Management of subsequent pregnancies following fourth-degree obstetric anal sphincter injuries (OASIS)

The management of subsequent pregnancy in women who sustained OASIS remains an enigma. Nearly all studies include all grades of OASIS including fourth-degree tears. In addition, most protocols require endoanal ultrasound and anal manometric assessment to provide advice regarding mode of delivery. In reality, most women who sustain an OASI do not undergo these investigations. The aims of our study were firstly to evaluate outcomes of fourth-degree OASIS in terms of sphincter defects, anal manometry and anal incontinence symptoms. Secondly, we wished to review recommendations made regarding subsequent mode of delivery after fourth-degree OASIS according to different published protocols.

An observational study of all women who had undergone a primary repair of a fourth-degree tear and seen in the perineal clinic of a tertiary urogynaecology unit between January 2006 and December 2017. Three-dimensional endoanal ultrasound and anal manometry were performed on all women, and symptoms assessed using the validated modified St Mark's Score for anal incontinence. Diagnostic test accuracy analysis was performed for use of symptoms in predicting abnormal investigations.

74 fourth-degree tears were identified (mean follow-up 5.9 months; SD 11.5). Endoanal scan showed an internal anal sphincter defect in 77 % and an external anal sphincter defect in 49 %. Only 18 % had no defect. The mean incremental pressure rise was 12.6 mmHg (SD 15.5). 61 % were asymptomatic with a mean St Mark's Score of 3.8 (SD 5.4). The presence of symptoms alone had poor accuracy in predicting abnormal investigations. Based on Royal College of Obstetricians and Gynaecologists guidance, only 7% would not be offered a caesarean section as they are asymptomatic women with normal scan and manometry findings and would be counselled for a vaginal delivery.

Given that only a few units offer these specialist investigations to their OASI population, it would be reasonable to offer caesarean section to all women who have sustained a fourth-degree tear. However, in centres where endoanal ultrasound and anal manometry are available, individualised counselling can be offered.