44 - Sex Differences in the Pelvic Floor

Poster Board Number: 44
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Abstract Body : One role of the pelvic floor is to support and raise pelvic viscera. Loss of structural integrity of the floor can result in descent of organs through it, a condition known as pelvic organ prolapse (POP). While POP occurs in both sexes, it is more common in females, likely due to childbirth, though other factors (e.g., greater organ mass or pelvic cavity volume) may play a role. Symptoms of POP range from discomfort to increased risk of fatal infection. Today, POP affects over 300 million people, yet, prior to modern gynecology or family planning, the severity and prevalence of POP would have been even greater. Considering the potential impact of POP on evolutionary fitness, females are predicted to have “larger” pelvic floors than males as an adaptation to better support organs and reduce rates of POP. This study tested for female-biased dimorphism in pelvic floor thickness (PFT), mass (PFM), and volume (PFV) while also exploring how pelvic organ mass, body mass, and pelvic cavity volume (PCV) influence these traits. Pelvic organs and floors of 20 female and 19 male anonymized Stony Brook University Hospital patients were segmented from magnetic resonance images using Amira Software. Volumes were generated from these segmentations and multiplied with corresponding organ densities to estimate mass. PFT was measured at the midpoint between the rectum and urethra, which is the closest anatomical point to where most prolapse occurs. The coefficient of determination reveals pelvic organ mass explains a maximum of 1% of variation in pelvic floor traits, indicating that, despite the pelvic floor’s role in supporting pelvic organs and contrary to predictions, larger pelvic organs are not associated with larger pelvic floors, minimizing the likelihood of a causal relationship between the two. Total body mass explains only 5% of the variation in PFV and PFM and < 0.01% in PFT. PCV (estimated from distances between skeletal landmarks) explains the most variation in pelvic floor traits at 13% for PFV and PFM but only 4% for PFT. One-tailed t-tests (no violations of assumptions found) of size-unadjusted values of PFM and PFV demonstrates that females are significantly larger (24%) than males in both measures. Adjusting PFM and PFV for differences due to PCV also yields female-biased dimorphism. The minimal female-biased dimorphism in PFT is not significant with or without size-adjustment, suggesting that the primary source of structural integrity of the female pelvic floor is muscle mass spread across larger volumes rather than thickness at this clinically relevant location. As skeletal musculature elsewhere in the body displays male-biased dimorphism, this absolute and size-adjusted female-biased dimorphism in PFM and PFV is unusual and supports the hypothesis of an adaptive function for a thicker pelvic floor in females, likely to reduce the risk of POP. As pelvic organ mass and pelvic cavity volume surprisingly do not explain large amounts of variation in the floor, the impact of the gravid uterus and floor distention caused by childbirth are hypothesized to explain more variation in the pelvic floor. Future work will explore this possibility through analyses of the pelvic floor and organs in the later stages of pregnancy and post-partum across primi- and multi-parous individuals as well as across individuals’ lifetimes.