Volume 39, Issue 6 p. 704-709
Original Paper
Free Access

Prevalence of levator ani muscle injury in Chinese women after first delivery

S. S. C. Chan

Corresponding Author

S. S. C. Chan

Department of Obstetrics & Gynaecology, Faculty of Medicine, The Chinese University of Hong Kong, Hong Kong

Department of Obstetrics & Gynaecology, Prince of Wales Hospital, 30-32 Ngan Shing Street, Shatin, New Territories, Hong KongSearch for more papers by this author
R. Y. K. Cheung

R. Y. K. Cheung

Department of Obstetrics & Gynaecology, Faculty of Medicine, The Chinese University of Hong Kong, Hong Kong

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A. K. W. Yiu

A. K. W. Yiu

Department of Obstetrics & Gynaecology, Faculty of Medicine, The Chinese University of Hong Kong, Hong Kong

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L. L. L. Lee

L. L. L. Lee

Department of Obstetrics & Gynaecology, Faculty of Medicine, The Chinese University of Hong Kong, Hong Kong

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A. W. L. Pang

A. W. L. Pang

Department of Obstetrics & Gynaecology, Faculty of Medicine, The Chinese University of Hong Kong, Hong Kong

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K. W. Choy

K. W. Choy

Department of Obstetrics & Gynaecology, Faculty of Medicine, The Chinese University of Hong Kong, Hong Kong

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T. Y. Leung

T. Y. Leung

Department of Obstetrics & Gynaecology, Faculty of Medicine, The Chinese University of Hong Kong, Hong Kong

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T. K. H. Chung

T. K. H. Chung

Department of Obstetrics & Gynaecology, Faculty of Medicine, The Chinese University of Hong Kong, Hong Kong

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First published: 01 November 2011
Citations: 82

Abstract

Objectives

To assess the prevalence of levator ani muscle injury in Chinese women after their first delivery and investigate associated factors.

Methods

A prospective observational study was conducted involving Chinese nulliparous women recruited in the first trimester of pregnancy. Translabial ultrasound was performed at 35–38 weeks' gestation and 8 weeks postpartum and three-dimensional volume datasets were obtained. Offline analysis to detect levator ani muscle injury was performed by investigators blinded to the delivery details.

Results

339 women, with a mean age of 30.6 ± 3.9 years, completed the study. Overall, 201 (59.3%) had a spontaneous vaginal delivery, 62 (18.3%) an operative vaginal delivery (48 ventouse extraction and 14 forceps delivery), 14 (4.1%) an elective Cesarean section and 62 (18.3%) an emergency Cesarean section. No levator ani muscle injury was detected in any woman antenatally. After vaginal delivery, 57 (21.7% (95% CI, 16.7–26.7%)) women had levator ani muscle injury. The rates of injury for spontaneous vaginal delivery, ventouse extraction and forceps delivery were 15.4%, 33.3% and 71.4%, respectively. There was no levator ani muscle injury in the Cesarean section groups. Logistic regression analysis showed that only operative vaginal delivery (odds ratio, 3.09) was associated with an independent increase in the likelihood of levator ani muscle injury. Intrapartum epidural analgesics, duration of second stage of labor and infant birth weight were not independently associated with levator ani muscle injury.

Conclusions

The prevalence of levator ani muscle injury in Chinese women after their first vaginal delivery was 21.7% (95% CI, 16.7–26.7%). Operative vaginal delivery was found to increase the likelihood of women suffering such injury. A longer follow-up of these women and future studies on the effects of episiotomy are proposed. Copyright © 2012 ISUOG. Published by John Wiley & Sons, Ltd.

Introduction

Levator ani muscle injury has been identified as a risk factor for pelvic organ prolapse and urinary incontinence in women1-4, and is reported in 13–36% of Caucasian women after their first vaginal delivery5, 6. Instrumental delivery, especially forceps delivery, is associated with a higher risk7. In addition, levator function has been found to be primarily affected by the length of the second stage of labor, indicating an effect caused by full engagement of the fetal head but not childbirth itself8.

