Peripartum Ultrasound in the Prediction and Diagnosis of OASIs

1.1.Obstetric Anal Sphincter Injuries (OASIs) Obstetric anal sphincter injuries (OASIs) are serious complications of vaginal birth with a reported incidence throughout Europe and globally, from 1-10% with recent UK data demonstrating a steadily rising incidence of this type of trauma over the past 12 years .

OASIs is one of the most significant risk factors for anal incontinence in young women as well as other complications of long-term dyspareunia and perineal pain.Despite optimal primary repair, approximately 39% of women who sustain an OASIs will suffer from anal incontinence. Several studies have reported a much higher risk of anal incontinence with subsequent deliveries and the potential for symptoms to progress with time.

1.2. Prediction of OASIs With the wealth of population based databases that are now available prediction models are being developed to assist in clinical decision making. Several large population based cohort studies have identified various independent risk factors for sustaining primary or recurrent OASIS. These include nulliparity, increased birth weight, operative vaginal delivery, persistent occipito-posterior position, prolonged pushing phase, induction of labour, use of epidural analgesia and shoulder dystocia, although findings across studies are contradictory.However there is paucity of information relating to the impact of risk factors that can only be known prior to the actual birth both on the likelihood of OASIs occurring de-novo and the usefulness of such information for counselling women at potential risk of sustaining a primary OASIs.

Although members of the investigators group have previously demonstrated that some of these prediction models have been suggested to have good discriminatory value, they do not necessarily confer a good clinical value. Although the model was useful in ruling out risk of OASIs in women with no risk factors, it was not as good in ruling-in an OASIS - that means that a women presenting with all of the known clinical risk factors cannot be differentiated at any higher risk than a women with a smaller subset of these risk factors. Hence limiting its clinical usability.

1.3. Ultrasound and OASIs management Transperineal and endoanal ultrasound scanning is currently used for the diagnosis and management of OASIs. Indeed, they form part of the recommended practice for determining the best mode of birth in pregnancies following OASIs.

Studies have also assessed some transperineal sonographic parameters, like the ano-vaginal distance (AVD), as a way of alerting clinicians to the possibility of OASIs. This measure has been used postnatally and a it was concluded that a short AVD could be a warning sign of possible external sphincter injury before suturing. The authors of this study suggested that an AVD of 20 mm seemed to indicate a cutoff level of the occurrence of external sphincter injury. Nevertheless, The authors of this study stressed on the need of further evaluation of their findings.

A However, to the investigators knowledge such parameter has not been assessed as an intrapartum tool, in combination with other radiological or clinical parameters, in the prediction of the risk of OASIs.