergo another SBI. Animals were sacrificed at acute, subacute, and long-term time points post-second injury (N=3-6/time point), and pubocaudalis (PCa) was harvested to assess ex vivo muscle function, histomorphological properties and gene expression. Results Acutely following the 1 st SBI, PCa force was decreased relative to controls. At 4 weeks, PCa force was recovered and remained unchanged at 8 weeks. Similarly, lower PCa force was observed immediately after repeated SBI. In contrast to functional recovery after 1 st SBI, PCa force remained lower at 4 weeks post-2 nd SBI and continued to be decreased even after 12 weeks after repeated injury. Fiber size was smaller at the long-term time points after 2 nd SBI compared to controls and single SBI groups. As opposed to the resolution of centralized nuclei at 8 weeks post-1 st SBI, regenerating myofibers persisted even at 12 weeks post-2 nd SBI. In contrast to the peak of collagen content at 4 weeks post-1 st SBI, this parameter raised progressively over 12 weeks after repeated SBIs. Prolonged inflammatory response, impairment in muscle anabolism, and sustained expression of ECM remodeling genes were observed after repeated SBIs. Conclusions Repeated birth injuries delay PFM regeneration and impair function in the pre-clinical rat model.