There have been few studies with adequate sample size to evaluate the prevalence of levator ani muscle injury in women after their first delivery. Some studies used magnetic resonance imaging as an assessment tool but these studies were hampered by small sample size, probably because of the cost of the equipment5-7, 9. In addition, most of the study subjects were Caucasian and whether these findings are generalizable to other ethnic groups is unknown. It has been shown that connective tissue quality varies between populations, for example between Chinese and Caucasian women10, 11. This ethnic difference has recently been supported by a study that showed that Asian women have significantly less pelvic organ mobility, both antenatally and postnatally, than do Caucasians. However, this study had only a few Asian subjects12. There is also considerable variation in obstetric practice between institutions nationally and internationally, e.g. differing rates of Cesarean section, use of intrapartum epidural analgesia and use of episiotomy. In primiparous women delivering vaginally, episiotomy rates can range from 20% to more than 70%6, 13, 14. All these are potential factors that may influence the prevalence of levator ani muscle injury following childbirth.

Three-dimensional (3D) translabial ultrasound has been established as a method for assessing pelvic floor anatomy as well as the levator ani muscle. It is non-invasive and well tolerated. The safety of ultrasonography in both women and fetuses in pregnancy has also been confirmed in a recent meta-analysis15.

The aim of this study was to use translabial ultrasound to assess the prevalence of levator ani muscle injury in Chinese women after their first delivery. Factors that may affect levator ani muscle injury, including different obstetric practices, were also studied.

Methods

This was a prospective observational study that was part of a larger study on pelvic floor symptoms during and after pregnancy. Women were recruited in a tertiary obstetric unit during the first trimester of pregnancy (9–12 weeks' gestation) from August 2009 to August 2010. They were assessed in each trimester, i.e. at 9–12, 26–28 and 35–38 weeks. Only the assessment at 35–38 weeks was used in this study. Inclusion criteria were being Chinese, ≥ 18 years, nulliparous with a singleton pregnancy, history negative for urinary or fecal incontinence, no symptoms of prolapse prior to pregnancy. Written informed consent was obtained and the study was approved by the institutional ethics committee for research on human subjects (CRE-2009.257). Demographic data and antenatal history were obtained and body mass index (BMI) evaluated.

A standard translabial ultrasound scan was performed to assess pelvic floor anatomy by one of two investigators (L.L. or A.P.). A GE Voluson 730 3D ultrasound system (GE Medical Systems, Zipf, Austria) with a 4–8-MHz 3D autosweep transducer was used for image acquisition. The transducer was placed on the perineum in a mid-sagittal plane with the woman in the supine position immediately after voiding. 3D ultrasound scans of the pelvic floor anatomy, with a sweep angle of 85°, were obtained at rest and at pelvic floor muscle contraction16. Women were asked to perform pelvic floor muscle contractions until a satisfactory contraction had been achieved. At most three pelvic floor contractions were required, the most effective contraction being used for evaluation. All women were able to perform pelvic floor muscle contraction satisfactorily. The volume datasets were saved and analyzed in a standard way.

The women were managed obstetrically according to standard departmental protocols. Data on the delivery were obtained from the medical records when the women were discharged. Mode of delivery was classified as spontaneous vaginal, operative vaginal involving either ventouse extraction or forceps and elective or emergency Cesarean section. The women were assessed at 8 weeks postpartum using translabial ultrasonography in the same manner as they were antenatally. All the translabial ultrasound volume datasets were analyzed independently by two investigators (S.C. and R.C.) who were blinded to the delivery information. Analysis of the ultrasound data was done offline, remote from the actual examination.

The levator ani muscle was assessed using tomographic ultrasound imaging on volumes obtained at maximal pelvic floor contraction at 2.5-mm slice intervals, from 5 mm below to 12.5 mm above the plane of minimal hiatal dimensions, which is the horizontal line between the pubic symphysis and the anorectal junction in the mid-sagittal plane17. Following this, measurements of the levator–urethral gap were made by placing calipers in the center of the hypoechogenic structure that indicates the urethra and on the most medial aspect of the muscle insertion. Levator ani muscle injury was defined according to the minimal criteria for complete avulsion of the levator ani muscle, using a cut-off of 23.65 mm for the levator–urethral gap in at least three central slices obtained at the level of minimal hiatal dimension and at 2.5 and 5 mm above18, 19 (Figure 1). A cut-off of 23.65 mm for the levator–urethral gap has been shown to have 92% sensitivity and 95% specificity for diagnosing levator avulsion injury in Chinese women suffering from pelvic organ prolapse19. Good reproducibility of the sonographic diagnosis of levator ani muscle injury (kappa ≥ 0.7) has been demonstrated1, 19, 20.

