Episiotomy characteristics and risks for obstetric anal sphincter injuries: a case-control study

Abstract

Objectives: To investigate the association between the geometrical properties of episiotomy and obstetric anal sphincter injuries (OASIS) because episiotomies angled at 40-60° are associated with fewer OASIS than episiotomies with more acute angles.

Design: Case-control study.

Setting: University Hospital of North Norway, Tromsø and Nordland Hospital, Bodø, Norway.

Sample: Seventy-four women who had one vaginal birth and episiotomy. Cases (n = 37) have sustained OASIS at birth, while controls (n = 37) had not. The groups were matched for instrumental delivery.

Methods: Two groups of women with history of only one vaginal birth were compared. Episiotomy scar was identified and photographed and relevant measures were taken. Data were analysed using conditional logistic analysis.

Main outcome measures: Mean episiotomy angle, length, depth, incision point.

Results: The risk of sustaining OASIS decreased by 70% (odds ratio [OR] 0.30; 95% CI 0.14-0.66) for each 5.5-mm increase in episiotomy depth, decreased by 56% (OR 0.44; 95% CI 0.23-0.86) for each 4.5-mm increase in the distance from the midline to the incision point of the episiotomy, and decreased by 75% (OR 0.25; 95% CI 0.10-0.61) for each 5.5-mm increase in episiotomy length. Lastly, there was no difference in mean angle between groups but there was a "U-shaped" association between angle and OASIS (OR 2.09; 95% CI 1.02-4.28) with an increased risk (OR 9.00; 95% CI 1.1-71.0) of OASIS when the angle was either smaller than 15° or >60°.

Conclusion: The present study showed that scarred episiotomies with depth > 16 mm, length > 17 mm, incision point > 9 mm lateral of midpoint and angle range 30-60° are significantly associated with less risk of OASIS. Shrinkage of tissue must be considered.

Figure 1

Adapted from Andrews et al.which illustrates the measurements taken. a = line drawn from the posterior fourchette to the outer edge of the anal epithelium; b = episiotomy depth, a line from the caudal end of the episiotomy bisecting line ‘a’ perpendicularly; c = the shortest distance from the caudal end of the episiotomy to the anterior outer edge of the anal epithelium; d = line from the posterior fourchette to the point of episiotomy incision and e = episiotomy length.

Figure 2

Digital picture visualising two episiotomies with different geometric properties. Left picture shows an episiotomy associated with an increased risk of sustaining OASIS, whereas the episiotomy in the right picture has the angle, length, depth and incision point associated with less risk.