Objective outcome measures of the extent of laceration at delivery are needed. In this study we evaluated and describe here a method for learning perineal ultrasound measurement of the anovaginal distance (AVD). The learning period needed for examiners proficient in vaginal ultrasound examination and the interobserver agreement after reaching proficiency in AVD measurement were determined. The hypothesis was that the method is feasible to learn and reproducible for use in further research. The method was taught by an examiner experienced in perineal ultrasonography. The distance between the mucosal margin of the internal anal sphincter was measured with a vaginal probe. The studied examiners measured the AVD until similar results (+/- 5 mm) were achieved. The AVD in 40 women was then measured and documented by two examiners who were blinded to each others results. Interobserver agreement was calculated using the kappa score. Examiners with previous experience in vaginal ultrasonography had learned the method after performing five sets of comeasurements. The AVD measurements after the learning period showed almost perfect agreement (kappa = 0.87) between the examiners. The method for perineal ultrasound measurement of AVD was learned quickly with high interobserver agreement. The method is feasible to learn and reproducible for use in further research.

Introduction and Hypothesis

No measurements are available for diagnosing the extent of obstetric lacerations. The primary aim of this study was to evaluate the relation between the anovaginal distance (AVD) measured with transperineal ultrasound immediately after delivery and external anal sphincter injury. A secondary aim was to assess whether the palpated perineal thickness was associated with the AVD.

Methods

A prospective observational study of 150 primiparous women at the University Hospital, Linköping, Sweden. After vaginal delivery, initial inspection and palpation of the perineal thickness were performed by the midwife. The women were then divided into subgroups depending on the degree of the suspected perineal laceration. Transperineal ultrasound of the AVD was performed by a physician. Diagnostics of the perineal laceration were done according to standard care.

Results

Women with an external sphincter injury had a shorter AVD and shorter palpatory perineal thickness compared with women without anal sphincter injury. No external sphincter injuries were diagnosed when the AVD and/or palpation height was > 20 mm. The mean AVD in the group with probable second-degree laceration (n = 85) was 18.8 mm (95% CI 17.8–19.8), in suspected third-degree laceration (n = 33) 15.7 mm (95% CI 13.7–17.7) and in probable third-degree laceration (n = 32) 11.8 mm (95% CI 9.7–13.9) (p < 0.001).

Conclusions

A short AVD could be a warning sign postpartum and should increase the awareness of possible external sphincter injury before suturing. An AVD of 20 mm seems to indicate a cutoff level of the occurrence of external sphincter injury, but this needs further evaluation.

Risk factors for obstetric external anal sphincter injury are well known. Maternal and obstetric risk factors for internal anal sphincter injury are not extensively studied.

The rationale of this study was to evaluate the proportion of internal anal sphincter injury in women with external anal sphincter injury, diagnosed immediately after delivery.

This study will assess whether there are additional risks for obstetric internal anal sphincter injury.

Objectives

The primary aim was to assess the proportion of internal anal sphincter injury immediately in women with an external sphincter injury and to evaluate maternal and obstetric risk factors for internal anal sphincter injury in women with an external anal sphincter injury only.

A secondary aim was to relate the diagnostic methods used for obstetric perineal lacerations to the presence of an internal anal sphincter injury.

Study Design

A registry study with data from the Swedish Perineal Laceration Registry 2014–2018.

From the registry, the maternal and obstetric characteristics of 3,333 primiparous women with isolated external (N = 2,236) versus both external and internal (N = 1,097) anal sphincter injuries were studied, as were the methods used for examining the obstetric anal sphincter injuries.

Results

In 32.9 % (1,097/3,333) of primiparous women with an external anal sphincter injury, an internal anal sphincter injury was diagnosed immediately after delivery. A perineal palpatory thickness of less than 10 mm was a diagnostic sign for internal sphincter injury. Well-known risk factors associated with obstetric anal sphincter injuries could not be confirmed as independent risk factors for internal sphincter injury. When the infant is born with an arm beside the head, there is an almost two-fold increased risk for internal sphincter injury.

Conclusions

Our main finding is that 32.9 % of women with external anal sphincter injury also have an internal anal sphincter injury. A palpable perineal thickness of less than 10 mm, a degree 4-laceration and an infant born with a hand by the head increases the risk of internal sphincter injury and should be a clinical warning sign.