Additional outcomes of interest included the anteroposterior diameter of the hiatus, area of the hiatus, and change in hiatal area from rest to pelvic floor muscle contraction and from rest to Valsalva. Anteroposterior hiatal diameter was measured as the shortest distance from the posteroinferior margin of the symphysis pubis to the rectal sling in the midsagittal plane at rest, Valsalva, and pelvic floor muscle contraction.14 We measured hiatal area at the plane of minimal hiatal dimension on rest, Valsalva, and pelvic floor muscle contraction volumes. The plane of minimal hiatal dimension is defined as the minimal distance between the hyperechoic posterior aspect of the pubic symphysis and the hyperechoic anterior margin of the levator ani muscle just behind the anorectal angle in midsagittal plane.15,16 The change in hiatal area from rest to pelvic floor muscle contraction was calculated by subtracting area at pelvic floor muscle contraction from area at rest. The change in hiatal area from rest to Valsalva was calculated by subtracting area at rest from area at Valsalva.
To improve quality control with the three-dimensional transperineal ultrasonography, we randomly selected 15 women from 192 participants of the Mothers Outcomes After Delivery study who delivered exclusively by cesarean without prior labor. These women served as negative control participants in the ultrasound protocol because they were not expected to have any levator trauma. Additionally, their inclusion blinded the investigators performing and interpreting the ultrasound volumes to women's obstetric history.
The primary analysis was a comparison of levator ani muscle avulsion between women with forceps-assisted delivery and women with vacuum-assisted delivery. Sample size was calculated for this aim. Prior publications suggest that incident levator ani muscle injury occurs in 50–65% of women after forceps-assisted vaginal delivery.10 The incidence of levator injury after a vacuum delivery is not as well established but we anticipated that 10–20% of women with a history of vacuum delivery would have a levator injury.9,10 For our power calculations, we assumed a type I error probability of .05. We also anticipated that participation would be similar in the two groups, and, therefore, the ratio of forceps to vacuum participants would be 7:5. Based on these assumptions, we calculated a sample size of 96 (56 women in the forceps delivery group and 40 women in the vacuum delivery group) for the purpose of having 80% power to reject the null hypothesis that there is no difference in the prevalence of levator ani muscle avulsion injuries between the two groups. We evaluated differences between the two delivery groups in secondary outcomes of hiatal dimensions as well as demographic and obstetric characteristics using Wilcoxon rank-sum test for continuous variables and Fisher exact test for categorical variables. Pelvic floor disorder outcomes were also compared between the two delivery groups using χ2 conditional exact test. A P value threshold of .05 was used for inference testing. All analysis was performed using SAS 9.3.
We identified a subset of 127 women from the participants of Mothers Outcomes After Delivery study who had history of either forceps or vacuum-assisted vaginal delivery but not both types of operative vaginal deliveries. Five of these women were excluded because the history of operative vaginal delivery was reported by the participant but could not be verified from the medical records. Two additional women were excluded because they were pregnant at the time of ultrasound study. A total of 120 women met our eligibility criteria and were offered participation. Among these eligible participants, a total of 75 women agreed to participate and underwent three-dimensional transperineal ultrasonograms. Two of these 75 participants were excluded from the final analysis as a result of missing or uninterpretable volumes. Thus, 73 women were included in the final analysis. This included 45 women with a forceps delivery history and 28 women with a vacuum delivery history. There were no differences in the demographic characteristics or obstetric factors of women who were included in the final analysis (n=73) and those who were not included (n=47), as shown in Appendix 1. Among 15 randomly selected women in the unlabored cesarean delivery group, 10 women agreed to participate.
Women in the forceps and vacuum delivery groups were comparable in most demographic factors, as summarized in Table 1. Women with a history of forceps delivery were slightly older at their index delivery as compared with women with a history of vacuum delivery, although this difference was not statistically significant. Both delivery groups had a similar time interval from index delivery to the ultrasound study (median interval 10.3 compared with 9.8 years, P=.73). Three participants experienced two operative vaginal deliveries.
In terms of the obstetric characteristics, the majority of women in both groups were multiparous. More women in the forceps delivery group had a history of a prolonged second stage of labor compared with women in the vacuum delivery group, but this difference was not statistically significant. There was no significant difference in the rates of episiotomy between the two groups. Most episiotomies performed at the time of operative delivery (36/48) were midline with no difference in episiotomy type between groups (P=.29). The only statistically significant difference between the two vaginal delivery groups was in history of anal sphincter laceration, which was more common among women in the forceps-assisted delivery group compared with women in the vacuum delivery group (53% compared with 18%, P=.006).
Substantial differences were observed in the proportion of women with levator trauma in the forceps compared with vacuum groups. We identified levator avulsions among 22 of 45 women (49%) who had undergone forceps delivery compared with 5 of 28 who had undergone vacuum delivery (18%; P=.012). Thus, the prevalence ratio was 2.74 (95% confidence interval [CI] 1.17–6.40) and the odds ratio was 4.40 (95% CI 1.42–13.62). Among the 10 unlabored cesarean delivery women serving as negative control participants, nine had interpretable ultrasound volumes, of which none were found to have levator injury.
