Obstetric Anal Sphincter Injuries (OASIS) are a common cause of maternal morbidity with an overall incidence in the UK of 2.9% (range 0-8%) (1). Women who have obstetric anal sphincter injuries diagnosed at the time of delivery undergo primary repair. However, a residual defect persists in many cases after this repair. This can contribute to a range of physical symptoms and psychological distress. Bowel symptoms caused by OASIS can include faecal urgency, faecal incontinence, post defecation soiling or flatus incontinence (2).

The aim of this study is to access the accuracy of clinical diagnosis of obstetric anal sphincter injuries using Anal Endosonography (EUAS), and the correlation between confirmed injury and change to anorectal physiology squeeze pressure, as well as the incidence of bowel symptoms.

We retrospectively reviewed data collected prospectively from 1135 women who attended the Third and Fourth Degree Tears Clinic at our unit, 12 weeks post delivery, between June 2008 and October 2019. Anal Endosonography and anorectal physiology tests were performed, and symptoms were documented using a validated questionnaire (St Mark’s Incontinence Score). Statistical analysis was carried out using Microsoft Excel and SPSS (Statistical Package for the Social Sciences).

Anal sphincter injury was confirmed in 876 (78.8%) women and 236 (21.3%) had no injury. Anorectal physiology was performed on 1018 women, of which 45.6% had a mean maximal resting pressure below the normal range, and 68.8% had a mean incremental squeeze pressure below the normal value. Women with confirmed OASIS had significantly lower squeeze and resting pressures (p<0.001) than those without a confirmed sphincter injury. 

Of the women who attended clinic, 393 (34.8%) reported bowel symptoms, with those with endosonographic evidence of injury more likely to develop flatus incontinence (p<0.001). However, there was no significant difference in the rates of passive faecal incontinence, urge faecal incontinence or post defecation soiling between those with and without confirmed OASIS. 

Women who received an instrumental delivery, epidural analgesia, or an episiotomy had a significantly higher (all p<0.001) incidence of confirmed OASIS compared to those with no confirmed injury. There was no significant difference between forceps and ventouse delivery (p=0.443).

The mean (±SD) age of patients referred to clinic was 31.9 (±4.8) years. The average duration of the second stage of labour in women who had a confirmed OASIS was 108 minutes, this was significantly longer than women without confirmed injury, who had an average duration 75 minutes (p<0.001). The mean (±SD) birthweight of the babies was 3507g, with no significant difference between the weight of those born to women with or without confirmed injury (p=0.710). There was no significant difference between the number of women who had injury confirmed on ultrasound between primiparous and multiparous women (p=0.407). Women of Asian ethnicity were significantly more likely to have an OASIS confirmed on ultrasound compared to Black women (p=0.01). However, there no other significant differences between other ethnicities (Asian, Black, White, Mixed or Other.)

All women who give birth at our unit who are clinically diagnosed with an obstetric anal sphincter injury, are referred to the Third Degree Tears Clinic. Cases confirmed on ultrasound are then discussed at MDM and patients are offered early treatment to encourage better long-term outcomes. This also allows women to be able to make an informed decision on any future mode of delivery, based on whether or not an anal sphincter injury was confirmed. 21.2% of the women in this study can be reassured that they do not have an injury and that there is no contraindication for further vaginal deliveries.  The fact that over 20% were clinically misdiagnosed suggests there is scope to improve clinical diagnosis and a large proportion of cases may be missed. There was limited evidence of vaginal births following obstetric anal sphincter injury so the potential effects of this could not be determined.

Those with confirmed obstetric anal sphincter injury are more likely to complain of bowel symptoms, particularly flatus incontinence. Anal ultrasound can aid in the confirmation of OASIS and may aid women when planning future deliveries.  Anal sphincter pressures were significantly reduced in those with an injury confirmed on EAUS compared with those with no injury. Further study following up the women with confirmed OASIS after an extended period of time would be pertinent in the assessment of the long-term impact of injuries.

Thiagamoorthy G, Johnson A, Thakar R, Sultan AH (2014) National survey of perineal trauma and its subsequent management in the United Kingdom. International Urogynecology Journal. 25, 1621–7.
Reid AJ, Beggs AD, Sultan AH, Roos A-M, Thakar R (2014) Outcome of repair of obstetric anal sphincter injuries after three years. International Journal of Gynecology & Obstetrics. 127(1), 47–50.