The prevalence of anal incontinence has been estimated at 4–17% among adults. The prevalence increases by age. More women than men suffer from anal incontinence.1–3 The principal cause of anal incontinence in women is obstetric trauma. Such trauma may be caused by mechanical damage, impairment of the innervations of anal sphincter muscles, or both.4–7 Mechanical damage of the anal sphincter muscles is assumed to be the most important risk factor for anal incontinence because reinnervation of the pelvic floor has been shown to occur in up to 80% of women with innervation impairment after delivery.4,6,8–10 There is also some evidence of subtle nerve injury to the anal sphincter after vaginal delivery. Such injury may accelerate the normal impact of aging and may therefore explain the appearance of anal incontinence many years after delivery.11
Studies have reported that 30–50% of women with third- or fourth-degree of perineal tears experience anal incontinence.5,12–17 Most of the women who report anal incontinence after severe perineal tears suffer from gas incontinence. The symptoms seem to prevail for years.17,18 Because symptoms of anal incontinence may affect a women's social, psychological, and sexual life,14 it is important to prevent these tears. Severe perineal tears occur in 0.5–3% of all women who deliver vaginally, and recent studies have shown that the incidence may be increasing (Bek KM. Obstetric anal sphincter rupture [PhD thesis]. University of Aarhus, 1993:9-23).5,19
The risk factors for severe perineal tears include null parity, median episiotomy, high birth weight, operative vaginal delivery, prolonged labor, induced labor, epidural anesthesia, and maternal age.4,5,12,20–25 Three recent studies have shown an increased risk of third- and fourth-degree tears of the anal sphincter in women who have a history of a severe laceration.26–28 These studies have not reported the absolute risk of anal sphincter laceration in second delivery in women with a history of such laceration. The prior studies also suffer from limited sample sizes. To gain a better understanding of the recurrence risk of anal sphincter laceration, large population-based studies are needed. A better understanding of these risks is necessary in counseling women with prior anal sphincter laceration.
In this study, which included all women in Norway with first and second vaginal deliveries during the period 1967–1998, the first aim was to estimate the impact of anal sphincter laceration during the first delivery on the risk of recurrence in the second delivery. The second aim was to calculate the absolute risk (during 1990–1998) of anal sphincter laceration as it relates to prior anal sphincter laceration and birth weight of the offspring.
MATERIALS AND METHODS
The study was a population-based cohort study. The data were extracted from the Medical Birth Registry of Norway, 1967–1998. Since 1967, all deliveries in Norway after 16 weeks of gestation, more than 1.8 million births, have been recorded in the Medical Birth Registry.29 The registration is based on standardized forms completed by midwives in the delivery ward shortly after delivery. The study population included all women with 2 consecutive (first and second) singleton deliveries (n = 547,405). The study sample was restricted to women who gave birth vaginally in both the first and second deliveries (n = 486,463; Fig. 1). Women with cesarean delivery at the first or the second delivery, and thus not at risk of anal sphincter laceration, were excluded (n = 60,942, 11.1% of the study population).
Information on all variables was obtained from the Medical Birth Registry of Norway. Anal sphincter laceration at the second delivery, coded “yes” or “no,” was the outcome variable. In the Medical Birth Registry, anal sphincter laceration is categorized as either “rupture of the anal sphincter” or “total rupture of the anal sphincter.” These 2 categories were combined as our definition of anal sphincter laceration.
The main explanatory variable was anal sphincter laceration at the first delivery, coded “yes” or “no.” Information on the other explanatory variables was obtained for the second delivery as follows:
- Birth weight: categorized as less than 3,000, 3,000–3,499, 3,500–3,999, 4,000–4,499, 4,500–4,999, or more than 5,000 g.
- Use of forceps: coded “yes” or “no.”
- Use of vacuum: coded “yes” or “no.”
- Maternal age: categorized as less than 25, 25–29, 30–34, 35–39, or 40 years of age or older.
- Period of delivery: categorized as 1967–1974, 1975–1979, 1980–1984, 1985–1989, 1990–1994, or 1995–1998.
- Use of epidural analgesia: coded “yes” or “no.”
- Prolonged labor: prolonged total duration of labor (> 24 hours) or prolonged second stage of labor (> 60 minutes), coded “yes” or “no.”
Crude and adjusted odds ratios of anal sphincter laceration at second delivery according to history of anal sphincter laceration were estimated with 95% confidence intervals in logistic regression models. Absolute risks of anal sphincter laceration at second delivery according to sphincter laceration at first delivery and birth weight were estimated as prevalence with 95% confidence intervals. All statistical analyses were performed using SPSS 10.0 (SPSS Inc, Chicago, IL).
