A Comparative MRI Study of Superficial Perineal Musculature in Women with Pelvic Organ Prolapse, and in Parous and Nulliparous Controls

The objective was to investigate morphological differences in the superficial perineal muscles—bulbocavernosus muscle (BCM), superficial transverse perineal muscle (STPM), and perineal body (PB)—in women with pelvic organ prolapse (POP) compared with parous and nulliparous controls, and their correlation with symptoms. This observational study included 99 women: 66 with POP, 14 parous controls, and 19 nulliparous controls. Participants completed Pelvic Floor Distress Inventory-20 and Pelvic Floor Impact Questionnaire-7 (PFIQ-7) questionnaires and underwent 3.0 T MRI at rest and during Valsalva. High-resolution T2-weighted images were used for 3D reconstruction and volumetric analysis. The analysis incorporated relevant metrics of the BCM, STPM, and PB, including the assessment of BCM integrity with a particular emphasis on identifying ruptures. Receiver operating characteristic curve analysis was applied to explore the normal reference value for STPM volume. The BCM volume was significantly smaller in the POP group versus nulliparous controls (469.0 mm 3 vs 1000.5 mm 3 , p < 0.001), but not compared with parous controls. Within the POP group, patients with BCM disruption (36.4%, 24 out of 66) had significantly worse quality-of-life scores (PFIQ-7, CRAIQ-7, POPIQ-7; p < 0.05). STPM volume was significantly reduced in the POP group (396.1 mm 3 ) compared with both the parous group (639.9 mm 3 ) and the nulliparous group (807.1 mm 3 ). A STPM volume of less than 705 mm 3 was relevant with POP in parous women (area under the curve = 0.75, 63.6% sensitivity, 81.8% specificity, p < 0.001). No significant differences were found in PB volume among three groups. Superficial perineal muscles, particularly the BCM and STPM, show significant morphological differences in women with POP. These differences include a reduction in the volume of BCM, as well as decreases in the length, width, and volume of the STPM. Rupture of the BCM is associated with colorectal-anal and prolapse symptom severity. Quantitative MRI assessment of these structures could serve as a valuable biomarker for evaluating pelvic floor support and POP risk.