The anal sphincter consists of two concentric rings of muscle fibers, the internal and external sphincters, which are under involuntary and voluntary control respectively. These fibers encircle the anal canal and prevent feces from escaping involuntarily until defecation is desired.
The major cause of most anal sphincter injuries is vaginal childbirth. These Obstetric Anal Sphincter Injuries (OASIS) have been related to several chronic maternal complications including sexual difficulties, anal incontinence, and overall reduced quality of life. It is noted that OASIS rates are on the rising side in many industrialized countries.
Risk Factors for OASIS
- Maternal age
- Fetal head circumference of 35 cm or more
- Birth weight of 4000 g or more
- Prolonged second stage
- Any presentation which does not have the occiput anteriorly
- The use of oxytocin to augment labor
- Prolonged pregnancy of 40 weeks or more
- Previous caesarean delivery
- Advanced maternal age
- Instrumental delivery especially forceps use
- Midline episiotomy
These factors can clearly be classified as modifiable, such as the use of episiotomy and of oxytocin augmentation, and non-modifiable, including maternal and fetal characteristics.
Birth Position and Obstetric Anal Tears
The lithotomy position used in the majority of hospital births in North America is associated with the highest incidence of anal injuries during childbirth. The use of the birth stool and squatting position also has a somewhat higher incidence of OASIS. The reasons for this phenomenon are complex.
It is thought that in some women, at least, the lithotomy position results in enhanced pressure sensation on the perineum which dulls the ability of the patient to regulate her pushing down. This may lead to a too-rapid expulsion of the baby with trauma to the anal sphincter complex. Another reason might be the in this position the sphincter is exposed to greater pressure than with other postures.
Birth Order and Obstetric Anal Tears
Birth order also plays a role in the incidence of obstetric anal injuries. Women undergoing vaginal birth after a Caesarean (VBAC) are found to have the greatest rate of OASIS, followed by nulliparous women and parous women. Only 1.3 percent of women who had already delivered a child had this traumatic complication, but 5.7 percent of nulliparas and 10.6 percent of women who were undergoing VBAC. Even so, the lowest injury rates are persistently found among those who delivered their babies standing up, irrespective of parity. The highest rates occurred in the lithotomy position, as well as an increased risk seen with squatting and the use of the birth seat.
Treatment and Complications
Careful primary repair of the torn sphincter should be done following the childbirth by an experienced caregiver, using absorbable long-lasting sutures. This should be followed by meticulous follow-up at the postpartum visits to elicit any further or worsening symptoms. Follow up is best done by someone who is skilled in this field.
In some patients, the injury is recognized late, and these cases may be dealt with by constipating drugs and physiotherapy. If these fail, and if the sphincter is significantly torn, elective surgical repair is attempted with long-lasting absorbable sutures. In all cases, the sutures will fall over time. For this reason, sacral nerve stimulation is often used to stimulate and strengthen rectal reflexes in these patients.
OASIS leads to anal or fecal incontinence in anywhere between 15 and 60 percent of cases. The prevalence of this complication increases with time in these women.
Women with OASIS often delay their next delivery from apprehension. It is found that a history of diabetes in the first pregnancy is associated with a higher chance of recurrent OASIS, as well as a second pregnancy with a birth weight above 3.5 kg. On the other hand, having a first baby weighing 4 or more kg at birth, with second delivery at 37-38 weeks reduced the chances of recurrence.
Women who underwent a planned Caesarean for their next gestation had the following risk factors;
- History of a fourth degree perineal tear in the previous delivery
- The use of epidural, spinal or general anesthesia
- Country of birth
- Maternal age
This may indicate that these women had a different risk profile from the ordinary population, which accounted for the increased OASIS incidence in this group.