Women in the forceps group also had wider hiatal areas and larger anteroposterior hiatal diameters at rest, squeeze, and Valsalva (Table 2). These women also had a larger change in the hiatal area from rest to Valsalva, indicating greater ballooning of the hiatus with Valsalva after a forceps delivery. Additionally, we observed less change in hiatal area from rest to squeeze among women with a history of forceps delivery, indicating less closure of the levator hiatus with voluntary levator contraction.
Although the prevalence of anal sphincter laceration (53%) and the levator ani muscle avulsion (49%) was similar among women in the forceps delivery group, these were not the same women. Only 11 of 24 (46%) women in the forceps group with a history of anal sphincter laceration had evidence of levator avulsion. In comparison, among 21 women in the forceps group with no history of anal sphincter laceration, 11 (52%) had levator avulsion. We did not find a statistically significant association between anal sphincter laceration and levator ani muscle avulsion (Fisher exact test, P=.77). In addition, on multivariate analysis, the association between levator ani muscle avulsion and forceps delivery did not change after adjusting for history of anal sphincter laceration. Of note, our analysis was not adequately powered to study the association between levator avulsion and anal sphincter laceration.
Table 3 summarizes the prevalence of pelvic floor disorders between women with levator ani muscle avulsion injury and those without levator ani after adjusting for the type of operative vaginal delivery. As shown in Table 3, irrespective of the delivery group, women with levator ani muscle avulsion injuries tended to have a higher prevalence of pelvic floor disorders (especially stress incontinence and prolapse) compared with women without levator ani muscle avulsion injuries. However, the only statistically significant difference found was for prolapse symptoms, which were significantly more common among women with levator avulsion (P=.036).
We observed a significant difference in the prevalence of levator avulsion between the forceps and vacuum delivery groups 10 years after operative vaginal birth. Other investigators have reported similar findings among women evaluated in the first year after delivery. Kearney et al10 reported levator muscle injury in 6 of 18 women 9–12 months after forceps birth compared with 2 of 12 after vacuum birth. Similarly, levator avulsions were more common at 4 months postpartum among Australian women who had forceps delivery compared with women who had vacuum delivery (7/20 compared with 3/34, P=.017).9 Finally, 8 weeks after delivery, levator avulsions were significantly more common among Chinese women delivered by forceps (16/48) compared with vacuum (10/14).8 Our study not only supports the findings of these studies, but it also provides evidence for a persistent effect of forceps delivery a decade after the operative vaginal birth. Although the natural history of levator avulsion after childbirth is not well known, it has been suggested that some avulsions resolve over time.17,18 Our study provides evidence for a persistent effect of forceps delivery on levator ani muscles.
We also identified differences in levator function between the groups. Women in the forceps group had a wider levator hiatus, a smaller decrease in hiatal area with pelvic floor contraction, and greater widening of the hiatus area with Valsalva. These findings suggest a decreased ability to close the hiatus during a levator contraction and an inability of the avulsed levator muscle to maintain hiatal dimensions with increased abdominal pressure.
We found that women with levator ani muscle avulsion were significantly more likely to report prolapse symptoms, independent of delivery type. Other pelvic floor conditions were marginally more common among women with levator ani avulsion, but differences were not statistically significant. Our findings raise the question of whether levator injury could explain the known association between forceps and pelvic floor disorders.2–4 However, this study was not adequately powered to test this hypothesis. Our results argue in favor of a study with a sufficient sample size to simultaneously investigate the effect of operative delivery and levator avulsion on the development of pelvic floor disorders.
There are several strengths of this study. The investigators were blinded to patients' obstetric exposures. We used well-defined and validated techniques for assessment of levator avulsion. Another strength is that only those participants whose operative vaginal delivery was confirmed from medical records were included in the study.
Although there was no difference between the forceps and vacuum delivery groups in terms of several potential confounding obstetric and demographic factors, the effects of unmeasured confounders cannot be eliminated as a result of the observational nature of this study. For example, the obstetrician's choice for forceps compared with vacuum may reflect unmeasured factors that could contribute to the higher proportion of levator injury after forceps delivery. This issue could hypothetically be addressed by conducting a randomized control study, but such study is unlikely to be feasible. Also, our study population was relatively small; thus, although the prevalence ratio of levator avulsion associated with forceps delivery is considerably greater than 1.0, the 95% CI is wide.
The overall rate of operative vaginal delivery has diminished in United States over the past two decades (with a parallel increase in the rate of primary cesarean delivery).19 However, in a recent statement, the American College of Obstetricians and Gynecologists recognized operative vaginal delivery as a safe practice that could potentially reduce primary cesarean deliveries.20 We believe that our research thus provides timely and clinically relevant information as obstetricians endeavor to use operative vaginal delivery as a strategy to reduce cesarean delivery during the second stage of labor. The relative increase in levator avulsion after forceps compared with vacuum delivery and the suggestion of an association between levator ani injury and pelvic floor disorders in this setting provides evidence that vacuum may be a safer alternative to forceps. However, pelvic floor injury is only one important outcome; other maternal and neonatal factors should be considered in this important decision-making.
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Demographic and Obstetric Characteristics of 120 Eligible Participants by Inclusion Status
© 2015 by The American College of Obstetricians and Gynecologists. Published by Wolters Kluwer Health, Inc. All rights reserved.