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Objective To determine the risk factors for anal sphincter injuries during operative vaginal delivery.
Setting and design A population-based observational study.
Population All 21 254 women delivered with vacuum extraction and 7478 women delivered with forceps, derived from the previously validated Dutch National Obstetric Database from the years 1994 to 1995.
Methods Anal sphincter injury was defined as any injury, partial or complete, of the anal sphincters. Risk factors were determined with multivariate logistic regression analysis.
Main outcome measures Individual obstetric factors, e.g. fetal birthweights, duration of second stage, etc.
Results Anal sphincter injury occurred in 3.0% of vacuum extractions and in 4.7% of forceps deliveries. Primiparity, occipitoposterior position and fetal birthweight were associated with an increased risk for anal sphincter injury in both types of operative vaginal delivery, whereas duration of second stage was associated with an increased risk only in vacuum extractions. Mediolateral episiotomy protected significantly for anal sphincter damage in both vacuum extraction (OR 0.11, 95% CI 0.09–0.13) and forceps delivery (OR 0.08, 95% CI 0.07–0.11). The number of mediolateral episiotomies needed to prevent one sphincter injury in vacuum extractions was 12, whereas 5 mediolateral episiotomies could prevent one sphincter injury in forceps deliveries.
Conclusions Primiparity and occipitoposterior presentation are strong risk factors for the occurrence of anal sphincter injury during operative vaginal delivery. The highly significant protective effect of mediolateral episiotomies in both types of operative vaginal delivery warrants the conclusions that this type of episiotomy should be used routinely during these interventions to protect the anal sphincters.
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Anal sphincter injury during delivery is considered to be one of the major risk factors for faecal incontinence in women.1 After anal sphincter injury, up to 50% women have complaints of faecal incontinence mainly because of persisting sphincter defects.2–4
Knowledge of possible risk factors for the occurrence of anal sphincter injuries may therefore reduce the likelihood of faecal incontinence. Operative vaginal delivery has been shown to be a significant contributor to the number of anal sphincter injuries.5–8 In daily obstetric practice, the use of operative vaginal deliveries is inevitable in case of fetal distress or prolonged second stage of labour. Knowledge and modification of attributive risk factors may help reduce the number of anal sphincter injuries during operative vaginal delivery.
Episiotomy is the most commonly performed obstetric operation and was traditionally thought to decrease the risk for major perineal trauma and pelvic floor dysfunction in later life. These claims were critically reviewed and questioned in two large reviews.8,9 Randomised controlled trials comparing the liberal use with the restricted use of mediolateral episiotomies showed no beneficial effect of liberal over restrictive use with regard to the prevention of anal sphincter damage.10,11 However, these trials were, because of their study design, unable to establish the possible protective effect of a mediolateral episiotomy itself.
In a large population-based observational study, an 80% risk reduction for the occurrence of anal sphincter injuries was associated with the restrictive use of mediolateral episiotomy.5 In the literature, only few studies addressing the effect of episiotomy on anal sphincter damage in operative vaginal delivery have been published, and most have dealt with one specific instrument or considered the use of midline episiotomies only.12–22
The Dutch National Obstetric Database (LVR) allows population-based studies on a variety of clinical variables associated with pregnancy, labour and delivery and has been used before to analyse risk factors for the occurrence of anal sphincter injury during vaginal delivery.5
The present study was designed to analyse the effect of the mediolateral episiotomy and to establish the presence of attributive risk factors on the occurrence of anal sphincter injury in instrumental vaginal delivery using the data from this database.
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Anal sphincter injury during delivery occurred in 3.0% of vacuum extractions and in 4.7% of forceps deliveries. Primiparity, fetal birthweight and occipitoposterior position were associated with a significantly increased risk for anal sphincter injury during both vacuum extractions and forceps deliveries. Duration of second stage was only associated with anal sphincter damage during vacuum extractions, whereas induction of labour showed no association with anal sphincter lesions in neither vacuum extraction nor forceps delivery. The use of a mediolateral episiotomy had a highly protective effect on the occurrence of anal sphincter injuries during both vacuum extraction and forceps delivery.
