Corresponding author:
Mads N Holten-Andersen, Department of Pediatrics, Innlandet Hospital
Trust, Anders Sandviksgate, 2619 Lillehammer, Norway.
Email: madsn.holten-andersen@sykehuset-innlandet.no
Telephone: +47 41487829
Running title:
Risk factors for anal sphincter injury
Abstract
Objective To identify modifiable and non-modifiable risk
factors for severe obstetric anal sphincter injury (OASI) following
vaginal delivery.
Design Retrospective case-control study.
Setting Single center maternity clinic in South-Eastern Norway
Population Women diagnosed with OASI following singleton
vaginal birth after 30 weeks’ gestation (n = 421) and matched controls
(n = 421) during 1990-2002.
Methods Data were extracted retrospectively from an
institutional birth registry. For each woman with OASI the first
subsequent vaginal singleton delivery matched for parity was elected as
control. Potential determinants for OASI were assessed by conditional
logistic regression analyses.
Main outcome measure OASI, defined as 3rd or
4th degree obstetric anal sphincter lesions.
Results Among modifiable factors amniotomy was the strongest
independent determinant for OASI in both primi- (adjusted odds ratio
[aOR] 4.84; 95% CI 2.60–9.02) and multiparous (aOR 3.76; 95% CI
1.45–9.76) women, followed by augmentation with oxytocin (primiparous:
aOR 1.63; 95% CI 1.08–2.46, multiparous: aOR 3.70; 95% CI
1.79–7.67). Vacuum extraction and forceps delivery were independently
associated with OASI in primiparous women (vacuum: aOR 1.91; 95% CI
1.03–3.57, forceps: aOR 2.37; 95% CI 1.14–4.92), and episiotomy for
OASI in multiparous women (aOR 2.64; 95% CI 1.36–5.14).
Conclusions Amniotomy may be a hitherto unrecognized
independent modifiable risk factor for OASI and should be further
investigated for its potential role in preventive strategies for OASI.
Funding Innlandet Hospital Trust research fund, grant number
150434.
Keywords Obstetric anal sphincter injury; OASI; Birth; Birth
injury; Modifiable risk factor; Amniotomy.