Complete title: When to assess residual anal sphincter defect after OASI by ultrasound

Objectives Obstetrical anal sphincter injuries (OASIS) are complications of vaginal delivery. Unrepaired anal sphincter after delivery increases the risk of anal incontinence. The aim of our study is to search for residual defect after OASI repair by 4D introital ultrasound (US). We hypothesised that imaging prior to hospital discharge would show the same number of defects as assessment at 3 months. Study design This is a retrospective analysis of 138 patients with immediate repair after OASIS. Since 2009, we have been routinely inviting all our patients with OASIS to the perineal clinic for postoperative follow-up. We scheduled the first visit before discharge from hospital and followed up with a second visit after three months. During both visits, patients underwent transperineal 4D ultrasound examination. We provided examination while at rest and during contraction; volumes were saved for further evaluation. Results Eighty-one patients (58.7%) completed both exams and were included in the analysis. Residual external anal sphincter defect was found in 17.3% at the first visit versus 13.6% at the second (delayed) visit. We were not able to assess the sphincter in 9.9% at early versus none in the delayed examination. In 9.9%, the residual defect was not confirmed by the second examination, and in 4.9% the residual defect was missed by the first examination. Conclusions In conclusion, we have found that a 3 month scan yields a smaller number of defects than a scan before discharge.