The pelvic floor consists of sling-like, levator ani muscles that support the pelvic organs and play a key role in urinary and fecal continence, as well as sexual function. Pelvic floor laxity refers to a weakening of the levator ani plate, leading to variable degrees of urinary or fecal incontinence and pelvic organ prolapse. Dynamic MR Defecography (MRD) allows a multicompartment evaluation of functional and structural pelvic floor disorders. The estimated Levator Ani Subtended Volume (eLASV), calculated as the volume contained between the pubococcygeal line and the pelvic hiatus, can be obtained using MRD to assess the volume of the pelvic basin and an indirect measurement of the laxity of the levator ani muscles. It has been shown that eLASV increases with advancing age, a well-established risk factor for the development of pelvic floor disorders. Whether the effect of age on pelvic floor laxity is due to age-related changes in the pelvic floor muscles, such as a loss of muscle mass is unclear. We recently showed that the severity of pelvic organ prolapse is associated with increasing psoas muscle fat fraction, a biomarker for reduced skeletal muscle mass. The cross-sectional area of the psoas muscle has been utilized in various studies as an indicator of lean muscle mass, offering an estimation of the overall skeletal muscle mass. In this study, we hypothesized that pelvic floor laxity is linked to a decrease in skeletal muscle mass and we investigated the relationship between the Psoas Muscle Index (PMI) to: (1) Hiatus length (H line), (2) Hiatus descent (M line), and (3) basin volume (eLASV), using MRD.

We performed a retrospective analysis of women presenting with anorectal symptoms who were evaluated by MRD, at our tertiary referral academic center. Clinical characteristics, including symptoms severity scores, were assessed using the Cleveland Clinic Fecal Incontinence (CCFI), and the Obstructed Defecation Syndrome (ODS) questionnaires. Pelvic floor laxity was measured using the pubococcygeal reference line. The H line was drawn from the inferior margin of the symphysis pubis to the posterior aspect of the anorectal junction measuring hiatus length, and M line was drawn perpendicularly, from the pubococcygeal line to the posterior end of the H line, measuring hiatus descent. We calculated eLASV as: −72.838 + 0.598 H-line + 1.217 M-line + 1.136 Levator Hiatus Width. An eLASV > 38.5 was considered abnormal. Image analysis included measurement of the cross-sectional area of the psoas muscle at the L4 level. The cross-sectional area was computed by manually drawing an ROI of the borders of the left and right psoas muscles, which were in turn averaged and divided by the square of the height, to calculate the psoas muscle index (PMI). Multivariable linear regression was used with adjustment for confounding covariates.

Our study cohort involved 165 women, median (IQR) age 53 (40-66) years, BMI 27 (23-29); 103 (62%) Caucasian. Forty-six (28%) had a history of smoking, 20 (12%) had diabetes, 71 (43%) had cardiovascular comorbidities, 102 (62%) had a history of neuropsychiatric disorder and 85 (53%) had a history of pregnancy with vaginal delivery. 
In 117 patients, median CCFI score was 5 (0-13) and ODS was 10 (6-14). There was a significant correlation between increasing CCFI score and both decreased PMI (coefficient: 1.069, p = 0.02), and increased eLASV (coefficient: 0.053 p = 0.03). There was no correlation ODS score and PMI (p = 0.07) or eLASV (p = 0.94).
 
The median (IQR) for resting H line was 5.7 (5.1-6.4), resting M line 1.6 (1.1-2.3), levator hiatus width 3.75 (3.2-4.3), and eLASV 23.301 (11.278-33.357). Fifty-seven women (35%) had abnormal hiatus length with an H line >6 cm, 60 (36%) had abnormal hiatus descent, or M line >2 cm at rest, and 28 (17%) had abnormal basin volume with an eLASV >38.5. There was a significant correlation between advancing age and increasing H line, M line, and eLASV (p<0.001).

Linear regression analysis indicated that decreased PMI was significantly associated with increased age (coefficient: 4.327, p < 0.001), decreased BMI (coefficient: 1.762, p < 0.001), and the development of abnormal M line (coefficient: 0.132, p = 0.02), H line (coefficient: 0.129 p = 0.045), and eLASV (coefficient: 4.486, p < 0.001). After adjusting for age and BMI, differences remained significant for M line (coefficient: 0.125, p = 0.047), and eLASV (coefficient: 3.643, p < 0.001).

In multiple regression analysis increase in eLASV was significantly associated with advancing age (coefficient: 0.265, p = 0.01), decreasing PMI (coefficient: 2.984, p = 0.01), and increasing parity (coefficient: 3.248, p = 0.01), but not BMI (coefficient: 0.090, p = 0.74).

Consistent with previous studies, we found that aging is associated with worsening pelvic floor laxity in women. Furthermore, PMI at L4 level, an MRD measure of skeletal muscle mass, correlated with advancing age, BMI, and measures of pelvic floor laxity. We showed that all measures of pelvic floor laxity, including increased hiatus descent (M Line), hiatus length (H line), and pelvic basin volume (eLASV) significantly increased with the loss of skeletal muscle mass. Although the decrease in skeletal muscle mass was significantly age-dependent and associated with decreased BMI, the increase in pelvic floor laxity, as shown with increased hiatus descent and pelvis basin volume was independent of age and BMI. Indeed PMI, age, and parity were each independent risk factors associated with increased pelvis basin volume.

Decline in skeletal muscle mass is associated with increased pelvic floor laxity, independent of age and BMI. Skeletal muscle mass may help to identify women who are at increased risk of pelvic floor disorders and could potentially guide management decisions. Future research is needed to delineate the clinical implications of interventions that may impact skeletal muscle mass and its role as a biomarker of female pelvic floor disorders.

Continence 7S1 (2023) 100991
DOI: 10.1016/j.cont.2023.100991