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Randomized trial of cesarean vs vaginal delivery for effects on the pelvic floor in squirrel monkeys

  • Fiona M. Lindo
    Correspondence
    Corresponding author: Fiona M. Lindo, MD, MPH.
    Affiliations
    Department of Obstetrics and Gynecology, Scott and White Healthcare, and Texas A&M Health Science Center College of Medicine, Temple, TX
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  • Emily S. Carr
    Affiliations
    Department of Obstetrics and Gynecology, Scott and White Healthcare, and Texas A&M Health Science Center College of Medicine, Temple, TX
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  • Michelle Reyes
    Affiliations
    Department of Obstetrics and Gynecology, Scott and White Healthcare, and Texas A&M Health Science Center College of Medicine, Temple, TX
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  • Jilene M. Gendron
    Affiliations
    Department of Radiology, Scott and White Healthcare, and Texas A&M Health Science Center College of Medicine, Temple, TX
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  • Julio C. Ruiz
    Affiliations
    Department of Veterinary Sciences, Michale E. Keeling Center for Comparative Medicine and Research, University of Texas MD Anderson Cancer Center, Bastrop, TX
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  • Virginia L. Parks
    Affiliations
    Department of Veterinary Sciences, Michale E. Keeling Center for Comparative Medicine and Research, University of Texas MD Anderson Cancer Center, Bastrop, TX
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  • Thomas J. Kuehl
    Affiliations
    Department of Obstetrics and Gynecology, Scott and White Healthcare, and Texas A&M Health Science Center College of Medicine, Temple, TX

    Department of Pediatrics, Scott and White Healthcare, and Texas A&M Health Science Center College of Medicine, Temple, TX

    Department of Molecular and Cellular Medicine, Radiology, Scott and White Healthcare, and Texas A&M Health Science Center College of Medicine, Temple, TX

    Department of Veterinary Sciences, Michale E. Keeling Center for Comparative Medicine and Research, University of Texas MD Anderson Cancer Center, Bastrop, TX
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  • Wilma I. Larsen
    Affiliations
    Department of Obstetrics and Gynecology, Scott and White Healthcare, and Texas A&M Health Science Center College of Medicine, Temple, TX
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Open AccessPublished:September 11, 2015DOI:https://doi.org/10.1016/j.ajog.2015.09.003

      Background

      Vaginal delivery is a risk factor in pelvic floor disorders. We previously described changes in the pelvic floor associated with pregnancy and parturition in the squirrel monkey, a species with a humanlike pattern of spontaneous age- and parity-associated pelvic organ prolapse.

      Objective

      The potential to prevent or diminish these changes with scheduled cesarean delivery (CD) has not been evaluated. In a randomized, controlled trial, we compared female squirrel monkeys undergoing spontaneous vaginal delivery with those undergoing scheduled primary CD for pelvic floor muscle volumes, muscle contrast changes, and dynamic effects on bladder neck position.

      Study Design

      Levator ani, obturator internus, and coccygeus (COC) muscle volumes and contrast uptake were assessed by magnetic resonance imaging in 20 nulliparous females examined prior to pregnancy, a few days after delivery, and 3 months postpartum. The position of bladder neck relative to bony reference line also was assessed with abdominal pressure using dynamic magnetic resonance imaging.

      Results

      Baseline measurements of 10 females randomly assigned to scheduled primary CD were not different from those of 10 females assigned to spontaneous vaginal delivery. Levator ani and obturator internus muscle volumes did not differ between groups, while volumes were reduced (P < .05) in the observation immediately after pregnancy. The COC muscles increased (P < .05) immediately after delivery for females in the spontaneous vaginal delivery group, but not for females in the scheduled CD group. Position of the bladder neck descended (P < .05) by 3 months postpartum in both groups.

      Conclusion

      Scheduled CD diminishes changes in COC muscle volume and contrast reported to be associated with spontaneous vaginal delivery in squirrel monkeys. However, pelvic support of the bladder was not protected by this intervention suggesting that effects of pregnancy and delivery are not uniformly prevented by this procedure.

