Modifiable and non-modifiable risk factors for obstetric anal sphincter injury in a Norwegian Region: a case-control study.

backgroundObstetric anal sphincter injury (OASI) is a common and severe complication of vaginal delivery and may have short- and long-term consequences, including anal incontinence, sexual dysfunction and reduced quality of life. The rate of OASI varies substantially between studies and national birth statistics, and a recent meta-analysis concluded that there is a need to identify unrecognized risk factors. Our aim was therefore to explore both potential modifiable and non-modifiable risk factors for OASI.

methodsWe performed a case-control study in a single center maternity clinic in South-Eastern Norway. Data were extracted retrospectively from an institutional birth registry. The main outcome measure was the occurrence of the woman's first-time 3

resultsThe mean incidence of OASI was 3.4% of vaginal deliveries, but it increased from 1.9% to 5.8% during the study period. In the final multivariate regression model, higher maternal age and birthweight for primiparous women, and higher birthweight for the multiparous women, were the only non-modifiable variables associated with OASI. Amniotomy was the strongest modifiable risk factor for OASI in both primi- (odds ratio [OR] 4.84; 95% confidence interval [CI] 2.60-9.02) and multiparous (OR 3.76; 95% CI 1.45-9.76) women, followed by augmentation with oxytocin (primiparous: OR 1.63; 95% CI 1.08-2.46, multiparous: OR 3.70; 95% CI 1.79-7.67). Vacuum extraction and forceps delivery were only significant risk factors in primiparous women (vacuum: OR 1.91; 95% CI 1.03-3.57, forceps: OR 2.37; 95% CI 1.14-4.92), and episiotomy in multiparous women (OR 2.64; 95% CI 1.36-5.14).

conclusionsAmniotomy may be an unrecognized independent modifiable risk factor for OASI and should be further investigated for its potential role in preventive strategies.