Racial differences in the levator ani muscle and levator hiatus in individuals of reproductive age

Abstract

Background: Most studies on pelvic floor muscle morphology (dimensions, shape) and its relationship with patient characteristic risk factors of pelvic floor dysfunction (demographics, medical history) have largely pertained to White individuals with vaginas. There is a need to establish normative data on pelvic floor muscle anatomy and identify morphological differences in racially diverse cohorts that may play a role in racial differences in the prevalence and pathophysiology of pelvic floor dysfunction.

Objective: This study aimed to compare levator ani muscle thickness and levator hiatal morphology and their association with patient characteristics, between asymptomatic Black and White women-identifying individuals with a vagina of reproductive age.

Study design: In this dual-site retrospective study, pelvic magnetic resonance images of asymptomatic individuals with a vagina aged 18 to 50 years with normal pelvic anatomy-defined as normal gastrointestinal, genitourinary, or reproductive systems-were collected and categorized by self-reported race (Black, White). All individuals were imaged in the axial plane at rest in the supine position for medical indications. From these images, the levator ani muscle thickness and levator hiatal dimensions were measured. The levator hiatal area was traced to create 2-dimensional models of levator hiatal shape. Statistical shape modeling with principal component analysis was performed to quantify levator hiatal shape given by principal component scores. One-way multivariate and univariate analysis of covariance tests evaluated racial differences in levator ani muscle and levator hiatal measures with adjustments for age, body mass index, and parity. Spearman rank correlation with Fisher's z transformation tests quantified and compared correlations between patient characteristics and levator ani muscle and levator hiatal measures by racial group.

Results: A total of 112 participants were analyzed, with 59 Black and 53 White individuals. The Black cohort, compared to the White cohort, had a larger levator hiatal area (difference, 1.31 cm²; 95% confidence interval, 0.15-2.47), perimeter (difference, 0.95 cm; 95% confidence interval, 0.32-1.59), and anteroposterior diameter (difference, 0.53 cm; 95% confidence interval, 0.23-0.83). Of the patient characteristics, only parity-related correlations differed by race and were stronger in the White cohort than the Black cohort. Among White individuals, higher parity was associated with a larger levator hiatal area (r=0.60, P<.001), perimeter (r=0.49, P<.001), anteroposterior diameter (r=0.41, P=.002), and transverse diameter (r=0.49, P<.001), whereas among Black individuals, parity had no correlation with levator hiatal dimensions (r range, -0.07 to 0.00; all P>.60). Statistical shape modeling demonstrated that a more ovular, narrower levator hiatus shape predominated in Black individuals (P=.004) and correlated with higher body mass index in the Black (r=-0.45, P<.001) and White (r=0.54, P<.001) cohorts. Posterior distension of the levator hiatus (a more U-shaped levator hiatus) was associated with aging in only the Black cohort (r=-0.40, P=.002).

Conclusion: After controlling for age, body mass index, and parity, larger levator hiatal dimensions and a more ovular, narrower, U-shaped levator hiatus were observed in Black individuals compared to White individuals. Future longitudinal studies are necessary to gain further insight into causative mechanisms for racial morphological variability in pelvic floor muscles that may contribute to racial disparities in pelvic floor dysfunction risk.

Keywords: age; body mass index; dimensions; levator ani muscle; levator hiatus; magnetic resonance imaging; parity; pelvic floor; race; statistical shape modeling.