Anal sphincter damage during delivery may lead to faecal incontinence in up to 50% women, mainly due to persisting sphincter defects after primary repair.2–4 Studies on risk factors for anal sphincter damage have pointed out that instrumental deliveries are an important contributor to these injuries, with obstetric forceps known to carry a higher risk than vacuum extraction.5,7 In daily obstetric practice, the use of instrumental deliveries is inevitable. Knowledge of potentially modifiable risk factors for anal sphincter lesions may therefore contribute to the prevention of faecal incontinence.
In both vacuum extractions and forceps deliveries, increasing fetal birthweight was associated with an increased risk for anal sphincter injuries. This confirms the results of previous studies on this subject.20,21 Hudelist et al. found a similar association of increasing birthweight with anal sphincter lesions in their cohort of women delivered with forceps. Although increasing fetal head circumference may seem to be a logical explanation for this association, no association of fetal head circumference with anal sphincter injuries during forceps deliveries was found in this study.
In vacuum extractions, duration of second stage was weakly associated with an increased risk for anal sphincter damage, whereas in forceps deliveries, no association was found. This is in contrast with two earlier studies in which a significant association of the duration of second stage was found in forceps deliveries but not in vacuum extractions.20,22 Comparison of the obstetric characteristics in these studies shows that the population described differs largely from the Dutch situation, with a much higher rate of induction of labour and epidural anaesthesia, which may have a significant effect on the duration of second stage. In both studies, the mean duration of second stage was significantly longer than in our study.
Whether the mildly elevated risk associated with an increased duration of second stage, as found in our study, plays a role in the prevention of anal sphincter lesions remains doubtful.
Our results on the risk increasing effect of primiparity in both vacuum extractions and forceps deliveries corroborate the results of Combs et al.12 on this subject. However, in this study, no distinction was made between vacuum and forceps deliveries. Our results show that primiparity itself carries a larger risk in vacuum extractions than in forceps deliveries. The exact mechanism for this phenomenon remains unclear.
The position of the fetal vertex appears to be an important factor in the occurrence of sphincter lesions in operative vaginal deliveries. In vacuum extractions with occipitoposterior position, this risk was doubled, whereas this risk was tripled in forceps deliveries. These results support previous studies of Wu et al.20 and Benavides et al.22 in which similar risks were reported. The relative increase of fetal head circumference when the fetal head passes through the birth canal in occipitoposterior position and the more dorsally directed extraction towards the anal sphincter complex, necessary during vacuum extraction and forceps delivery, may explain this association.
Studies on the role of midline episiotomies in operative vaginal deliveries from the USA show that this type of episiotomy is strongly associated with an increased risk for the occurrence of third- and fourth-degree perineal tears.13,15,16,20,22 However, the role of mediolateral episiotomies in operative vaginal deliveries is debated. Youssef et al.17 reported a risk increasing effect of the use of episiotomies in operative vaginal deliveries, but after subdivision in vacuum extractions and forceps deliveries, this risk was no longer present. Bodner-Adler et al.18 reported a protective effect of mediolateral episiotomies in forceps deliveries, and Aukee et al.19 reported a similar effect in vacuum extractions. Combs et al.12 showed a strong protective effect of mediolateral episiotomies in operative vaginal delivery without distinction between forceps and vacuum deliveries. In our study, the vast majority of all episiotomies were mediolateral episiotomies. In both vacuum extractions and forceps deliveries, this type of episiotomy had a strongly protective effect for the occurrence of sphincter lesions.
The strength of this study compared with other studies is the large number of forceps and vacuum deliveries, allowing the determination of the impact of other obstetric factors and minimising the risk of unknown confounders. The weakness of this study is that it is retrospective and not randomised. A randomised trial to establish the effect of a mediolateral episiotomy will be very difficult to perform because it will require a large number of cases against the background of all variables associated with sphincter injuries during delivery.
In conclusion, risk factors for anal sphincter injury during operative vaginal delivery with either vacuum extraction or forceps are primiparity, increasing fetal weight and occipitoposterior position. Duration of second stage is only weakly associated with sphincter injuries in vacuum extraction. A mediolateral episiotomy appears to be highly protective for these injuries in operative vaginal delivery. Twelve mediolateral episiotomies are required to avoid one case of anal sphincter injury during vacuum extraction and only five in forceps delivery. In view of the persisting high morbidity after anal sphincter injury during delivery, liberal use of a mediolateral episiotomy during operative vaginal delivery is advocated.