Direct mechanical injury. Direct external or internal anal sphincter muscle disruption can occur, as with a clinically obvious third- or fourth-degree perineal laceration or an occult injury subsequently noted on ultrasound.
Neurologic injury. Neuropathy of the pudendal nerve may result from forceps delivery or persistent nerve compression from the fetal head.14 Traction neuropathy may also occur with fetal macrosomia and with prolonged pushing during Stage 2 in successive pregnancies, or with prolonged stretching of the nerve due to persistent poor postpartum pelvic floor tone. Injured nerves often undergo demyelination but usually recover with time.
Combined mechanical and neurologic trauma. Isolated neurologic injury, as described above, is believed to be rare. Neuropathy more commonly accompanies mechanical damage.15
Who is at risk?
Several risk factors are unavoidable. One of these is primiparity, a consistently reported independent variable also associated with other risk factors for obstetric anal sphincter injury, such as instrument delivery (TABLE 2).
Major risk factors for obstetric anal sphincter injury
|RISK FACTOR||ODDS RATIO|
|Short perineal body||8|
|Instrumental delivery, overall||3|
|Forceps vs vacuum||2.88*|
|Forceps with midline episiotomy||25|
|Prolonged second stage of labor (>1 hour)||1.5–4|
|Intrapartum infant factors:|
|Birthweight over 4 kg||2|
|Persistent occipitoposterior position||2–3|
|Previous anal sphincter tear||4|
|All variables are statistically significant at P<.05.|
|*Relative risk of altered fecal symptoms based on RCT findings, vacuum vs forceps.17 Data from randomized controlled trials are lacking for most labor variables. Due to differing methods of analysis (univariate vs regression) and outcome measures, risk ratios reported in the literature vary considerably. This table presents the approximate odds ratios for risk factors that have been reported most consistently from 1 prospective cohort study,16 1 randomized controlled trial,14 and, otherwise higher-quality retrospective analyses.18-23|
Preventing obstetric anal sphincter injury
Sphincter injury can occur even when obstetrical management is optimal. Although evidence from RCT data is often lacking, sufficient observational and retrospective data support the following recommendations to reduce the likelihood of injury.
Choose vacuum delivery before forceps
Any form of instrument delivery increases the risk of obstetric anal sphincter injury and altered fecal continence by between 2- and 7-fold.2,16,24 An RCT found clinical third-degree tears in 16% of women with forceps-assisted deliveries, compared with 7% of vacuum-assisted deliveries; the authors concluded that, when circumstances allow, vacuum delivery should be attempted first (acknowledging however that 23% of vacuum deliveries failed and proceeded to a forceps extraction, a sequence associated with increased injury).17 A meta-analysis confirmed that vacuum extraction is preferred when instrumental delivery is necessary (SOR: A).25
When midline episiotomy was performed during instrument delivery, the risk of obstetric anal sphincter injury approximately doubled again, such that, in one study, forceps delivery with episiotomy caused a 25-fold increase in obstetric anal sphincter injury.24
Any steps that may safely reduce the need for instrument delivery should be supported. Toward this end, the Canadian Clinical Practices Obstetrics committee has recommended evidence-based labor interventions such as one-to-one support in labor, the increased use of a partogram in labor and appropriate oxytocin use, all in an effort to reduce needs for operative interventions.26
If episiotomy necessary, mediolateral less risky than midline
Episiotomy was long promoted as a means of preserving the integrity of the perineal musculature and of avoiding damage to the anal sphincter, and it has been practiced routinely by some.27 Strong evidence now indicates that routine episiotomy (midline or mediolateral) is unhelpful and should be abandoned.25,27-29
Observational evidence overwhelmingly shows that midline episiotomy is strongly associated with obstetric anal sphincter injury.19,22,23,30,31 One of the few RCTs comparing midline with mediolateral episiotomy, although flawed in its design, noted that a clinical third-degree laceration occurred as an extension of episiotomy in 11.6% of midline incisions compared with just 2% of mediolateral cuts.32
Another RCT, designed to examine routine versus restrictive episiotomy, noted that all but 1 (98%) of the 47 third- or fourth-degree lacerations in a group of 700 women followed midline episiotomy.29 A retrospective database analysis noted a 6-fold higher risk of third-degree perineal lacerations for women undergoing midline episiotomy compared with mediolateral incision.23 Elsewhere, midline episiotomy was associated with a 5-fold increase in symptoms of fecal incontinence at 3 months postpartum when compared with women with an intact perineum.24
Even when midline episiotomies do not extend into clinical third-degree lacerations, the incidence of resultant postpartum fecal incontinence triples when compared with spontaneous second-degree perineal lacerations.30 The authors postulate that a perineum cut by midline episiotomy allows for more direct contact to occur between the fetal hard parts and the anal sphincter complex during delivery, thereby increasing occult obstetric anal sphincter injury.