      Key words

      Introduction

      Pelvic floor disorders are a major women’s health issue with significant negative effects on quality of life.
      • Luber K.M.
      • Boero S.
      • Choe J.Y.
      The demographics of pelvic floor disorders: current observations and future projections.
      The etiology of pelvic organ prolapse is multifactorial, however, vaginal delivery has been consistently shown to be a risk factor.
      • MacLennan A.H.
      • Taylor A.W.
      • Wilson D.H.
      • Wilson D.
      The prevalence of pelvic floor disorders and their relationship to gender, age, parity and mode of delivery.
      • Lukacz E.S.
      • Lawrence J.M.
      • Contreras R.
      • Nager C.W.
      • Luber K.M.
      Parity, mode of delivery, and pelvic floor disorders.
      This disorder occurs gradually and is most commonly seen in aging women who have delivered ≥1 infants vaginally. The overarching hypothesis is that injury to the integrity of the nerve muscle units of the pelvic floor initiates this process, leading to a reduction in the muscle’s ability to oppose intraabdominal forces. It also has been thought to result from direct damage to the muscles or associated connective tissues during parturition.
      • Shek K.L.
      • Dietz H.P.
      Intrapartum risk factors for levator trauma.
      Cesarean deliveries (CD) have been proposed as being a protective factor.
      • Cacciatore A.
      • Giordano R.
      • Romano M.
      • Rosa B.
      • Fonti I.
      Putative protective effects of cesarean section on pelvic floor disorders.
      The squirrel monkey is an excellent animal model developing dystocia during parturition requiring CD and changes in the pelvic support structures resulting in prolapse.
      • Coates K.W.
      • Gibson S.
      • Williams L.E.
      • et al.
      The squirrel monkey as an animal model of pelvic relaxation: an evaluation of a large breeding colony.
      Little is known about the specific effects of pregnancy alone on the pelvic floor muscles. We have previously identified changes in the pelvic floor associated with pregnancy and parturition in the squirrel monkey model.
      • Lindo F.M.
      • Reyes M.
      • Gendron J.M.
      • Runge V.
      • Shull B.
      • Kuehl T.J.
      Differentiating the effect of pregnancy from parturition on the pelvic floor in the squirrel monkey.
      The coccygeus (COC) muscle, within the posterior pelvis in squirrel monkeys, is correlated to the pubo-COC (levator ani [LA]), anteriorly located in human beings, as they both undergo significant strain, stretch, and injury during parturition. The monkeys have a fixed pubic symphysis so the pelvic floor has to widen toward the posterior pelvic rim, to accommodate the fused fetal skull producing forces that would inherently affect the COC. We examined animals prepregnancy, immediately prior to delivery, and a few days postpartum. Both pregnancy and delivery affected the COC muscle volume, but there was no consistent change in either the obturator internus (OI) or LA muscles following pregnancy or parturition. The study, however, was performed on multiparous females.
      • Lindo F.M.
      • Reyes M.
      • Gendron J.M.
      • Runge V.
      • Shull B.
      • Kuehl T.J.
      Differentiating the effect of pregnancy from parturition on the pelvic floor in the squirrel monkey.
      To determine if parturition-related changes in pelvic floor muscle volume and contrast changes could be prevented or diminished by CD we conducted a randomized controlled trial of scheduled primary CD on nulliparous females.

