MATERIALS AND METHODS
The Childbirth and Pelvic Symptoms study (CAPS)13 was performed by the Pelvic Floor Disorders Network, a multicenter network supported by the National Institute of Child Health and Human Development. The CAPS study was a prospective cohort study of primiparous women designed to study the relationship between vaginal delivery with a sphincter laceration and subsequent incontinence. Women in this study were recruited from the 921 participants in CAPS. Methods of the CAPS study have been reported in detail13 and are briefly summarized here. Enrollment into this study was conducted from September 2003 to February 2005. Three cohorts of primiparous women were recruited while the women were hospitalized after a singleton delivery. The primary cohort of interest consisted of women with an anal sphincter tear (n=104). Two comparison groups were recruited: women who delivered vaginally without a clinically recognized anal sphincter tear (n=94) and women who underwent cesarean delivery without labor (n=36). We attempted to include all women who delivered with a sphincter laceration. For each woman with an anal sphincter tear recruited for this study, we recruited the next consecutive woman who delivered vaginally without a clinically recognized sphincter tear. We attempted to include all women who delivered by cesarean without labor.
At the time of (or shortly after) their 6-month telephone interviews for the CAPS study, CAPS participants were approached to join the CAPS Imaging Study,14 which correlated standardized imaging (MRI and endoanal ultrasonography), physical examination findings, and symptom assessment. The MRIs obtained for the CAPS Imaging Study provided the data for this secondary analysis.
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This research protocol was approved at the institutional review boards at all clinical sites and the central data coordinating center. All women provided informed consent for participation. Data for this investigation were obtained 6-12 months after delivery. Weight and height were measured, and body mass index was calculated for each subject. Race was self-reported. Subjects were allowed to report more than one race but were asked to select a primary racial category if more than one race was indicated.
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The MRI protocol was standardized at a 1-day training session, led by the expert consulting radiologist at the central site before study initiation. After centralized training, images were acquired using a 1.5T magnet with the patient in a supine position and a surface array coil wrapped around the pelvis. Ultrasound gel (60 mL) was placed in the rectum. After localizer images, we obtained sagittal ultra-fast T2-weighted images (rest and strain), and transverse and coronal T2-weighted images (rest). For straining images, participants were coached to strain without elevating the lumbosacral spine or thighs. Each dynamic image required 2 seconds for acquisition.
On sagittal images, the pubococcygeal line was used to represent the normal location of the pelvic floor. Rest and maximal strain midsagittal images were obtained to evaluate the descent of the bladder neck and anorectal junction, anteroposterior length of the hiatus, and angle of the levator plate with the pubococcygeal line. The angle of the posterior rectal wall relative to the pubococcygeal line was measured at rest and during Valsalva. The H line, the distance from the inferior posterior aspect of the symphysis to the posterior rectal wall, was calculated. This represents the anteroposterior width of the genital hiatus. The distance from the posterior end of the H line, measured perpendicular to the pubococcygeal line, represented the M line. On the midsagittal image, we also obtained the following bony measures15: sacral length and depth, the obstetric conjugate (from the sacral promontory to the superior symphysis), and the anteroposterior outlet (from the last vertical joint of the coccyx to the inferior symphysis) (Fig. 1).
Axial measurements of levator muscle thickness were obtained at the level of the constrictor urethrae muscle. The width of the genital hiatus was obtained at the cranial-most image that included the symphysis. Bony measurements obtained on axial images included the angle of the pubic arch (in degrees, with the symphysis as the apex), the intertuberous diameter (measured from the posterior and medial cortex of the ischial tuberosities), and the interspinous diameter (measured from the posterior ischial spines).
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Using the coronal image that included the femoral heads and fovea, we measured the transverse inlet (from the inner aspect of the ischial cortex at the level of the fovea on each side). The transverse diameter of the pelvic inlet was measured at the level of the fovea. On oblique coronal images obtained in the plane of the sacrum, the maximum transverse inlet diameter was measured again.
Standardized images were obtained at six clinical sites. Images were reviewed by the site radiologist and a central radiologist. Image interpretation was standardized through a full day of in-person training for research radiologists. The radiology investigators were masked to the subjects’ obstetric characteristics and race. Our prior research (personal communication: Mark E. Lockhart, Julia R. Fielding, Holly E. Richter, Linda Brubaker, Caryl G. Salomon, Wen Ye, et al. Reproducibility of Dynamic MRI pelvic measures: a multi-site study. Submitted to Radiology, 2007) suggested high variability among readers of pelvic MRI measurements, particularly with respect to soft-tissue parameters. As a result, this research used the measures obtained by the central reader in all cases.
The mean and standard deviations for each dimension were calculated for African-American and white women. There were too few women of other races for meaningful comparisons. The initial analysis compared the two racial groups, adjusting only for cohort. Since the African-American participants were younger than the white participants (5% and 95% percentiles were 16.6-32.7 years of age for African-American women and 19.9-38.8 years of age for white women, P<.001), we then examined the potential confounding effect of age by adding it as a covariate to each analysis. When adjustment for age significantly changed the result or age was significant (in either the African-American or white population), we performed a second analysis restricted to the subpopulation of women under the age of 30, adjusting for cohort; the limit of age 30 was chosen because there were too few African-Americans above the limit to provide a reliable estimate of the age effect. Otherwise, we report results from the initial analyses. For all measures, the interaction effect between cohort and race was also examined using only subjects under the age of 30 in the two larger cohorts; there were insufficient observations in the cesarean delivery cohort for inclusion in this analysis.
Normal support was defined as the bladder neck above the pubococcygeal line with strain. In cases of normal support, descent of the bladder neck was not quantified. If the bladder neck descended below the pubococcygeal line, the descent was measured in centimeters. A similar strategy was used for the angle of levator plate with rest and with straining. Again, descent was measured only if the levator plate was below the pubococcygeal line. When the angle of the levator plate extended below the pubococcygeal line, the angle was measured in degrees. To compare these measures between African-American and white women, we first used the ϰ2 test to compare the proportions of women with abnormal descent across races. When no significant difference was detected, analysis of variance was used to test for a difference in the severity of descentbetween races (eg, among women with abnormal support). In all analyses, we adjusted for cohort effect. We did not adjust for height, body mass index, or site because they had no effect on the inferences.
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