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OverviewAetiologyPathophysiologyClinical features and red flagsInvestigationsDifferential diagnosisManagementComplicationsMonitoring and follow-upSpecial situationsCommon pitfallsQuestionsSources
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Educational draft · awaiting clinical reviewThe full textbook explains uncertainty but does not replace live national or local guidance, specialist advice, or current prescribing information.

Third- and fourth-degree perineal trauma

Obstetrics and gynaecology · UK scope · Sources checked 7 Sept 2026 · Clinical review pending

RapidFull
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Do not miss sphincter or rectal injury

A deep perineal tear may involve the external sphincter, internal sphincter or anorectal mucosa, even when the surface appearance understates the injury.

Action: Explain and perform systematic examination with adequate analgesia, including rectal assessment, seek an experienced obstetrician and arrange repair under appropriate anaesthesia. Escalate heavy bleeding and same-day review for later wound breakdown.

Overview

Identify and classify obstetric anal sphincter injury accurately, organise skilled repair, and support bowel function, wound recovery and future birth counselling.

Obstetric anal sphincter injury, often abbreviated OASI, includes third- and fourth-degree perineal tears. Accurate recognition is important because injury to continence structures needs a different repair from a second-degree tear. The external sphincter contributes voluntary squeeze, while the internal sphincter contributes resting tone. A small external wound can coexist with deeper damage, so inspection alone is insufficient.

Care continues beyond suturing. The woman needs understandable information about the injury, effective analgesia, prevention of hard stools, infection surveillance and a route to discuss continence or sexual symptoms without embarrassment. Most women improve substantially after appropriate repair and healing, but persistent symptoms deserve active investigation. Follow-up should address function and quality of life, not simply whether the skin looks closed.

Key points

  • Grade 3a involves less than half the external anal sphincter thickness; 3b involves more than half.
  • Grade 3c includes both external and internal sphincter injury; fourth degree also involves anorectal mucosa.

Aetiology

01

Mechanical birth injury

Stretching and tearing during vaginal birth can extend from the perineal muscles into the anal sphincter complex. The depth and layers involved determine classification, rather than the visible surface length alone.

02

Risk context

Assisted vaginal birth, a larger baby, shoulder dystocia and first vaginal birth are associated with increased risk. These factors support preventive attention but do not predict all injuries reliably.

Pathophysiology

  1. 1
    External sphincter disruption

    Damage to the external sphincter reduces the integrity of voluntary anal closure. Accurate approximation of the injured muscle aims to restore the continuity needed for functional recovery.

  2. 2
    Internal sphincter injury

    The internal sphincter contributes to resting continence pressure. Its involvement distinguishes grade 3c from injuries limited to the external layer and requires deliberate identification at repair.

  3. 3
    Mucosal breach and contamination

    Fourth-degree injury extends through anorectal mucosa as well as the sphincters. An isolated buttonhole also breaches mucosa, creating a route for contamination and possible fistula if unrecognised.

Clinical features and red flags

Examine systematically after vaginal birth

Explain what examination involves, obtain consent and ensure adequate analgesia, lighting and positioning. Assess the tear's apex, bleeding and involved structures, including digital rectal examination. Refer to a more experienced clinician if the depth or sphincter anatomy is uncertain.

Classify the anatomical damage

Record external sphincter thickness disrupted and whether the internal sphincter or anorectal mucosa is involved. If uncertain whether external injury exceeds half the thickness, classify as 3b rather than underestimate it. Clear operative description supports subsequent counselling.

Look for a rectal buttonhole

An isolated tear through anorectal mucosa with an intact anal sphincter complex is not fourth degree. It can be missed if the clinician relies on perineal appearance or assumes intact sphincters exclude rectal injury; prompt recognition and appropriate repair prevent fistula.

Ask directly about bowel function

At follow-up, ask about urgency, leakage of stool or wind, incomplete evacuation, pain and impact on daily life. Women may not volunteer these symptoms because of embarrassment or assumptions that they are expected after birth.

Red flags requiring action

  • Uncertain sphincter integrity or rectal mucosal involvement requires experienced assessment before routine suturing.
  • Persistent severe pain, fever, offensive discharge or a separating wound needs urgent review; wound breakdown requires same-day maternity assessment.