      Materials and Methods

      Squirrel monkeys were obtained from the National Squirrel Monkey Breeding and Research Resource in Bastrop, TX. Sixteen were housed at the Bastrop facility and 4 at the Scott and White Healthcare animal facility in Temple, TX. Both facilities are Association for Assessment and Accreditation of Laboratory Animal Care (AAALAC) International accredited and institutional animal care and use committees with oversight for each facility reviewed and approved the project prior to initiation. Squirrel monkey breeding activity is seasonal with ovulatory cycles occurring from December through April in Texas even as animals are housed inside in light- and temperature-controlled rooms. Gestation lasts about 152 days. All females were evaluated with magnetic resonance imaging (MRI) prior to pregnancy. Once conception was confirmed, the pregnancies were dated and monitored using ultrasound. Spontaneous delivery was estimated to occur >147 days of gestation, so CD was planned for 144-147 days of gestation. All CD were performed by the veterinarian (J.C.R.) involved with the study, who is experienced with this procedure in squirrel monkeys. All pregnant animals within the study were randomized by computer-generated sequence number so that 10 females were assigned to spontaneous vaginal delivery and 10 assigned to scheduled CD. Assignments were made as pregnancies were confirmed and dated.
      MRI was performed of the pelvic floor from L7-C4 using the validated procedure, previously described by Kramer et al
      • Kramer L.A.
      • Gendron J.M.
      • Pierce L.M.
      • Runge V.M.
      • Shull B.L.
      • Kuehl T.J.
      Magnetic resonance imaging of the levator ani in the squirrel monkey: a comparison of muscle volume between a cohort with pelvic organ prolapse and matched normals.
      and modified by Bracken et al.
      • Bracken J.N.
      • Reyes M.
      • Gendron J.M.
      • Pierce L.M.
      • Runge V.M.
      • Kuehl T.J.
      Alterations in pelvic floor muscles and pelvic organ support by pregnancy and vaginal delivery in squirrel monkeys.
      Using 3-dimensional fast low-angle shot MRI sequences, 224 sagittal images were acquired. A neonatal blood pressure cuff was placed around the animal’s abdomen to provide transabdominal pressure. A dynamic series (using 2-dimensional fast low-angle shot) of images was obtained during 25 seconds while inflating the cuff with a volume of air (40 mL) to reproduce prolapse as evaluated by perineal exam, if present, or voiding of bladder contents as described by Pierce et al.
      • Pierce L.M.
      • Coates K.W.
      • Kramer L.A.
      • Bradford J.C.
      • Thor K.B.
      • Kuehl T.J.
      Effects of bilateral levator ani nerve injury on pelvic support in the female squirrel monkey.
      Twenty sagittal images along the midline were obtained to evaluate the position of the bladder neck relative to bony landmarks. A bony reference line was created from the anterior aspect of the pubic symphysis to the anterior aspect of the first tail vertebra. The position of the bladder neck relative to this reference line in millimeters was measured as an objective description of the degree of support of this pelvic structure. The females that had undergone CD did not have a dynamic series performed in the immediate postpartum exam. Instead, the position of bladder neck was measured in sagittal images without abdominal pressure. These are reported for completeness, but not used in analysis. As a monitor of sedation status, animals were visually observed for motion of limbs during the entire MRI process, which typically lasted <15 minutes. The high-resolution gradient echo sagittal views were reformatted as 224 axial views to obtain muscle volume and contrast intensity measurements. Animals were returned to the facility and monitored during recovery of sedation. All females were examined during the breeding season, prior to conception for their first MRI, then between 1-5 days after delivery and finally at 3 months postpartum.
      The Digital Imaging and Communications in Medicine (DICOM) image files were processed using software (3D-Doctor; Able Software Corp, Lexington, MA) for review, manipulation, measurement, and analysis. The left and right sides of the OI, LA, and COC muscles were traced by authors (E.S.C., F.M.L., W.I.L., and T.J.K.) who were blinded to mode of delivery of the animal subjects. Each tracing was completed twice by different examiners and the averages used for analysis. The volumes were calculated by the software algorithm from the surfaces and thicknesses of the image series. The average volume for each paired muscle group is reported. For all of the muscles, high-contrast areas were traced separately. Recording and analysis of images using 3D-Doctor software has been established in our laboratory with good interobserver correlations for pelvic floor muscle volume measures.
      • Pierce L.M.
      • Coates K.W.
      • Kramer L.A.
      • Bradford J.C.
      • Thor K.B.
      • Kuehl T.J.
      Effects of bilateral levator ani nerve injury on pelvic support in the female squirrel monkey.
      The pelvic outlet diameter is the distance between the inferior lateral margins of the obturator foramina at the level of the pubic arch. This measure was made in an axial MRI as reported previously for this species.
      • Bracken J.N.
      • Reyes M.
      • Gendron J.M.
      • Pierce L.M.
      • Runge V.M.
      • Kuehl T.J.
      Alterations in pelvic floor muscles and pelvic organ support by pregnancy and vaginal delivery in squirrel monkeys.
      • Joyce J.S.
      • Dornak S.
      • Gendron J.M.
      • Reyes M.
      • Ruiz J.C.
      • Kuehl T.J.
      Lack of association between pelvic outlet diameter and pelvic organ prolapse in squirrel monkeys.
      Analysis of variance with repeated measures design was utilized for parametric measures of muscle volumes and percentage of contrast (Statistica software; StatSoft, Tulsa, OK). Post hoc testing of means was performed using Duncan test with P < .05 indicating significance. Power analysis of 0.80 was calculated to detect a 24% difference in COC muscle volume (P < .05).