Details are in the caption following the image

Tomographic ultrasound imaging using a volume obtained at maximal pelvic floor contraction, with 2.5-mm slice intervals from 5 mm below to 12.5 mm above the plane of minimal hiatal dimensions, showing right levator ani muscle injury (*) in a woman 8 weeks postpartum.

Assuming the prevalence of levator ani muscle injury is 20% in vaginal delivery, with an accepted error of 5%, means that 236 subjects would be required for the study to be statistically significant. In addition, we estimated that only 75% of the subjects would achieve a vaginal delivery and that there would be a drop-out rate of 20%, implying that in all we would need to recruit 390 subjects for the study.

Statistical analysis

Descriptive analysis was used to study the prevalence of levator ani muscle injury after delivery. Indices measured in two groups of subjects were compared using Student's unpaired two-tailed t-test. For comparison of frequencies, the chi-square test or two-sided Fisher exact test was used where appropriate. Logistic regression analysis was performed to assess the impact of delivery factors on the likelihood of levator ani muscle injury, and P < 0.05 was considered to be statistically significant.

Results

A total of 398 women were recruited during the first trimester and 339 (85.2%) completed the study; mean postnatal follow-up was 8.5 (range, 7.2–10.8) weeks after delivery. A total of 59 (14.8%) women withdrew from the study: one suffered a second-trimester miscarriage, 26 did not attend postnatal follow-up and 32 were delivered in other hospitals. The mean age of those who completed the study was 30.6 ± 3.9 years and mean BMI in the first trimester was 21.1 ± 2.83 (range, 15.5–34.1) kg/m2. There was no difference between those who completed the study and those who dropped out of the study in relation to maternal age, BMI, gestational age at delivery, mode of delivery, birth weight and head circumference.

Overall, 201 women (59.3%) had a spontaneous vaginal delivery, 62 (18.3%) an operative vaginal delivery (48 ventouse and 14 forceps), 14 (4.1%) an elective Cesarean section and 62 (18.3%) an emergency Cesarean section. The indications for operative vaginal delivery and Cesarean section are listed in Tables 1 and 2. Three of the six women who suffered arrest of labor in the first stage did so at a cervical dilatation of 8 cm, with the presenting part at the level of the ischial spines. Two women had arrest of labor during the second stage; both were delivered by Cesarean section without any attempt at operative vaginal delivery. The mean gestational age at delivery was 39.2 ± 1.9 weeks and mean birth weight was 3070 ± 460 g. Intrapartum epidural analgesia was used by 26 (9.9%) of those having a vaginal delivery and a left mediolateral episiotomy was performed in 191 (95.0%) women who underwent spontaneous vaginal delivery and all women who underwent operative vaginal delivery.

Table 1. Indications for operative vaginal delivery in the study population
Operative vaginal delivery (n (%))
Indication Ventouse extraction (n = 48) Forceps delivery (n = 14)
Prolonged second stage of labor 27 (56.2) 7 (50.0)
Fetal distress 20 (41.7) 6 (42.9)
Chorioamnionitis 1 (2.1)
Maternal indication (convulsions) 1 (7.1)
Table 2. Indications for Cesarean section in the study population
Cesarean section (n (%))
Indication Elective (n = 14) Emergency (n = 62)
Malpresentation 7 (50.0) 3 (4.8)
Macrosomia 2 (14.3) 1 (1.6)
Placenta previa 1 (7.1) 1 (1.6)
Unstable lie (n = 1)/high head at term (n = 2) 3 (21.4)
Maternal cardiac disease 1 (7.1)
Chorioamnionitis 18 (29.0)
Failed induction 14 (22.6)
Fetal distress 13 (21.0)
Arrest of labor at first stage 6 (9.7)
Arrest of labor at second stage 2 (3.2)
Severe pre-eclampsia 3 (4.8)
Placental abruption 1 (1.6)

Levator ani muscle injury was not detected in any woman antenatally. After delivery, levator ani muscle injury was detected in 57/263 women (21.7% (95% CI, 16.7–26.7%)) in the vaginal delivery group. The rates of levator ani injury for those who had a spontaneous vaginal delivery, ventouse extraction and forceps delivery were 31/201 (15.4%), 16/48 (33.3%) and 10/14 (71.4%), respectively.

