This stretching could result in a tearing of the muscle body, detachment from its bony insertions, and neuropathic injury.
Direct damage to pelvic floor musculature has been suggested by computer models of parturition, which simulate the fetal head causing stretching of the levator ani complex to potentially injurious lengths. Various investigators have identified connective tissue disruption, muscle fiber hypoxia, nerve compression injury, among others.
The marvelous complexity of obstetrical labor and delivery has made it difficult to determine the exact nature of the injury. While the etiology of urinary and fecal incontinence and pelvic organ prolapse is almost certainly multifactorial, vaginal parity is the most consistently associated risk factor. Pelvic floor dysfunction is a major women's health problem with significant negative economic and quality of life impact. Subjects with injury to both the puborectalis and ileococcygeous at 6 weeks did not recover to normal at 6 months, whereas those with injury only to the puborectalis tended to have normal MR at 6 months. In those subjects recovering to normal MR by 6 months, an average of nearly 60% increase in R puborectalis muscle thickness compared to that seen at 6 weeks indicated the extent of the injury. Frequency of abnormal muscle by race and age is shown in the table. Incidence of any abnormality at any of the 4 sites was considered abnormal. Initial review indicated only 3 subjects not of African-American (AA) or Caucasian (W) race, and only 1 AA primipara of age ≥30yrs, therefore statistical analysis was limited to 45 primiparas and 25 nulliparas.
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