At the second delivery, a total of 3,378 women (0.7%, 3,378/486,463) had an anal sphincter laceration. Of the 9,558 women with anal sphincter laceration at the first delivery, 357 (3.7%) had recurrent anal sphincter laceration. Of the 495,903 women without anal sphincter laceration at the first delivery 3,021 (0.6%) had a sphincter laceration at the second delivery. Hence 10.6% (357/3,378) of all cases of anal sphincter laceration at the second delivery were recurrent.
The crude odds ratio of anal sphincter laceration at the second delivery was 6.5 (95% confidence interval [CI] 5.8–7.3) when there was sphincter laceration at the first delivery (Table 1). The crude odd ratios of the other risk factors are also presented in Table 1.
The adjusted odds ratio (OR) of sphincter laceration at the second delivery was 4.3 (95% CI 3.8–4.8) (Table 1). For women having a child with birth weight greater than 5,000 g, the adjusted OR of anal sphincter laceration was 23.6 (95% CI 16.5–33.6) compared with having a child with birth weight less than 3,000 g. Also, use of forceps increased the risk of anal sphincter laceration at the second delivery (adjusted OR 5.1, 95% CI 4.3–6.0). Use of epidural anesthesia seemed to be protective (adjusted OR 0.8, 95% CI 0.6–0.9). Giving birth during the most recent years (1995–1998) gave an adjusted OR of 4.3 (95% CI 3.7–5.0) when compared with deliveries during the beginning of the observation period (1967–1975), which was the reference period.
The estimated impact of prior anal sphincter laceration was almost the same in each period (1967–1974: adjusted OR 4.4, 95% CI 2.1–9.3; 1995–1998: adjusted OR 3.7, 95% CI 3.0–4.5). Data from the other periods are not shown.
In women with anal sphincter laceration during the first delivery, the absolute risk of a recurrent laceration increased from 1.3% (95% CI 0.4–3.2%) (Table 2) for birth weight less than 3,000 g to 23.3% (95% CI 11.8–38.6%) for birth weight more than 5,000 g. In women without anal sphincter laceration during the first delivery, the risk for laceration at the next delivery increased from 0.2% (95% CI 0.1–0.3) to 3.7% (95% CI 2.5–5.4). Hence, the increase in relative risk according to birth weight of the offspring was 17.9 (23.3%/1.3%) for women with sphincter laceration at the first delivery and 18.5 (3.7%/0.2%) for women without a laceration at the first delivery. The increase in absolute risk according to birth weight was 22% (23.3% –1.3%) for women with a sphincter laceration at the first delivery and 3.5% (3.7% –0.2%) for those women without a laceration.
A history of anal sphincter laceration was associated with a 4.3-fold increased risk of a severe obstetrical laceration at second delivery compared with no history of sphincter laceration. Only 10.6% (357/3,378) of all cases of anal sphincter laceration at the second delivery were in women with a history of laceration in the first pregnancy. The absolute risk of anal sphincter laceration increased according to the birth weight of the offspring, in particular in women with prior anal sphincter laceration. In this group the risk was 1.3% (95% CI 0.4–3.2%) for birth weight less than 3,000 g and 23.3% (95% CI 11.8–38.6%) for birth weight greater than 5,000 g.
To our knowledge, the impact of an anal sphincter laceration during the first delivery on the risk of a sphincter laceration in the second delivery has been reported in 3 studies only.26–28 These studies have reported an increased risk: Payne et al26 (crude OR 3.4, 95% CI 1.8–6.4), Peleg et al27 (adjusted OR 2.5, 95% CI 1.8–3.4), and Martin et al28 (adjusted OR 5.3, 95% 3.9–7.1). None of these studies have calculated the absolute risk of anal sphincter lacerations at second delivery according to birth weight. In these studies women who had their second delivery in a different hospital were excluded, and the size of this proportion was not given. Selection bias in follow-up may therefore have caused biased risk estimates.
As in our study, prior studies have shown an impact of high birth weight and use of forceps on the risk of sphincter laceration.5,12,30–32 The observed increased risk of anal sphincter laceration during the study period, even when controlling for these risk factors, could be explained by differences in reporting over time. There has, however, been little focus on sphincter laceration in Norway. A true increase in the risk of sphincter laceration is therefore more likely. Also, other studies have suggested that the risk of severe perineal tears may be increasing (Bek KM. PhD thesis, 1993).5,19 It is difficult to explain the increase over time in anal sphincter laceration, but we believe that changes in clinical practice during the period may have occurred. A Swedish retrospective study has shown a difference in the incidence of anal sphincter rupture between Sweden and Finland, and this may be due to the difference in manual control of the baby's head when crowning.33 Changes in pregnant women over time, for example, changes in body mass index, muscle strength in the pelvic floor, or mental health, may be other possible explanations for the increasing occurrence of anal sphincter laceration.