      Results

      Two groups of 10 females were randomized to either spontaneous vaginal delivery or scheduled primary CD upon diagnosis of pregnancy with serial ultrasound. The baseline evaluations of both groups were similar (Table 1) for age, body size, and pelvic floor measurements including muscle volumes, bladder neck position with abdominal pressure, and width of the bony pelvic outlet. No vaginal lacerations were found on exam and no detected anal sphincter tears observed on MRI. Pregnancy outcomes were similar (Table 2) except that animals undergoing scheduled CD were delivered about 3 days earlier than those delivering spontaneously. There was an impression that the first CD animal may have had the beginnings of labor, however the head was not engaged. Perinatal mortality was equal in both groups. The first 2 infants delivered by CD developed respiratory depression related to maternal anesthesia. The anesthetic regimen was modified and no other complications were noted. One of the females in the spontaneous vaginal delivery group had perinatal fetal demise related to a spontaneous preterm delivery at 139 days, about 2 weeks premature. The other perinatal mortality event was related to protracted labor due to an abnormal presentation resulting in dystocia. This infant died in utero, but was delivered by CD. The female was included in the analyses as a vaginal delivery based on the intention-to-treat principle.
      Table 1Comparison of animals assigned to scheduled primary cesarean vs spontaneous vaginal delivery
      VariablePrimary cesarean delivery
      Mean ± SD
      Spontaneous vaginal delivery
      Mean ± SD
      P value
      Student t test.
      N1010
      Maternal age at conception of first pregnancy, y3.9 ± 0.43.9 ± 0.4.99
      Maternal body weight prior to first pregnancy, g760 ± 53732 ± 90.41
      Parity (all nulliparous)00
      Baseline pelvic floor measures
      Muscle volumes, mm3
       Levator ani–right427 ± 59427 ± 67.98
       Levator ani–left422 ± 54425 ± 54.92
       Obturator internus–right394 ± 42412 ± 45.37
       Obturator internus–left377 ± 37403 ± 37.14
       Coccygeus–right234 ± 36239 ± 38.75
       Coccygeus–left221 ± 44223 ± 27.90
      Bladder neck position relative to reference line (caudal of line is positive), mm1.8 ± 2.31.9 ± 3.3.91
      Bony outlet diameter, mm17.7 ± 1.217.9 ± 1.4.64
      Lindo. Short title: RCT mode of delivery and effects on pelvic floor. Am J Obstet Gynecol 2015.
      a Mean ± SD
      b Student t test.
      Table 2Delivery outcomes for first pregnancies of females assigned to 2 treatment groups
      VariablePrimary cesarean deliverySpontaneous vaginal deliveryP value
      N1010
      Gestational age at delivery, d
      Mean ± SD
      146 ± 2149 ± 4.015
      Mann-Whitney U test for nonparametric variables
      Birthweight, g
      Mean ± SD
      102 ± 10100 ± 19.77
      Student t test
      CRL, mm
      Mean ± SD
      122 ± 6122 ± 8.96
      Student t test
      Ponderal index (100 × weight/CRL3)
      Mean ± SD
      1.39 ± 0.101.36 ± 0.23.82
      Mann-Whitney U test for nonparametric variables
      Female infant gender3/10 (30%)4/10 (40%).64
      χ2 test
      Perinatal mortality2/10
      Two newborns died shortly after delivery due to respiratory depression from maternal anesthesia
      2/10
      One fetus delivered as stillbirth at 139 d gestation and 1 fetus died in utero from dystocia. Dead infant was delivered by cesarean delivery after failed labor on day 146 of pregnancy.
      1.0
      χ2 test
      CRL, crown-rump length.
      Lindo. Short title: RCT mode of delivery and effects on pelvic floor. Am J Obstet Gynecol 2015.
      a Mean ± SD
      b Mann-Whitney U test for nonparametric variables
      c Student t test
      d χ2 test
      e Two newborns died shortly after delivery due to respiratory depression from maternal anesthesia
      f One fetus delivered as stillbirth at 139 d gestation and 1 fetus died in utero from dystocia. Dead infant was delivered by cesarean delivery after failed labor on day 146 of pregnancy.
      The results on muscle volumes for 20 female squirrel monkeys are summarized in Table 3. All 3 pairs of pelvic floor muscles have changes associated with parturition. For LA and OI muscles, these changes are seen as a reduction in volume immediately after delivery that returns to prepregnancy values by 3 months postpartum. These changes were not different between the delivery groups. In contrast, the COC muscles increased in volume following spontaneous vaginal delivery, but not CD. This results in a significant difference (P < .0001) between delivery modes. When the COC muscles were traced, the percent of muscle volume associated with high levels of contrast accumulation was increased in the same manner (Figure 1) (Videos 1 and 2).
      Table 3Comparison of average muscle volumes (mm3, means with SE) for 3 pelvic floor muscles in 2 groups of females including 10 with primary scheduled cesarean deliveries and 10 with spontaneous vaginal deliveries measured at 3 times relative to first pregnancy
      Delivery groupPrior to pregnancy1–5 d Postpartum3 mo PostpartumP values for group differencesP values for time differences
      Average of left and right levator ani muscles
      Spontaneous vaginal426 ± 26378 ± 24410 ± 26.67
      Cesarean425 ± 26366 ± 24394 ± 26
      Total425 ± 18
      Analysis of variance with repeated measures were used to compare muscle volumes. Letter superscripts are used to label the means. Those with different superscripts differ (P < .05 using Duncan post hoc test after analysis of variance to examine group and time differences)
      372 ± 17
      Analysis of variance with repeated measures were used to compare muscle volumes. Letter superscripts are used to label the means. Those with different superscripts differ (P < .05 using Duncan post hoc test after analysis of variance to examine group and time differences)
      401 ± 18
      Analysis of variance with repeated measures were used to compare muscle volumes. Letter superscripts are used to label the means. Those with different superscripts differ (P < .05 using Duncan post hoc test after analysis of variance to examine group and time differences)
