APPEARANCE OF THE LEVATOR ANI MUSCLE IN PREGNANCY AS ASSESSED BY 3-D MAGNETIC RESONANCE IMAGING

M. Boreham, M. Zaretsky, M. Corton, J. Alexander, D. McIntire, D. Twickler

University of Texas Southwestern Medical Center at Dallas, Dallas, TX

OBJECTIVE:  To describe normal levator ani (LA) anatomy in postterm nulliparous women using 3-dimentional (3-D) magnetic resonance (MR).  
MATERIALS AND METHODS: Nulliparous women with uncomplicated pregnancies scheduled for postdate inductions were enrolled.  Women underwent an MR (4mm slices, 0 gap) SSFSE acquisition aligned axial to the maternal pelvis at an angle parallel to the obstetric conjugate, which included the maternal pelvis.  Utilizing 3-D volume processing software, LA volume and arch were evaluated from coronal images, while LA sling, hiatus type and distance were measured from the reformatted axial images.  To quantify the subjective decision regarding levator hiatus (“U” vs. “V”), the distance between the lateral borders of the LA at the level of the midurethra was measured across the mid-vagina (levator hiatus, LH).  Shapiro-Wilkes test for normality, linear regression trend analysis, Chi-square evaluation, and student’s t-test of Fisher’s transformation were performed.  
RESULTS:  Data were available in 74 (80%) of 92 women.  The cohort was 75% Hispanic ethnicity, 11% African-American and 3% Caucasian with a mean [SD] maternal age of 22[5] y and body mass index (BMI) of 31.6 [5.0].  The mean LA volume was 13.5 [3.7] cm3. The levator sling arms were intact in 93% of women and assumed a convex shape (arch) in the majority (92%).  Six women (8%) had discontinuous LA. The levator ani shape was characterized as a “U” in 45% and a “V” in 54%.  The width of the LH was 44.4 [6.6] mm.  The appearance of a “U” configuration was associated with a significantly (p=0.01) wider LH (46.7 [7.8] mm) than the “V” (42.6 [4.8] mm) configuration. There was a positive association between LA volume and higher fetal station (p=0.02).  

Station	n	LA Volume
-5,-4 -3 -2 -1,0,+1	15 26 22 5	14.8 14.0 13.5 10.5

Similarly, increasing BMI was associated with increased LA volume (p=<0.001).  There was no association between LA volume and fetal weight.  Station, BMI, birthweight and LA volume were not associated with LH shape.  
CONCLUSIONS:  Body habitus and fetal station affected levator volume assessed by 3D-MR in nulliparous, post term women. The levator sling and arch were well preserved in this large cohort, and 8% were found to have a discontinuous LA.   Morphometry of the LA was variable. The significance of these findings during pregnancy is unknown in relation to risk of obstetric pelvic floor injury.

Key Words: levator ani, magnetic resonance imaging, nulliparous, levator hiatus, fetal station

Disclosure – Speakers Bureau: M. Boreham, Pfizer, Watson, Ortho-McNeil.