This review presents the available data on the diagnosis of obstetric anal sphincter injury (OASI) by postnatal ultrasound imaging. There is increasing evidence that anal sphincter tears are often missed after childbirth and, even when diagnosed, often sub- optimally repaired, with a high rate of residual defects after reconstruction. Even after postpartum diagnosis and primary repair, 25-50% of patients will have persistent anal incontinence (AI). As clinical diagnosis may fail in the detection and classification of OASI, the use of imaging has been proposed in order to improve detection and treatment of these lacerations. Three-dimensional endoanal ultrasound (3D-EAUS) is considered the Gold Standard in the detection of OASI, and recently four-dimensional transperineal ultrasound (4D-TPUS), commonly available in obstetric and gynaecological settings, has proven to be effective as well. Avoidance of forceps delivery when possible, performance of a rectal examination after vaginal delivery and before repair of any significant perineal tear, and the offer of a sonographic follow up at 10-12 weeks after vaginal delivery in high-risk women (maternal age ≥ 35, vaginal birth after cesarean (VBAC), forceps, prolonged second stage, overt OASI, shoulder dystocia and macrosomia) may help reduce morbidity arising from anal sphincter tears.
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