      <.0001
      Average of left and right obturator internus
      Spontaneous vaginal407 ± 17371 ± 21419 ± 23.38
      Cesarean385 ± 17364 ± 21398 ± 23
      Total396 ± 12
      Analysis of variance with repeated measures were used to compare muscle volumes. Letter superscripts are used to label the means. Those with different superscripts differ (P < .05 using Duncan post hoc test after analysis of variance to examine group and time differences)
      368 ± 15
      Analysis of variance with repeated measures were used to compare muscle volumes. Letter superscripts are used to label the means. Those with different superscripts differ (P < .05 using Duncan post hoc test after analysis of variance to examine group and time differences)
      409 ± 16
      Analysis of variance with repeated measures were used to compare muscle volumes. Letter superscripts are used to label the means. Those with different superscripts differ (P < .05 using Duncan post hoc test after analysis of variance to examine group and time differences)
      <.0001
      Average of left and right coccygeus
      Spontaneous vaginal231 ± 15
      Analysis of variance with repeated measures were used to compare muscle volumes. Letter superscripts are used to label the means. Those with different superscripts differ (P < .05 using Duncan post hoc test after analysis of variance to examine group and time differences)
      338 ± 26
      Analysis of variance with repeated measures were used to compare muscle volumes. Letter superscripts are used to label the means. Those with different superscripts differ (P < .05 using Duncan post hoc test after analysis of variance to examine group and time differences)
      240 ± 21
      Analysis of variance with repeated measures were used to compare muscle volumes. Letter superscripts are used to label the means. Those with different superscripts differ (P < .05 using Duncan post hoc test after analysis of variance to examine group and time differences)
      .030
      Cesarean227 ± 15
      Analysis of variance with repeated measures were used to compare muscle volumes. Letter superscripts are used to label the means. Those with different superscripts differ (P < .05 using Duncan post hoc test after analysis of variance to examine group and time differences)
      225 ± 26
      Analysis of variance with repeated measures were used to compare muscle volumes. Letter superscripts are used to label the means. Those with different superscripts differ (P < .05 using Duncan post hoc test after analysis of variance to examine group and time differences)
      226 ± 21
      Analysis of variance with repeated measures were used to compare muscle volumes. Letter superscripts are used to label the means. Those with different superscripts differ (P < .05 using Duncan post hoc test after analysis of variance to examine group and time differences)
      Total229 ± 11282 ± 19233 ± 15<.0001
      Interaction term also significant (P < .0001) indicating group by time factor interaction.
      Lindo. Short title: RCT mode of delivery and effects on pelvic floor. Am J Obstet Gynecol 2015.
      a,b,c Analysis of variance with repeated measures were used to compare muscle volumes. Letter superscripts are used to label the means. Those with different superscripts differ (P < .05 using Duncan post hoc test after analysis of variance to examine group and time differences)
      d Interaction term also significant (P < .0001) indicating group by time factor interaction.
      Figure thumbnail gr1
      Figure 1Effect of delivery on COC muscle volume
      Comparison of percentage of coccygeus (COC) muscle volume with high contrast within groups using analysis of variance with repeated measures. Groups differ (P<.0001), times differ (P<.0001) and interactions are significant (P<.0001). Means labelled with different letters, differ (P<.05) using Duncan's post-hoc test. Contrast levels for animals in scheduled primary cesarean delivery (CD) group remained unchanged throughout the study interval.
      Lindo. Short title: RCT mode of delivery and effects on pelvic floor. Am J Obstet Gynecol 2015.
      Measurements in this study (Figure 2) demonstrate that the outlet diameter is increased after completion of pregnancy regardless of mode of delivery.
      Figure thumbnail gr2
      Figure 2Effect of delivery on pelvic outlet width
      Comparisons of diameter of bony outlet dimension. Groups did not differ (P = .50) while values at both postpartum evaluations differed (P < .05) from those prior to pregnancy.
      C-section, cesarean delivery.
      Lindo. Short title: RCT mode of delivery and effects on pelvic floor. Am J Obstet Gynecol 2015.
      The bladder neck was used as a site to evaluate the extent of descent when abdominal pressure is provided for 20 seconds during a dynamic MRI procedure. Measurement of the position of the bladder neck in the 2 groups of females at 3 times is presented in Figure 3. All animals were noted to have descent of the bladder neck, regardless of delivery mode, with no prolapse visualized on exam. Both postpartum examinations of the females with spontaneous vaginal delivery demonstrate a significant descent following delivery. In the animals with CD, the evaluation 3 months after delivery showed a similar descent. Measurements at the time of the postpartum evaluation immediately after delivery in the CD group were obtained without application of abdominal pressure due to recent abdominal surgery and thus were not included in the analysis.
      Figure thumbnail gr3
      Figure 3Effect of delivery on bladder descent
      Comparison of bladder neck position within groups using analysis of variance with repeated measures. Groups did not differ prior to pregnancy (P=.97) or 3 months postpartum (P=.19). Means labelled with different letters, differ (P<.05) using Duncan's posthoc test.
      Lindo. Short title: RCT mode of delivery and effects on pelvic floor. Am J Obstet Gynecol 2015.

