Anal sphincter injury

Last revised by Daniel J Bell on 22 Feb 2024

Anal sphincter injury is a form of perineal trauma that can involve internal or external anal sphincters and may extend to the anorectal mucosa in severe cases.

This article is focusing on the most common type of anal sphincter injury that is associated with vaginal delivery, and represents third and fourth-degree perineal tears, which is referred to as obstetric anal sphincter injury (OASI) 7.

Perineal injuries may occur secondary to vaginal delivery, sexual activity, external pelvic trauma or anorectal surgeries 1,3,5.

Vaginal delivery is the most common cause of anal sphincter injury in women 2,3. About 85% of vaginal deliveries are associated with mild perineal injury 8, while 10% are associated with severe injuries 4.

Anal sphincter injury can be asymptomatic but usually presents with fecal urgency, fecal incontinence, flatus incontinence or dyspareunia.

High-risk factors associated with obstetric anal sphincter injuries can be categorized as follows 3,4,8:

  • maternal causes: nulliparity, Asian ethnicity and short perineum

  • fetal causes: macrosomia (>4 kg) and persistent occipitoposterior position

  • intrapartum causes: instrumental delivery (e.g. forceps), epidural anesthesia, induced labor, prolonged second stage of labor (>1 hour), and episiotomy (midline and mediolateral incisions)

The internal anal sphincter is mainly responsible for anal closure and continence during rest, while the external anal sphincter plays the main role during increased intra-abdominal pressure e.g. during coughing 4.

Perineum stretching during vaginal delivery can cause anal sphincter injury by direct trauma to the muscles or injury to the pudendal nerve

The Sultan classification is widely used (c. 2023) to grade obstetric perineal trauma 6,7:

  • first degree: laceration of vaginal mucosa or perineal skin only

  • second degree: involvement of the perineal muscles without the anal sphincter

  • third degree: tear of the anal sphincter muscles

    • 3a: <50% thickness of external anal sphincter (EAS)

    • 3b: >50% thickness of EAS

    • 3c: torn EAS and internal anal sphincter (IAS)

  • fourth degree: a third degree tear extending to anal mucosa

N.B. obstetric anal sphincter injury (OASI) correlates to third and fourth degree tears 7. Perineal trauma not included in the Sultan classification includes labial laceration, periurethral tear, vaginal sidewall tear, cervical tear, etc 7.

Clinical assessment at the time of vaginal delivery plays a major role in the early diagnosis of OASIS and imaging is usually used for late-onset presentations 3.

Different techniques of ultrasound can be used to evaluate the anal canal for sphincter disruption or discontinuity, in transverse and longitudinal orientations. IAS appears hypoechoic while EAS appears hyperechoic. 3D techniques can be used for volume measurements and multiplanar reconstructions 9. Patients can be placed in left lateral or lithotomy positions.

  • transrectal (endoanal): considered the gold standard imaging modality in the evaluation of anal sphincter injury 3,8

  • transvaginal (endovaginal): the transducer is placed in the fourchette of the vaginal introitus 10

  • transperineal (perianal): done using high-frequency transducers

Both endoluminal and external phased-array coils can be used. Anal sphincter injury appears as hypointense disruption of the circular sphincter 13.

Treatment can be conservative or surgical depending on the degree of injury and presentation of the patient. Conservative management may include zinc-aluminum ointment and pelvic floor physical therapy 3. Third and fourth-degree perineal tears should be surgically corrected to regain anal continence 3,8. Repair techniques include end-to-end and overlap repairs 3,8. Sacral nerve modulation can be used to treat fecal incontinence whether due to anal sphincter injury or pudendal nerve injury 3.

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