The indications for operative vaginal delivery were not associated with levator ani muscle injury (13/34 of those performed for a prolonged second stage, P = 0.52; 11/26 for fetal distress, P = 0.96). No women who underwent a Cesarean section, including emergency Cesarean section for arrest of labor in the first or second stage, suffered levator ani muscle injury. Among those with levator ani muscle injury, 36 were unilateral and 21 bilateral. Of the group with unilateral injury, 21 (58.3%) were on the right side and 15 (41.7%) were on the left side. All 10 women who did not have an episiotomy had no levator ani muscle injury.

Univariable analysis of factors possibly associated with levator ani muscle injury in women who delivered vaginally is shown in Table 3. Direct logistic regression analysis was performed to assess the impact on the likelihood of levator ani muscle injury of five factors, in combination, that were found to be significant on univariable analysis. The full model was statistically significant (n = 263, χ2 (5) = 26.8, P < 0.001), indicating that the model was able to distinguish between women who had levator ani muscle injury and those who did not. Only operative vaginal delivery had an independent statistically significant contribution to the model, with an odds ratio (OR) of 3.09 (Table 4).

Table 3. Maternal and delivery characteristics according to presence or absence of postpartum levator ani muscle injury in Chinese women delivered vaginally
Parameter Levator ani muscle defect (n = 57) No levator ani muscle defect (n = 206) P
Maternal age (years) 30.2 ± 3.7 30.3 ± 3.7 0.82
Maternal body mass index, first trimester (kg/m2) 20.4 ± 2.2 21.0 ± 2.8 0.16
Maternal body mass index, third trimester (kg/m2) 25.3 ± 2.7 25.8 ± 2.9 0.28
Maternal height (m) 1.60 ± 0.1 1.58 ± 0.1 0.11
Gestational age at delivery (weeks) 39.7 ± 1.2 39.2 ± 1.5 0.04
Occipitoposterior position 6 (10.5) 18 (8.7) 0.22
Intrapartum epidural analgesia 8 (14.0) 18 (8.7) 0.24
Operative vaginal delivery 26 (45.6) 36 (17.5) < 0.001
Forceps delivery 10 (17.5) 4 (1.9) < 0.001
Ventouse extraction 16 (28.1) 32 (15.5) 0.03
Duration of second stage of labor (min) 51.5 ± 36.4 32.8 ± 29.1 < 0.001
Duration of active second stage of labor (min) 44.4 ± 29.5 31.2 ± 26.4 < 0.001
Birth weight (g) 3168 ± 331 3041 ± 421 0.04
Infant head circumference (cm)* 33.6 ± 1.5 33.1 ± 1.4 0.01
  • Data are given as mean ± SD or n (%).
  • * n = 220 only.
Table 4. Multivariable logistic regression in the prediction of postpartum levator ani muscle injury in Chinese women delivered vaginally
Parameter Odds ratio (95% CI) P
Operative vaginal delivery 3.09 (1.46–6.52) 0.003
Birth weight (kg) 1.43 (0.55–3.74) 0.47
Gestational age at delivery (days) 1.02 (0.98–1.07) 0.28
Duration of second stage of labor (min) 1.02 (1.00–1.04) 0.05
Duration of active second stage of labor (min) 0.99 (0.96–1.01) 0.20

Discussion

Levator ani muscle injury is prevalent in urogynecological patients1. Women suffering from such an injury have a higher risk of developing pelvic organ prolapse2, and there is ample evidence that levator ani muscle injury occurs in childbirth5-7. Studying the prevalence of this injury and the risk factors in women after their first delivery is therefore important.

This prospective observational study had an adequate sample size. There was no statistical difference in the clinical characteristics of the women who completed the study compared to those who defaulted on follow-up, and the default rate was lower than reported in previous studies6. Moreover, the women's age and BMI, use of intrapartum epidural analgesia, mode of delivery and episiotomy in this study were comparable with those in the background population for which our department provides obstetric services, as reported in audit reports and previous publications13, 21. This means that the data obtained in the study group were unlikely to be biased and could be generalized to Chinese women delivering in institutions with similar obstetric practice.