      Comment

      Vaginal delivery has been found to be a consistent risk factor associated with pelvic organ prolapse. Several studies evaluating the pelvic floor in its postpartum state have outlined visible changes that occur, some irreversible.
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      Anatomic variations in the levator ani muscle, endopelvic fascia, and urethra in nulliparas evaluated by magnetic resonance imaging.
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      Magnetic resonance imaging of the pelvic floor in the postpartum patient.
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      The appearance of levator ani muscle abnormalities in magnetic resonance images after vaginal delivery.
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      • Miller J.M.
      Pelvic structure and function at 1 month compared to 7 months by dynamic magnetic resonance after vaginal birth.
      Prospective, randomized trials to attribute specific causes of pelvic floor injury have remained difficult to conduct in human beings. Female squirrel monkeys have been found to develop defects in the pelvic floor as a result of risk factors of increased parity and age.
      • Coates K.W.
      • Gibson S.
      • Williams L.E.
      • et al.
      The squirrel monkey as an animal model of pelvic relaxation: an evaluation of a large breeding colony.
      Like human beings, they also deliver large babies through small pelvises. This similarity makes them an excellent species for human comparison of normal progression of pelvic floor changes after pregnancy and parturition. Their structures can be evaluated using similar methods as used in human beings such as MRI and ultrasound, they can be sacrificed for histological and other tissue evaluation, and we can propose and test interventions appropriately.
      • Pierce L.M.
      • Rankin M.R.
      • Foster R.T.
      • et al.
      Distribution and immunohistochemical characterization of primary afferent neurons innervating the levator ani muscle of the female squirrel monkey.
      • Pierce L.M.
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      • Rankin M.R.
      • et al.
      Levator ani muscle and connective tissue changes associated with pelvic organ prolapse, parity, and aging in the squirrel monkey: a histological study.
      Previous studies have shown similar structural framework in the squirrel monkey pelvis including pelvic floor, muscles, fascia, and nerve innervation, revealed by histology and anatomic dissections.
      • Coates K.W.
      • Gibson S.
      • Williams L.E.
      • et al.
      The squirrel monkey as an animal model of pelvic relaxation: an evaluation of a large breeding colony.
      • Kramer L.A.
      • Gendron J.M.
      • Pierce L.M.
      • Runge V.M.
      • Shull B.L.
      • Kuehl T.J.
      Magnetic resonance imaging of the levator ani in the squirrel monkey: a comparison of muscle volume between a cohort with pelvic organ prolapse and matched normals.
      • Stratford R.R.
      • Kuehl T.J.
      • Coates K.W.
      • Thor K.B.
      • Shull B.L.
      • Pierce L.M.
      Evaluation of the squirrel monkey model of pelvic organ prolapse: anatomical and histological comparisons of the pelvic floor between women and squirrel monkeys.
      Within the species OI, LA, and COC muscles are important for pelvic floor support. Evaluations focused on muscle volumes and identified volume changes in the COC muscle associated with pregnancy and delivery.
      • Bracken J.N.
      • Reyes M.
      • Gendron J.M.
      • Pierce L.M.
      • Runge V.M.
      • Kuehl T.J.
      Alterations in pelvic floor muscles and pelvic organ support by pregnancy and vaginal delivery in squirrel monkeys.
      We evaluated a cohort of 7 multiparous squirrel monkeys and found that pregnancy and delivery were not associated with consistent changes in the volume of OI and LA muscle groups.
      • Lindo F.M.
      • Reyes M.
      • Gendron J.M.
      • Runge V.
      • Shull B.
      • Kuehl T.J.
      Differentiating the effect of pregnancy from parturition on the pelvic floor in the squirrel monkey.
      We also found that COC muscle volume in multiparous females began to increase near to term gestation when compared with measurements made prior to pregnancy.
      • Lindo F.M.
      • Reyes M.
      • Gendron J.M.
      • Runge V.
      • Shull B.
      • Kuehl T.J.
      Differentiating the effect of pregnancy from parturition on the pelvic floor in the squirrel monkey.
      We hypothesized that the greatest change would be seen in the COC muscles at the time immediately after spontaneous vaginal delivery with an improvement by 3 months postpartum. The muscle is located in the region of the pelvis between the posterior aspects of the pelvic bones (Figure 4). The paired COC muscles attach to the fused vertebra of the sacrum in an area that has been reported to expand in the pelvic-cephalic dimension during parturition, producing forces that would inherently place excessive stretch to the pelvic portion of the COC muscles. Removing the element of pelvic stretch and dystocia, CD was found to protect the squirrel monkey pelvis from COC muscle injury. We anticipated that scheduled CD without labor would prevent COC injury and the subsequent effects on bladder descent. Those with CD did have preservation of COC muscle, however effects on bladder descent were not prevented. We saw an increase in COC volume and corresponding increase in contrast accumulation consistent with tissue edema
