Introduction
Obstetric anal sphincter injury (OASI) is the most common cause of anal incontinence in women and can have a devastating effect on a woman’s mental and physical health as well as their social wellbeing (1). In the United Kingdom, the rate of OASIs in primiparous women is 6.1%. The rates have risen three-fold from 2000 to 2012 (2). A similar trend has also been observed in Nordic countries and Canada (3, 4).
It is recommended that all women should be examined by a midwife or doctor following a vaginal delivery to assess perineal trauma and anal sphincter disruption (5) by performing a digital rectal and vaginal examination to avoid missing OASI. It has been shown that when a trained clinician re-examines women after their initial assessment, that the detection rate of OASIs doubles (6).
Women with missed OASIs are more likely to develop anal incontinence (7, 8). Endoanal ultrasound (EAUS) has been evaluated to improve the detection of OASIs at delivery. Andrews et al (13) assessed the anal sphincter by performing an EAUS immediately postpartum. Women also had a clinical examination by the accoucher to assess perineal trauma and were re-examined by a trained clinical research fellow immediately after delivery. They found that 58% of OASIs were missed by doctors and midwives (13) and all of these were identified using EAUS. EAUS also had a false positive rate of 2% based on anal sphincter defects on scan that were not clinically identifiable. The feasibility of using EAUS immediately after delivery however is limited as it is expensive, requires training and is not readily available in most maternity units. EAUS is also known to cause more discomfort than other imaging modalities (9) including transperineal ultrasound (TPUS). Over the last decade there has been an interest in using TPUS to image the anal sphincter (9-12). The equipment is universally available and enables visualisation of the anal sphincter complex without disrupting anatomy. It also has the advantage of multiplanar imaging and three-dimensional (3D) volume storage which allows one to review images offline.
Although previous research highlights that OASIs can be missed by doctors and midwives (6, 13, 14), many of these studies were performed more than 10 years ago. Training in both diagnosis and repair of OASIs has improved subsequently (15-17). The aim of this study was to establish whether 3D TPUS improves the detection of OASIs in women having their first vaginal delivery compared with clinical examination.