RESUMEN
INTRODUCTION: Obstetric anal sphincter injuries (OASIS) may be underdetected in primiparous women. This study evaluated the prevalence of OASIS in primiparous women after normal vaginal delivery or instrumental delivery using endoanal ultrasound (US) during postnatal follow-up. METHODS: This study retrospectively analysed endoanal US data collected during postnatal follow-up (6-12 months after vaginal delivery) at a tertiary hospital in Hong Kong. Offline analysis to determine the prevalence of OASIS was performed by two researchers who were blinded to the clinical diagnosis. Symptoms of faecal and flatal incontinence were assessed with the Pelvic Floor Distress Inventory. RESULTS: Of 542 women included in the study, 205 had normal vaginal delivery and 337 had instrumental delivery. The prevalence of OASIS detected by endoanal US was 7.8% (95% confidence interval [CI]=4.1%-11.5%) in the normal vaginal delivery group and 5.6% (95% CI=3.1%-8.1%) in the instrumental delivery group. Overall, 82.9% of women with OASIS on endoanal US did not show clinical signs of OASIS. Birth weight was significantly higher in the OASIS group (P=0.012). At 6 to 12 months after delivery, 5.5% of women reported faecal incontinence and 17.9% reported flatal incontinence, but OASIS was not associated with these symptoms. CONCLUSIONS: Additional training for midwives and doctors may improve OASIS detection.