      • Joyce J.S.
      • Dornak S.
      • Gendron J.M.
      • Reyes M.
      • Ruiz J.C.
      • Kuehl T.J.
      Lack of association between pelvic outlet diameter and pelvic organ prolapse in squirrel monkeys.
      in the spontaneous vaginal delivering females. Figure 1 shows the vaginal delivery group had some resolution of the edema, but not back to baseline. We do not yet know the relationship of COC muscle changes to pelvic organ prolapse, however repeated injury due to additional pregnancies could possibly show irreversible damage over time. This will be further examined as we follow our cohort through additional pregnancies. Our findings translate to changes we see in the LA musculature in the human female. In the human parturition process, the shearing forces of the presenting part impact the LA muscles, allowing the pelvis to stretch to accommodate the delivering fetus. The resulting LA avulsion leads to loss of support of the pelvic floor resulting in stress urinary incontinence and prolapse in women.
      • Heilbrun M.
      • Nygaard I.
      • Lockhart M.E.
      • et al.
      Correlation between levator ani muscle injuries on magnetic resonance imaging and fecal incontinence, pelvic organ prolapse, and urinary incontinence in primiparous women.
      This prospective trial shows that avoiding engagement into the pelvis and vaginal delivery prevents this muscle injury (Figure 5).
      Figure thumbnail gr4
      Figure 4Location of COC within squirrel monkey pelvis
      Three-dimensional (3-D) reconstruction of coccygeus (COC) muscle (purple) connections depicts area likely to be injured due to stress and strain during labor and parturition. Located in posterior pelvic rim, on top where muscle connects to bony elements of pelvis and vertebral processes. COC muscle appears to be area where effect of stretching and thinning actually induce edematous change. Anterior pelvis where levator ani (LA) and obturator internus (OI) muscles are in squirrel monkey may be protected by fixed pubis symphysis so that even though there is thinning and elongation or compression of muscle seen, it is not sufficient to produce trauma. Thereby LA and OI muscles are not noted to have any detrimental effects in comparison to COC, which is vulnerable to injury due to widening of pelvis to accommodate fetus that occurs in posterior pelvis.
      dark gray, pelvic bone; green, COC ligament; light gray, sacrum.
      Lindo. Short title: RCT mode of delivery and effects on pelvic floor. Am J Obstet Gynecol 2015.
      Figure thumbnail gr5
      Figure 5Fetal interaction with pelvic structures during delivery
      In squirrel monkeys, coccygeus (COC) muscles, because of location, may be more subject to stretch-induced trauma than levator ani muscle. A, X-ray of squirrel monkey pelvis with fetus in cephalic position. B, Pelvis of squirrel monkey. Pubic symphysis (PS) is fixed, resulting in accommodation of fetal head by expansion of posterior pelvis. C, Squirrel monkeys deliver in mentum anterior presentation resulting in affected cephalic end of COC muscles.
      Lindo. Short title: RCT mode of delivery and effects on pelvic floor. Am J Obstet Gynecol 2015.
      A second outcome of pregnancy and parturition in the nulliparous squirrel monkey is postpartum descent of the bladder neck, which is most prominent 3 months postpartum.
      • Lindo F.M.
      • Reyes M.
      • Gendron J.M.
      • Runge V.
      • Shull B.
      • Kuehl T.J.
      Differentiating the effect of pregnancy from parturition on the pelvic floor in the squirrel monkey.
      CD did not improve this change, demonstrating that pregnancy itself has measurable effects on the pelvic support structures that cannot be prevented by elective primary CD. This is an important factor to consider when making decisions for mode of delivery in women.
      It is interesting to note that the other muscles in the squirrel monkey pelvic floor LA and OI were similarly affected in both the vaginal delivery and CD group. The muscles were found to decrease in volume. We are unsure if this was due to stretch or atrophy. The COC muscle is the area where effects of stretch actually induce edematous change. The LA and OI muscles may be protected by the fixed symphysis so the stretch that occurs is not sufficient to produce trauma. We can hypothesize that the pregnancy may lead to decrease in muscle size and predispose the pelvic floor to injury. While pregnant, the animals are not moving or jumping as much as in the nonpregnant state. The decreased activity may limit blood flow and muscle building during pregnancy resulting in atrophy. These muscles volumes were found to return to baseline at 3 months postpartum in both groups, when the animals are back to their normal activity level. Another proposed theory is that as the fetus grows and takes up space in the pelvis there is a thinning and stretching of the muscles, allowing more room in the pelvis to accommodate the large fetus. If this was the case, we may have been unable to capture the full muscle volume because of inability to visualize it during our tracing due to the excessive thinning of the muscle.
      The pelvic floor muscles, bony connections, and connective tissue provide support for the pelvic organs. The pelvic outlet as measured in the axial view has been shown to be related to dystocia and stillbirth in the squirrel monkey and previously reported to increase after vaginal delivery.
      • Lindo F.M.
      • Reyes M.
      • Gendron J.M.
