Objective
We sought to determine whether an optimal level of lateral episiotomy use can be found by assessing the correlation between the hospital-based variations in episiotomy use and rates/odds ratios of obstetric anal sphincter injuries (OASIS).
Study Design
This was a retrospective population-based register study. The study group, comprising women with spontaneous singleton vaginal deliveries, contained all 154,175 primiparous and all 234,236 multiparous women. The correlations between lateral episiotomy use and incidence/risk of OASIS (n = 1659) were assessed using nonlinear and linear regression modeling.
Results
The rates of episiotomy were inversely correlated with the risk of OASIS among both groups of women. OASIS rates increased from 0.5-1.0% as episiotomy rates decreased from 80-40%.
Conclusion
Restricting lateral episiotomy use may result in higher OASIS rates. However, we could not determine the optimal level of episiotomy use since individual hospitals deviated substantially from the correlation curves.
There is a clear worldwide trend toward restricting the use of episiotomy, since its routine use produces not only additional clinical work, but also episiotomy-related complications such as the need for more suturing and healing complications, and further, there is greater probability of the perineum remaining intact. The role of episiotomy in reducing the risk of obstetric anal sphincter injuries (OASIS), however, is still controversial. This controversy originates from the many confounding factors, including the technique’s nonstandardized use and the lack of clear, self-explanatory indications. Another major confounding factor is the type of episiotomy, whether lateral, mediolateral, or medial. Most of the evidence so far has focused on medial or mediolateral episiotomies, while data on the effects of lateral episiotomy are sparse. The lateral type of episiotomy is the only technique in use in Finland.
We have previously reported that fetal macrosomia, vacuum extraction, and a prolonged active second stage of birth increase the risk of OASIS in both primiparous and multiparous women, and that episiotomy appears to be a protective factor in primiparous but not in multiparous women. Among multiparous women, episiotomy tends to be performed prophylactically more often in those who are at a high risk of OASIS than in low-risk women, consequently there may be confounding by indication. In Finland, the incidence of OASIS has risen constantly from 0.2% in 1997 to 0.9% in 2007 while lateral episiotomy has, in turn, been in constant decline over the same period of time. We have found that a policy leading to high rates of episiotomy provides protection from OASIS among both groups of women supporting the idea that a reduction in episiotomy use may have played a role in the observed trend. However, the difference in episiotomy rate between high and low episiotomy use groups of primiparous women was found to be 30% while that for OASIS was 0.4% (0.7% vs 1.1%) indicating poor efficiency in preventing OASIS. Delivering hospital appeared to have a clear effect on the OASIS incidence: we found up to 7-fold interhospital differences in OASIS incidence, and the use of episiotomy varied substantially. Therefore we conducted further analyses to find out whether it would be possible to identify the optimal level of lateral episiotomy use.
Materials and Methods
The source of data used in this study was the Medical Birth Register (MBR), which is a compilation of the clinical records of all of the obstetric care units in Finland; it is currently maintained by the National Institute for Health and Welfare (THL). The data for the period 1997 through 2007 were obtained from the THL, which also gave us the required authorization for the use of sensitive health register data in scientific research, as required by national data protection legislation. Only anonymized data were used and thus the informed consent of the registered individuals was not needed.
The MBR includes information on maternal and neonatal birth characteristics and perinatal outcomes (all live births or stillbirths delivered after the 22nd gestational week or weighing ≥500 g). Information on OASIS has been collected as part of the MBR since 2004. For the years 1997 through 2003, the information about OASIS was taken from the hospital discharge register (HDR), based on the International Statistical Classification of Diseases, 10th Revision codes O70.2 (third degree) and O70.3 (fourth degree). The 2 data sources were linked together using encrypted unique personal identification numbers. The degree of OASIS was classified according to standard definitions: a third-degree rupture involves the external anal sphincter, and a fourth-degree rupture affects both the anal sphincter and the anorectal mucosa. The perineum was checked by midwives who asked obstetricians to review the assessment if necessary.
We assessed the correlation between the hospital-based variation in lateral episiotomy use and incidence/odds ratio (OR) of OASIS in spontaneous vaginal deliveries. To eliminate the possibility that data from small hospitals may have biased the results, we included data only from the hospitals with at least 1000 deliveries annually. After excluding births in the 11 hospitals that closed their birth units during the study period, there were 34 birth hospitals operating in Finland during the 11-year study period (mean annual number of births, 1626; range, 222–5112). Of these, 15 birth units had <1000 births each year, so only the remaining 19 hospitals were included in the analyses. Primiparous and multiparous women were analyzed separately since our preliminary assessment of the data showed that nulliparous women were at a 7.1-fold (95% confidence interval [CI], 6.3–7.9) risk of OASIS compared to multiparous women. Women who were admitted for vaginal delivery after a previous cesarean section for their first birth were classified as primiparous (n = 9534, 6.2% of all primiparous women). Furthermore, to minimize the influence of known risk factors of OASIS, such as assisted deliveries, we limited the analyses to women with spontaneous singleton vaginal deliveries.
To quantify the correlation between episiotomy use and the occurrence of OASIS, we calculated the rates for both, and the ORs for OASIS, adjusted for maternal age, birthweight, and episiotomy for each study hospital by using logistic regression. Continuous variables were transformed into categorical variables, and all the variables were entered simultaneously into the model. Hospitals G and L had the lowest OASIS rates and were, therefore, used as references for both primiparous and multiparous women. Nonlinear (inverse model) and linear regression analyses were used to assess the relationships between episiotomy and OASIS by birth hospital. Differences were deemed to be significant if P < .05. In all of the analyses, data on third- (89.3%) and fourth- (10.7%) degree obstetric anal sphincter ruptures were pooled. The data were analyzed using SPSS for Windows 17.0 (SPSS Inc, Chicago, IL).
Results
The study data consisted of 154,175 primiparous and 234,236 multiparous women of whom 1275 (0.8%) and 384 (0.2%), respectively, experienced OASIS. Episiotomy was performed on 62% of the primiparous women and 15% of the multiparous women. The episiotomy rates in individual hospitals varied substantially, ranging from 38-86% for primiparous and 6-30% for multiparous women ( Tables 1 and 2 ). The occurrence of OASIS varied from 0.1-1.7% in primiparous and from 0.02-0.33% in multiparous women. Adjusted ORs for OASIS reached values of 6.73 (95% CI, 3.56–12.70) and 22.64 (95% CI, 3.13–163.63), respectively, in the hospital with highest rate (C), compared to hospital G or L (the reference hospitals with the lowest OASIS rates in primiparous and multiparous women, respectively) as shown in Tables 1 and 2 . In a total of 14 hospitals, primiparous women had a statistically significant increased background-adjusted risk of OASIS. For multiparous women, the equivalent number of hospitals was 11.
