Which characteristics of the episiotomy and perineum are associated with a lower risk of obstetric anal sphincter injury in instrumental deliveries.

Objetive: Operative vaginal delivery (OVD) is the most important risk fact for obstetric anal sphincter injury (OASI). Knowledge of possible risk factors for their occurrence may therefore reduce the likelihood of faecal incontinence. The aim is to analyse the effect of mediolateral episitomy and perineum characteristics on the occurrence of OASI in OVD.

Study Design: Case-control study, which included 958 OVD that were reviewed in Pelvic Floor and Puerperium Clinic. The episiotomy and perineum characteristics of those women who experienced OASIs (n = 150) were compared with those who had no evidence of anal sphincter injury (n = 788).

Results: In multivariate logistic regression analysis the factors which were independently associated were nulliparity, persistent occipitoposterior position, birthweight >3500 g, an angle of episiotomy <30°, a distance episiotomy-fourchette <5 mm and a distance of perineal body <30 mm. The analysis of subgroups show that only the multiparous women does not benefit from any feature of the episiotomy, and an angle greater than 30° and a distance episiotomy-fourchette >5 mm are associated with a risk reduction of OASI in nulliparous, perineal bodies ≤30 mm and occipitoanterior position.

Conclusions: Two modifiable risk factors at the time of performing the episiotomy, the angle and distance episiotomy-fourchette, have been identified as the risk modification of OASI. It is necessary to achieve an adequate angle to reduce the probability of OASIs in OVD, and in nulliparous women with an anterior position and a distance of perineal body ≤30 mm could benefit from increasing the episiotomy-fourchette distance.