Introduction
Obstetric anal sphincter injuries (OASIS) are serious complications to vaginal deliveries. They represent the most significant risk factor for developing female anal incontinence and might also cause perineal pain, dyspareunia, faecal urgency and urinary incontinence, leading to significant long-term suffering and potential stigmatisation.
Perineal injuries are classified according to a definition published by Sultan. First and second-degree injuries involve the vaginal epithelium and perineal muscles respectively. OASIS include third- and fourth-degree injuries to the perineum involving the external anal sphincter (EAS), internal anal sphincter (IAS), and the anal epithelium to varying extent.
There are several well-established risk factors associated with a higher risk of OASIS, including primiparity, foetal macrosomia, instrumental vaginal delivery, and prolonged second stage of delivery. Other aspects that have been proposed to affect the risk include maternal age, height, weight, smoking, ethnicity and educational level, but the evidence has either been sparse or not consistent in published material.
The prevalence of sphincter injury has fluctuated over the last decades and varies across countries. The incidence varies between 0.7-4.2% in the Nordic countries, with Finland representing the lowest and Denmark the highest numbers. An increasing trend has been observed where several explanations have been proposed; larger infant size, increased maternal age, increase in instrumental deliveries, improved diagnostic accuracy, and changed clinical practice of management of second stage of delivery.
Despite OASIS being recognised as a significant cause of both long-, and short-term morbidity there is no international consensus regarding the use of preventative measures. Several factors modifiable during labour have been investigated. Controlled delivery of the head, warm compresses held against the perineum, and choice of delivery position all have shown to decrease the risk of OASIS. The evidence regarding perineal protection manoeuvres has not been proven to have an effect in clinical studies but is used on a routine basis in clinical practice in Sweden and has been part of interventional programs that have successfully reduced the incidence of OASIS. For women at very high risk of OASIS a planned caesarean section might be offered as a preferred route of delivery.
Identifying risk factors in order to provide optimal preventative measures for high-risk women could lead to a significant decrease in long-term suffering and stigmatised symptoms. The aim of this nationwide register-based study was therefore to assess maternal, foetal and socio-demographic risk and protective factors for OASIS across the Swedish population.