The prevalence of occult obstetric anal sphincter injury following childbirth—literature review

OBJECTIVE: To calculate the published prevalence of ultrasound-detected occult anal sphincter damage associated with different modes of delivery. METHODS: A search of the English language literature for articles using keywords describing the prevalence of ultrasound-diagnosed anal sphincter injury following childbirth. The weighted mean prevalence of occult anal sphincter injury was calculated in the following groups: (1) primiparous women (unselected); (2) primiparous women after an unassisted normal vaginal delivery; (3) multiparous women (unselected); (4) following forceps delivery; (5) following ventouse delivery; (6) following cesarean section. RESULTS: Nineteen articles described ultrasound-diagnosed occult anal sphincter injury. The prevalence in unselected primiparous women (excluding cesarean section) was 29.2% (288/983). After unassisted vaginal delivery in primiparae the prevalence was 21.7% (74/341). The incidence in multiparous women (unselected) is 32.3% (107/331); following forceps delivery 49.1% (131/267) and with ventouse delivery it is 45.2% (66/146). Only one woman (in 173 cases) had anal sphincter injury following cesarean section. CONCLUSIONS: After a review of the literature, occult anal sphincter injury is mostly associated with the first vaginal delivery and is particularly high following instrumental deliveries. Ventouse is less traumatic than forceps. Cesarean section is protective to the anal sphincter.