Are pelvic floor muscle thickness and size of levator hiatus associated with pelvic floor muscle strength, endurance and vaginal resting pressure in women with pelvic organ prolapse stages I-III? A cross sectional 3D ultrasound study.

AIMS: To investigate if pelvic floor muscle (PFM) thickness and area of levator hiatus (LH) are associated with manometry measured PFM function in 109 women with pelvic organ prolapse (POP) stages I-III.
METHODS: In this cross-sectional study pubovisceral muscle thickness and LH area were assessed with three-dimensional transperineal ultrasound at rest and analyzed in the axial plane. PFM function was assessed with manometry and included strength, endurance, and vaginal resting pressure. Relationships were investigated using univariate linear logistic regressions models, Pearson product-moment correlation coefficient and hierarchical multiple regression.
RESULTS: The mean age was 49 (SD 12). There was a significant positive moderate association between muscle thickness and PFM strength (r = 0.49, P < 0.001) and endurance (r = .45, P < 0.001). A moderate negative association was found between LH area and vaginal resting pressure (r = -0.46, P < 0.001), strength (r = -0.41, P < 0.001) and endurance (r = -0.40, P < 0.001). Multivariate analyses included PFM strength, endurance, vaginal resting pressure, age, parity, BMI and socioeconomic status. Muscle thickness was best explained by PFM strength and LH area was best explained by vaginal resting pressure. However, PFM function explained only 20.0% and 26.4% of the variance in muscle thickness and LH area after controlling for age, parity, BMI, and socioeconomic status.
CONCLUSION: There are moderate associations between measurements using ultrasound and manometry in POP women. Thicker muscles and smaller LH were associated with higher strength and endurance. Smaller LH was additionally associated with higher vaginal resting pressure. Ultrasound and manometry measure different aspects of the PFM and cannot be used interchangeably.