Overall, 21.7% (95% CI, 16.7–26.7%) of the women participating in the study had levator ani muscle injury after their first vaginal delivery. This was comparable with the results of previous studies, although the obstetric practices were different5, 6. Gestational age at delivery, operative vaginal delivery, duration of second stage and active second stage of labor, infant birth weight and head circumference were found to be associated with levator ani muscle injury. However, only operative vaginal delivery—with an OR of 3.09—was found to be significant when factors were analyzed in combination in a multivariable model. The prevalence of levator ani muscle injury in forceps delivery has previously been reported to be between 35 and 52.6%6, 7, with no apparent difference between forceps delivery for fetal distress or second stage arrest7. Our study also found no difference in rates of levator ani muscle injury between these two indications for operative vaginal delivery. In our study, levator ani muscle injury was significantly more likely after ventouse extraction and the risk of injury was greater again with forceps delivery (P = 0.01). This implies that operative vaginal delivery is an independent risk factor for levator ani muscle injury and that the risk is particularly high for forceps delivery. Forceps delivery has been reported in the past to cause more trauma to the vagina than does ventouse extraction22. It would appear that the same is true of the pelvic floor.

Compared to Western practice, there are two major differences in obstetric practice in our population. The intrapartum epidural analgesia rate was low and the episiotomy rate was high. Intrapartum epidural analgesia has been found to have a protective effect on levator ani muscle injury6. This could be one of the factors contributing to a significantly higher risk of injury (22% in the current study vs. 13% in a similar study6, P = 0.01). The effect of episiotomy on levator ani muscle injury remains unclear. Shek and Dietz6, studying a predominantly Caucasian population, found it to be associated with an increased risk of levator ani muscle injury on univariable analysis but not on multivariable analysis. In the current study there was a very high episiotomy rate, and these episiotomies were done at the discretion of the attending midwife or doctor. Only 10 women who delivered vaginally did not have an episiotomy. This made evaluation of the effect of episiotomy on levator ani muscle injury impossible in our current study. Our reluctance to change practice is informed by the extremely low prevalence (0.03%) of third-degree tears in our population, which is associated with a very high episiotomy rate13, 23. However, in women with unilateral levator ani muscle injury, more occurred on the right side even though all episiotomies were done on the left side. Whether this is a chance finding requires further study.

The comparative rarity of genital prolapse in Chinese immigrant women noted by gynecologists in Western countries has been observed for some decades. This prompted Zacharin10 to conduct a cadaver study in Chinese women in an Asian location, and he found stronger pelvic support structures among this group. Similarly, urinary incontinence has been reported to be rare in Asian women, although this has been at least partially refuted in more recent studies24, 25. Whether this is due to racial differences or differences in lifestyle is uncertain. However, Dietz12 also demonstrated that Asian women have less mobility of the anterior and posterior vaginal compartments than do Caucasians, both antenatally and postnatally. Ideally, to elucidate the question of ethnicity as a factor in pelvic floor injury during childbirth, a multi-ethnic study of women having a similar lifestyle and who were cared for obstetrically by similar protocols would be needed. We are not in a position to do this study because only a small proportion of our patient population is non-Asian.

The long-term outcomes and implications of levator ani muscle injury are still unclear, although identifiable injury was found to reduce from 48.9% at 6 weeks postpartum to 17.8% at 6 months postpartum in one study of 57 subjects9. In addition, only one parameter of pelvic floor assessment was used in this study and the influence of other factors remains to be elucidated.

In conclusion, the prevalence of levator ani muscle injury in Chinese primiparous women after vaginal delivery was 21.7% (95% CI, 16.7–26.7%). Operative vaginal delivery, especially forceps delivery, was found to increase the likelihood of women having such trauma. A longer follow-up of these women and future studies of the effects of episiotomy are indicated. It is noteworthy that in a population with a very low prevalence of third-degree perineal tear following childbirth, the prevalence of levator ani muscle injury appears to be very similar to that in populations with much higher rates of tears13, 23.

Acknowledgements

This study obtained a grant from the Health and Health Services Research Fund from the Food and Health Bureau of Hong Kong SAR.