      • Runge V.
      • Shull B.
      • Kuehl T.J.
      Differentiating the effect of pregnancy from parturition on the pelvic floor in the squirrel monkey.
      Similarly, architectural differences in the bony pelvis of women with pelvic floor disorders have been found to include a wide transverse inlet and narrow obstetrical conjugate.
      • Handa V.L.
      • Pannu H.K.
      • Siddique S.
      • Gutman R.
      • VanRooyen J.
      • Cundiff G.
      Architectural differences in the bony pelvis of women with and without pelvic floor disorders.
      This study shows that regardless of mode of delivery, the measurements of the outlet diameter increased after pregnancy. It has been thought that the pelvic outlet is affected by the delivery of the fetus through the birth canal. However, we have seen a widening of this outlet, which is likely due to the laxity of the connective tissue occurring during pregnancy prior to any delivery process. The increase in the outlet diameter regardless of delivery mode implies that this change is more of a characteristic of pregnancy and not trauma to the birth canal.
      There are 2 important factors identified from our study. There may be direct insults to the muscles that are caused by application of shearing forces of the fetal head during vaginal delivery that could be prevented by CD, however risks associated with surgery must be considered. Secondly, there is evidence that connective tissue, supportive ligaments, and structures are affected by pregnancy, and are not protected by CD. We are not sure which of these factors contribute to the course of prolapse development. Pregnancy in conjunction with vaginal delivery may cause an additive effect on the damage to the pelvic floor resulting in disorders developing over time. However pregnancy alone with CD may also lead to the development of prolapse over time.
      Historically many studies have been done quantifying the changes in the pelvic floor in postpartum women using various imaging methods.
      • Heilbrun M.
      • Nygaard I.
      • Lockhart M.E.
      • et al.
      Correlation between levator ani muscle injuries on magnetic resonance imaging and fecal incontinence, pelvic organ prolapse, and urinary incontinence in primiparous women.
      • Rizk D.E.
      • Czechowski J.
      • Ekelund L.
      Dynamic assessment of pelvic floor and bony pelvis morphologic condition with the use of magnetic resonance imaging in a multiethnic, nulliparous, and healthy female population.
      These studies looked at the differences in the pelvic floor during nulliparous and postpartum states using matched controls for comparison, which results in confounders in data.
      • Hoyte L.
      • Schierlitz L.
      • Zou K.H.
      • Flesh G.
      • Fielding J.R.
      Two- and 3-dimensional MRI comparison of levator ani structure, volume, and integrity in women with stress incontinence and prolapse.
      • DeLancey J.O.
      • Sorensen H.C.
      • Lewicky-Gaupp C.
      • Smith T.M.
      Comparison of the puborectal muscle on MRI in women with POP and levator ani defects with those with normal support and no defect.
      There have not been any MRI studies of evaluating the pelvic floor muscles in a nulliparous female that also followed the same subjects through pregnancy and parturition. Also, prospective, randomized controlled trial of elective primary CD have not been reported.
      Strengths of this study include the ability to compare muscles at several time intervals within each subject and to randomize animals to either treatment group, which decreased selection bias and eliminated various confounders. Randomization of human subjects to vaginal delivery or CD is inappropriate. Therefore, using a species that develops the same pathologic process is a constructive way to answer questions of the progression of pelvic floor muscle insults in a female human pelvis. Being able to evaluate the same muscle over linear time points in the same animal improves accuracy of the measurements because any changes due to environmental factors would remain consistent in the individual animal. We have established a reliable evaluation of female squirrel monkeys as an appropriate comparison for the human female pelvis. Even though there are differences in human pelvic floor anatomy compared to that of the squirrel monkey species,
      • Coates K.W.
      • Galan H.L.
      • Shull B.L.
      • Kuehl T.J.
      The squirrel monkey: an animal model of pelvic relaxation.
      the similarities in the development of pelvic floor dysfunction allow us to observe the progression of pelvic floor disorders and document changes that occur over time. Our findings may be extrapolated to that of the human species.
      Our study identifies significant changes to the pelvic floor muscles as a result of vaginal delivery that can be prevented by elective primary CD in this animal model. However, we also observed that loss of support associated with pregnancy and vaginal delivery is not prevented.

      Acknowledgment

      The authors thank Dr Lawrence Williams for project oversight and Bethany Brock for technical assistance with ultrasound evaluations of pregnancy, transport of animals for magnetic resonance imaging, and infant care at the MD Anderson animal facility. Authors also thank Dr Christina Du and Angie Hitt for animal care and project assistance at the Scott and White Healthcare animal facility.

      Supplementary Data

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