Investigations

  1. 01
    Clinical perineal and digital rectal examinationFirst step
    Why
    Define the injured layers before planning appropriate repair.
    Interpretation and limitations
    Examination should identify external and internal sphincters and mucosal integrity. Imaging does not replace skilled immediate assessment, and uncertainty should prompt senior review before closing a potentially missed injury.
  2. 02
    Rectal examination after completing repair
    Why
    Check that unintended sutures have not penetrated anorectal mucosa.
    Interpretation and limitations
    If an inadvertently transfixing suture is found, it must be removed and the repair reassessed. Document the examination and ensure instruments and swabs are accounted for.
  3. 03
    Blood count and infection assessment when clinically indicated
    Why
    Evaluate significant blood loss or suspected postoperative infection.
    Interpretation and limitations
    Tests are selected according to bleeding, fever, pain and wound findings rather than requested routinely to grade the tear. Systemic illness or spreading infection needs urgent clinical treatment and escalation.
  4. 04
    Endoanal ultrasound and anal manometry in specialist follow-up
    Why
    Assess persistent symptoms and inform later birth counselling.
    Interpretation and limitations
    Ultrasound evaluates structural defects and manometry assesses function. Results must be interpreted with symptoms and preferences; they are not routine bedside requirements before an acute repair.

Differential diagnosis

01

Second-degree perineal tear

A second-degree tear involves perineal muscles without anal sphincter damage. Thorough examination is necessary because an apparently similar external wound may conceal a deeper injury.

02

Rectal buttonhole tear

Anorectal mucosa can be torn while the anal sphincter complex remains intact. This is documented separately from fourth-degree injury and needs repair to prevent persistent communication with the vagina.

03

Perineal haematoma

Increasing pain and swelling with limited external bleeding may reflect a haematoma. Its assessment and treatment differ from simply adding sutures to a visible tear.

Management

01Immediate assessmentRecognise and arrange skilled repairFirst stepA tear may involve the sphincter complex or anorectal mucosa.+
  1. 1Explain the need for a detailed examination and ensure effective analgesia before assessing the full extent of injury.
  2. 2Classify and document the tear, including internal sphincter involvement and any mucosal defect, and ask for senior help when uncertain.
  3. 3Arrange repair by an appropriately trained clinician or supervised trainee, normally in theatre under regional or general anaesthesia.
  4. 4If bleeding is excessive, control it temporarily as appropriate and expedite theatre while assessing the need for haemorrhage resuscitation.
02Operative principlesRestore each injured layer appropriatelyThe trained operator has defined the injury under suitable anaesthesia.+
  1. 1

Complications

01

Anal incontinence and urgency

Loss of control of wind or stool and urgency can persist despite healing. Direct questioning, specialist assessment and appropriate rehabilitation are necessary because symptoms may otherwise remain hidden.

02

Wound infection or dehiscence

Pain, discharge, fever or separation can signal impaired healing and infection. Wound breakdown needs same-day specialist maternity review, while severe systemic illness requires immediate emergency assessment.

03

Fistula, pain and emotional effects

An unrecognised mucosal defect can lead to a rectovaginal fistula, while pain and fear may affect intimacy and wellbeing. Follow-up should address these consequences alongside physical wound recovery.

Monitoring and follow-up

  • Assess pain, bleeding, swelling, urinary retention and wound appearance during early recovery; explain how and when the woman can obtain help after discharge.
  • Review stool consistency and laxative tolerance, aiming for soft formed stool rather than either constipation or diarrhoea.
  • Ask about continence, urgency, sexual pain and emotional wellbeing at follow-up, with a low threshold for referral when symptoms affect function.
  • Arrange same-day maternity review for wound breakdown and urgent specialist assessment for persistent healing concerns, systemic infection or suspected fistula.

Special situations

Internal sphincter identification matters

A documented 3c injury carries information beyond the percentage of external sphincter torn. Identifying and repairing the internal layer separately helps restore anatomy and explains why careful assessment under good conditions is essential.

Overlap requires free full-thickness ends

A partial tear does not provide two detached ends suitable for a tension-free overlap. End-to-end approximation avoids creating unnecessary tension; for a complete external tear, either recognised approach may be appropriate.

Suture choice follows the tissue

RCOG recommends 3-0 polyglactin for anorectal mucosa. For sphincter muscle, 3-0 PDS or 2-0 polyglactin can be used; burying knots beneath the superficial perineal muscles helps reduce later irritation and migration.

