Does the angle of episiotomy affect the incidence of anal sphincter injury?

Objective Mediolateral episiotomy is associated with lower rates of significant perineal tears than midline episiotomy. However, the relationship between precise angle of episiotomy from the perineal midline and risk of third-degree tear has not been established. This study quantifies this relationship. Design Case-control study. Setting National Maternity Hospital, Dublin, Ireland. Sample One hundred primiparous women who had undergone right mediolateral episiotomy 3 months previously. Methods Two groups of primiparous women were compared. Cases had sustained clinically apparent anal sphincter injury during delivery, while controls had not. The angle of episiotomy measured from the midline was marked on a superimposed sheet of transparent plastic film and measured using a protractor. Data were analysed using Student's t test, chi-square test and logistic regression analysis. Main Outcome measures Angle of mediolateral episiotomy from the perineal midline. Results Fifty-four cases and 46 controls were assessed. Cases were more likely to have undergone assisted delivery and consequently to have been delivered by an obstetrician than by a midwife. The mean angle of episiotomy measured significantly smaller in cases (30 degrees, 95% CI 28-32 degrees) than in controls (38 degrees, 95% CI 35-41 degrees; P < 0.001). Analysis showed a 50% relative reduction in risk of sustaining third-degree tear for every 6 degrees away from the perineal midline that an episiotomy was cut. Conclusions These results show that a larger angle of episiotomy is associated with a lower risk of third-degree tear and mediolateral episiotomy incisions should be made at as large an angle as possible to minimise the risk of sphincter disruption.