Outcome of anal symptoms and anorectal function following two obstetric anal sphincter injuries (OASIS)-a nested case-controlled study

Introduction and hypothesis Obstetric anal sphincter injury (OASI) is a significant risk factor for developing anal incontinence. It can therefore be hypothesised that recurrent OASI in a subsequent delivery may predispose women to further anal sphincter dysfunction. Methods A nested case-controlled study based on data collected prospectively between 2006 and 2019. Women matched for age and ethnicity, with a history of one OASI and no sphincter damage in a subsequent delivery (control) were compared to women sustaining a second OASI. Assessment was carried out using the St Mark's score (SMIS), anorectal manometry and endoanal ultrasound scan (findings quantified using the modified Starck score). Results Eighty-four women were included and equally distributed between the two groups, who were followed up 12 weeks postnatally. No difference in SMIS scores was found. Maximum resting pressure (MRP, mmHg) and maximum squeeze pressure (MSP, mmHg) were significantly reduced in the study group. Median (IQR) MRP in the study group was 40.0 (31.3-54.0) versus 46.0 (39.3-61.5) in the control group (p = 0.030). Median (IQR) MSP was 73.0 (58.3-93.5) in the study group versus 92.5 (70.5-110.8) (p = 0.006) in the control group. A significant difference (p = 0.002) was found in the modified Starck score between the study group (median 0.0 [IQR 0.0-6.0]) and control group (median 0.0 [IQR 0.0-0.0]). Conclusions We have demonstrated that women with recurrent OASI do not have significant anorectal symptoms compared to those with one OASI 12 weeks after delivery, but worse anal sphincter function and integrity. Therefore, on long-term follow-up, symptoms may possibly develop. This information will be useful when counselling women in a subsequent pregnancy.