Hypothesis / aims of study
Obstetric anal sphincter injury (OASIS) occurs in up to 2.9% of women. OASIS involves disruption of the anal sphincters and may cause injury to the ano-rectal mucosa during childbirth. The reported rate of OASIS in England have tripled in recent years. OASIS is associated with anal incontinence (AI) and poor quality of life. Racial variations, particularly Asian maternal ethnicity and other risk factors such as primiparity, instrumental delivery, foetal occipito-posterior position, high birthweight, prolonged second stage of labour, short perineal body, and midline episiotomy, have also been linked to OASIS.
The aim of the present study was to explore the impact of ethnicity on the severity of obstetric anal sphincter injuries (OASI), associated risk factors, and subsequent pelvic floor symptoms.
Study design, materials and methods
This retrospective study was carried out at a tertiary colorectal pelvic floor unit in the UK, examining patients who suffered from an Obstetric Anal Sphincter Injury (OASI) between 2008 and 2023. Patients were assessed in our specialised interdisciplinary OASIS clinic 12 weeks after childbirth, where we conducted evaluations of pelvic floor symptoms and performed endoanal ultrasound scans.
The variables evaluated included age, number of pregnancies (gravida), number of births (parity), history of episiotomy, extent of tear as determined by endoanal ultrasound, 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.
Binary categorical variables were analysed across different groups using the Chi-square test. For categorical variables that included three or more categories, which were all ordinal, comparisons between groups were made using the Kruskal-Wallis test. The sole continuous variable in the study, age, was examined using analysis of variance (ANOVA).
Results
Initially, 1497 women were considered for the study. However, 20 women were excluded due to incomplete data, and an additional 63 were excluded for lacking documented ethnicity information. After these exclusions, 1415 women remained for inclusion in the analysis. Comparisons were then made between various ethnic groups, with White British constituting the largest group at 32%, followed by White Other at 24% (Figure 1).
The analysis revealed statistically significant differences across the six ethnic groups in terms of age, number of pregnancies (gravida), number of births (parity), history of episiotomy, birthweight, flatus incontinence, the St. Mark’s incontinence score, and stress urinary incontinence. Conversely, no statistically significant differences were found between ethnic groups regarding the extent of tear, passive faecal incontinence, urge faecal incontinence, urge urinary incontinence, or urinary frequency, as detailed in tables 1 and 2.
Interpretation of results
The White British and White Other groups recorded the highest mean age of 32.8 years. Conversely, the Black group presented the lowest average age at 30.4 years. In terms of gravidity and parity, Black women showed the highest figures. Specifically, 25% of this group had a gravidity of 3 or more, markedly higher compared to 6% of the Mixed group and just 9% of the White Other group. Furthermore, only 66% of Black women had a parity of 1, significantly less than the 85% observed in both the White Other and Other ethnic categories. The prevalence of previous episiotomies was highest within the Other ethnic group, where 51% had experienced this procedure, while the lowest incidences were noted among Black (32%) and Asian (39%) women.
White British women experienced the highest birthweights, with 22% of these women delivering babies weighing 4 kg or more, and only 11% having babies weighing 3 kg or under. In stark contrast, Asian women had 29% of births with babies weighing 3 kg or under, and only 9% of babies weighing 4 kg or over.
Flatus incontinence was observed to be most prevalent among Mixed-race women at 42% and in the Other ethnic group at 38%, contrasting sharply with Black women, in whom it was least common at only 19%. Additionally, the St. Mark’s score, which assesses anal incontinence severity, was found to be lowest among Black women, with 91% scoring between 0 and 4. This figure significantly differed from Asian women, where only 80% fell within this range. Stress urinary incontinence showed a higher incidence in Mixed-race women, affecting 33%, and was considerably less prevalent in Black women at just 15%.
Concluding message
This retrospective analysis has highlighted a greater occurrence of OASIS among White British women. While numerous risk factors for OASIS have been previously identified, our study reveals that women from diverse ethnic backgrounds exhibit distinct risk factors for OASIS:
• Black women exhibited higher numbers in terms of gravida (total pregnancies) and parity (number of deliveries).
• The White British and White Other groups were characterised by older maternal age and higher birthweights.
• Women from Other ethnic backgrounds were more likely to have experienced an episiotomy.
The symptoms associated with OASIS appear to vary according to the patient’s ethnicity:
• Flatus incontinence was predominantly observed in women of mixed race and those classified in the Other ethnic group.
• The St. Mark’s incontinence score, which assesses the severity of anal incontinence, was found to be lowest among Black women.
• Stress Urinary incontinence was most frequently reported in mixed-race women.
Recognising the ethnic differences in the impact and risk factors of OASIS is essential. This knowledge enables the adjustment of risk factors, supports collaborative and informed decision-making regarding future mode of delivery, and helps set realistic expectations for potential pelvic floor symptoms following an OASIS.