EP331 - Exploring The Correlation Between Risk Factors And Severity Of Obstetric Anal Sphincter Injuries: A Multivariable Analysis

Purpose/Background: Obstetric Anal Sphincter Injury (OASIS) refers to perineal trauma involving the anal sphincter that occurs during vaginal delivery. The aim of this study is to assess and compare women with OASIS to identify factors associated with the severity of perineal damage.

Methods/Interventions: Women with OASIS diagnosed via endoanal ultrasound from 2008 to 2024 were evaluated in this retrospective study conducted in a tertiary colorectal pelvic floor unit. Patients were assessed in our specialized interdisciplinary OASIS clinic 12 weeks after childbirth, where we conducted evaluations of pelvic floor symptoms and performed endoanal ultrasound scans and anorectal manometry.
First analysis examined the associations between risk factors, symptoms, manometry data and the ultrasound OASIS score.
The variables evaluated included age, number of pregnancies, number of births, history of episiotomy, ethnicity, induction of labour, mode of delivery, birthweight, and various types of incontinence—flatus, passive faecal, and urge faecal incontinence. Additionally, the assessment covered the St. Mark’s incontinence score for anal incontinence, along with symptoms of stress and urge urinary incontinence and urinary frequency. Furthermore, two other variables were resting pressure and squeeze pressure, evaluated through anorectal manometry.
Further analyses examined the joint association between factors and OASIS in a multivariable analysis.

Results/Outcomes: 1428 women were considered for this study. The participants were categorized based on the severity of their perineal tears: 384 (26.9%) had no OASIS, 131 (9.2%) had a 3a perineal tear, 648 (45.4%) had a 3b perineal tear, 264 (18.5%) had a 3c perineal tear, while one woman (0.1%) had a 4th degree perineal tear. As there was only one woman with a grade 4 tear, this was combined with grade 3c for the purposes of analysis.
Significant differences were found between the different OASIS groups for various variables, as summarized in Table 1. The findings indicated a statistically significant variation among the groups in terms of gravida, previous episiotomy, induction of labour, mode of delivery, flatus incontinence, resting pressure and squeeze pressure. Conversely, no statistically significant differences were found regarding the other variables.
In terms of gravida, women with more serious injuries had more previous pregnancies. A significant difference in previous episiotomy between groups was observed. However, there was no clear trend. The results for induction of labour were also not entirely consistent with increasing severity of injury. Flatus incontinence increased with the level of injury. In terms of manometry data, a low resting and squeeze pressure were most more common with high levels of injury.
The multivariable analyses indicated that mode of delivery, flatus incontinence, resting pressure and squeeze pressure were associated with the occurrence of OASIS. As these are all significant in the multivariable analysis it suggests that are factors independently associated with the outcome (table 2).

Conclusion/Discussion: Assisted delivery has been associated associated with a higher incidence of OASIS. Flatus incontinence has emerged as one the key symptoms of OASIS. Low resting and squeeze pressure were more common in patients with OASIS compared to those without OASIS.
These findings contribute to a better understanding of the factors associated with OASIS and can help inform strategies for prevention and management of these injuries in obstetric practice.