Levator ani muscle subdivisions length and their relationship to prolapse and hiatal size.

Objectives: The levator ani muscle plays a crucial role in maintaining pelvic organ support. There is robust data on the relationship between pubovisceral injury and levator hiatus (LH), urogenital hiatus (GH) size and pelvic organ prolapse. However, changes in muscle length for the pubovisceral (PVM), puborectal (PRM) and iliococcygeal (ICM) muscle subdivisions of the levator are unknow as are the relationships between these elements; and how they relate to prolapse.

Objective: to develop a technique to evaluate muscle subdivision length and examine their correlations and relationship to LH, GH, and prolapse size.

Methods: A secondary analysis of MRIs was performed as part of ongoing research made at rest of 15 women with prolapse and 15 controls with normal support of similar age, BMI and parity. 3D-Slicer was used to measure LH and UGH areas according to previous studies. In the region of the ICM, PRM and PVM muscles 3D-curves were drawn from origin to insertion on planes oriented along muscle lines of action (Figure 1). The ICM plane was chosen to pass through points 2cm ventral to the ischial spines and ICM region of the levator plate. PVM and PRM planes passed through muscle insertions on the inner surface of the pubic bones and perineal body (PMV) and the posterior limit of the LH (PRM). In cases with complete avulsion, the origin/insertion distance was traced from the normal origin location along the expected muscle path for the PVM.

Results: PVM was 25% longer in women with prolapse (10.9±1.9 cm vs 8.7±1.5 cm, p=0.002), PRM was 20% longer (16.4±2.2 cm vs 13.7±1.3 cm, p< 0.001) and ICM was 18% longer (12.9±1.4 cm vs 11.5±0.7 cm, p=0.002) (Figure 2). All differences had large effect sizes ( >0.8) with LH and GH being the two largest, followed by PRM, PVM and ICM length. GH area was 61% larger in cases compared with controls (19.2±4.3 cm² vs 11.9±3.7 cm², p < 0.001), and LH area 66% larger (25.4±7.1 cm² vs 15.3±2.9 cm², p < 0.001). Correlations between hiatus size, muscle components length, and prolapse size were analyzed (Figure 2). PVM, PRM and ICM have similar correlation coefficient with prolapse size (~0.65-0.67). ICM has moderate correlation with GH and LH (0.53 and 0.58) while PVM and PRM have the strongest correlation with GH and LH.

Conclusions: Results from this pilot study shows similar elongation of LAM subdivisions in prolapse and their relation to LH and GH. The three muscle subdivisions are all correlated, suggesting those changes are part of the overall change in pelvic floor shape. ICM muscle length is as strongly related to prolapse size as the other subdivisions, suggesting that ICM may contribute to POP with a different mechanism than hiatal enlargement.

Funding: NIH-R01-HD-38665/NIH-R01-HD-094954