No validation of “rupture of the anal sphincter” or “total rupture,” as categorized in the Norwegian Medical Birth Registry, has been performed. In the Norwegian clinical language used by doctors and midwives, these categories include third and fourth degree of anal sphincter laceration. Less severe tears have other terms and are not included in the above definition and therefore not reported as such on the standardized forms. We are therefore confident that when “rupture of the anal sphincter” or “total rupture of the anal sphincter” is reported, these represent severe tears of the anal sphincter. The prevalence of reported severe ruptures in our study is also in agreement with other studies, which support the validity of our classification. There has been no change in categorization during the observation period, and there has been very little focus on obstetric trauma in vaginal deliveries among the public or the professionals in Norway. An underreporting of severe rupture during the observation period may have caused an underestimate of the prevalence.
Anal sphincter laceration in women with a prior laceration may be more likely to be reported to the Norwegian Medical Birth Registry than anal sphincter laceration in women without such history. In such a case, the impact of prior anal sphincter laceration may be overestimated.
All women in Norway with 2 consecutive vaginal deliveries during 1967–1998 were included in our study. Cesarean delivery at the second delivery was more prevalent in women with prior anal sphincter laceration than in women without such history (6.2% versus 3.7%). Since women with a prior laceration have been more likely to have a cesarean delivery than women without laceration, the estimated impact of prior laceration may have been underestimated.
Most of the known risk factors of sphincter laceration are controlled for in our analyses. Episiotomy is a potential risk factor, but has not been reported to the Medical Birth Registry. However, numerous studies have shown that women with a midline episiotomy carry a higher risk of sphincter laceration than women with a mediolateral episiotomy or than those with no episiotomy.34–37 In Norway, the mediolateral episiotomy has, during the whole observation period, been the procedure of choice, and since the middle of the 1980s, the use of episiotomy has not been recommended as a routine procedure. During the period 1980–1998, the prevalence of sphincter laceration at second delivery has more than doubled. It is, however, not known whether this increase is associated with decreased use of episiotomy.
Prior studies have shown epidural anesthesia to be a risk factor of anal sphincter laceration.20,21 However, other studies have not confirmed such finding.38,39 Epidural anesthesia has been associated with instrumental vaginal delivery and prolonged labor.38–40 In our study epidural anesthesia seemed to protect against sphincter laceration after control for known risk factors. This was a surprising finding and has to our knowledge not been shown in any other study. However, our finding may be spurious, or the increased risk of anal sphincter laceration with epidural anesthesia in other studies could also be explained by insufficient control for confounding factors.
Other potential risk factors of anal sphincter laceration include induction of labor, maternal body mass index, and delivery position. In the initial data analyses, we included both induction of labor and use of oxytocin but did not find these variables to be significantly associated with anal sphincter laceration. Information on maternal body mass index and delivery position were not available in the Medical Birth Registry and could therefore not be controlled for. There have been several randomized controlled trials of delivery position in the second stage of labor, but no significant difference in the risk of perineal trauma has been shown.41–44 We have little reason to believe that lack of control for the above factor has caused biased estimates of the impact of prior sphincter laceration
Prior anal sphincter laceration increases the risk of laceration at the next delivery. Few women with anal sphincter laceration at the first delivery had recurrent laceration (3.7%). This suggests that sphincter laceration at the first delivery has little impact on future risk. Only 10% of anal sphincter laceration at second delivery was in women with prior laceration. Eliminating this 10% of the cases through cesarean delivery would have little impact on the prevalence of sphincter laceration at the second delivery. One would have to perform a total of 25 cesarean deliveries to prevent one case. Our absolute risk estimates of repeated anal sphincter laceration according to birth weight may be of help in counseling women about mode of delivery. For birth weight less than 4,000 g, the risk for a repeated sphincter laceration was less than 4.1%. This risk is similar to risk at the first delivery. Thus, for women with predelivery birth weight estimated at less than 4,000 g, it may be reasonable to recommend vaginal delivery. In delivery counseling, presence of symptoms of anal dysfunction should also be considered. Such symptoms may be aggravated by subsequent vaginal delivery.45 For women delivering a child with a birth weight of 4,000–5,000 g, the risk of a new sphincter laceration ranged from 8.0% to 23.3%. Hence, in women with a history of sphincter laceration and with estimated birth weight more than 4,000 g, cesarean delivery may be an appropriate recommendation. It must, however, be kept in mind that exact predelivery birth weight estimates often are difficult to make. Only a minor proportion in our study, less than 1% of the entire cohort, had a child with a birth weight more than 4,000 g and a prior laceration. Intervention in this group will, however, have very little impact on the prevention of sphincter laceration in the second delivery in general.
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