Background Pelvic organ prolapse has two components; 1) protrusion of the pelvic organs beyond the hymen and 2) descent of the levator ani. The Pelvic Organ Prolapse Quantification system measures the first component, however, there remains no standard measurement protocol for the second mechanism. Objectives Test the hypotheses that 1) difference in the protrusion area is greater than the area created by levator descent in prolapse patients compared with controls and, 2) Prolapse is more strongly associated with levator hiatus compared to urogenital hiatus. Study Design Mid-sagittal MRI scans from 30 controls, 30 anterior predominant and 30 posterior predominant prolapse patients were assessed. Levator area was defined as the area above the levator ani and below the sacrococcygeal inferior pubic point line. Protrusion area was defined as the protruding vaginal walls below the levator area. The levator hiatus and urogenital hiatus were measured. Bivariate analysis and multiple comparisons were performed. Bivariate logistic regression was performed to assess prolapse as a function of levator hiatus, urogenital hiatus, levator area, and protrusion. Pearson correlation coefficients were calculated. Results The levator area for the anterior (34.0±6.5cm2) and posterior (35.7±8.0cm2) prolapse groups were larger during Valsalva compared to controls (20.9±7.8cm2, p<.0001 for both); similarly, protrusion areas for the anterior (14.3±6.2cm2) and posterior (14.4±5.7cm2) were both larger than controls (5.0±1.8cm2, p<.0001 for both). The levator hiatus length for the anterior (7.2±1cm) and posterior (6.9±1cm) were longer during Valsalva compared to controls (5.2±1.5cm, p<.0001 for both); similarly, urogenital hiatus lengths for the anterior (5.7±1cm) and posterior (6.3±1.1cm) were both longer than controls (3.8±0.8cm, p<.0001 for both). The difference in levator area in prolapse patients compared with controls was greater than the difference in protrusion area (14.0 ± 7.2cm2 v. 9.4 ± 5.9cm2, p<.0002). The urogenital was more strongly associated with prolapse than the levator hiatus (OR: 12.9, 95% CI: (4.1–39.2), OR: 4.3, 95% CI: (2.3–7.5)). Levator hiatus and urogenital hiatus are both correlated with levator and protrusion areas, and all were associated with maximum prolapse size (p≤0.001, for all comparisons). Conclusions In prolapse, the levator area increases more than the protrusion area and both the urogenital hiatus and levator hiatus are larger. The odds of prolapse for an increase in the urogenital hiatus are three times larger than for the levator hiatus, which leads us to reject both the original hypotheses.

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