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.