Levator morphology and strength after obstetric avulsion of the levator ani muscle

Objectives Obstetric levator avulsion may be an important risk factor for prolapse. This study compares the size of the levator hiatus, the width of the genital hiatus, and pelvic muscle strength between vaginally parous women with or without levator avulsion, 5 to 15 years after delivery. Methods Parous women were assessed for levator ani avulsion, using 3-dimensional transperineal ultrasound. Women with and without levator ani avulsion were compared with respect to levator hiatus areas (measured on ultrasound), genital hiatus (measured on examination), and pelvic muscle strength (measured with perineometry). Further analysis also considered the association of forceps-assisted birth. Results At a median interval of 11 years from first delivery, levator avulsion was identified in 15% (66/453). A history of forceps-assisted delivery was strongly associated with levator avulsion (45% vs 8%; P < 0.001). Levator avulsion was also associated with a larger levator hiatus area (+7.3 cm²; 95% confidence interval [CI], 4.1-10.4, with Valsalva), wider genital hiatus (+0.6 cm; 95% CI, 0.3-0.9, with Valsalva), and poorer muscle strength (-14.5 cm H₂O; 95% CI, -20.4 to -8.7, peak pressure). Among those with levator avulsion, forceps-assisted birth was associated with a marginal increase in levator hiatus size but not genital hiatus size or muscle strength. Conclusions Obstetric levator avulsion is associated with a larger levator hiatus, wider genital hiatus, and poorer pelvic muscle strength. Forceps-assisted birth is an important marker for levator avulsion but may not be an independent risk factor for the development of pelvic muscle weakness or changes in hiatus size in the absence of levator avulsion.