Future caesarean is an option for selected women

Women with persistent symptoms or abnormal endoanal ultrasound or manometry should be counselled about elective caesarean. A previous OASI alone does not create a universal requirement for caesarean in every subsequent pregnancy.

Common pitfalls

  1. 01

    Labelling a deep tear second degree without rectal and sphincter assessment can leave a major injury untreated.

  2. 02

    Calling every mucosal tear fourth degree overlooks the distinct rectal-buttonhole diagnosis when sphincters are intact.

  3. 03

    Routine addition of a bulking agent to lactulose after repair can worsen early bowel symptoms and is not recommended.

  4. 04

    Waiting until the routine six-week review for wound separation or new faecal leakage delays assessment of a potentially serious complication.

Practice

Two practice questions

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Obstetrics and gynaecologyOriginal SBA

Classifying internal sphincter involvement

After an assisted vaginal birth, examination under anaesthesia shows disruption of both the external and internal anal sphincters. The anorectal mucosa is intact. How should the injury be classified?

Choose one answer, then submit
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Sources and review status5 sources · checked 7 Sept 2026 · clinical review pending+
Sources

Sources and review status

National guidance is shown before implementation-dependent detail. Typical adult dose examples remain subject to patient factors, contraindications and the live BNF or specialist protocol. Source check completed 7 Sept 2026; clinical approval remains outstanding.

  • RCOG Green-top 29: management of third- and fourth-degree tears
  • RCOG: care following obstetric anal sphincter injury
  • NICE NG194: perineal healing and urgent wound assessment
  • BNF: lactulose adult dosing
  • NHS inform: paracetamol adult dose and safety

Editorial contact: Doctor’s Passport editorial team. Source-check date is separate from clinical approval. Report a content problem · Content changes

Authoring stateComplete draftClinical stateAwaiting reviewJurisdictionUnited Kingdom
A rectal buttonhole with an intact sphincter complex is a distinct injury, not a fourth-degree tear.
  • When uncertain about third-degree extent, classify to the higher grade and seek senior help.
  • Repair is normally performed in theatre by a trained clinician under regional or general anaesthesia.
  • Use end-to-end repair for partial external sphincter tears; full-thickness tears may use end-to-end or overlap.
  • Provide broad-spectrum antibiotics, a stool-softening laxative and appropriate pain relief after repair.
  • Arrange specialist review at approximately six to twelve weeks, with earlier assessment for symptoms or wound concerns.
    • New faecal urgency, incontinence, inability to control flatus or stool passing vaginally must be actively assessed rather than dismissed as normal recovery.
    Repair torn anorectal mucosa when present using an appropriate continuous or interrupted technique and suitable absorbable suture.
  • 2Identify and repair the internal sphincter separately when torn, without attempting to overlap it.
  • 3Use end-to-end approximation for partial-thickness external tears; either end-to-end or overlap is acceptable for full-thickness external injury.
  • 4Avoid figure-of-eight sutures because their haemostatic effect can compromise tissue perfusion.
  • 5Complete a rectal examination after repair, removing any unintended mucosal suture, and document the reconstruction clearly.
    • 03RecoveryProtect healing and restore functionPrimary repair is complete and the woman is recovering after birth.+
    1. 1Provide broad-spectrum antibiotic cover according to the local OASI protocol and prescribe a stool-softening laxative to avoid straining.
    2. 2Provide appropriate analgesia and bladder care, explain hygiene and discuss gentle pelvic-floor rehabilitation with physiotherapy support.
    3. 3Give clear advice on urgent symptoms, including wound separation, fever, increasing pain or new continence problems.
    4. 4Arrange review usually at six to twelve weeks with clinicians interested in OASI; refer persistent pain or incontinence for specialist gynaecological or colorectal assessment.
    5. 5Counsel about future birth choices using symptoms, previous injury and any specialist investigation results, documenting the discussion.
    Key medicines and prescribing safety3 treatments · regimens, roles and cautions+
    Reduces hard-stool passage and straining that can place tension on the healing repair.

    Lactulose after sphincter repair

    A usual adult starting regimen is 15 mL orally twice daily, adjusted to produce soft formed stools; RCOG recommends stool-softening treatment for about 10 days after repair.

    Reduce the dose if stools become loose or diarrhoea occurs. Avoid routine combination with a bulking agent after OASI; check for suspected obstruction and seek review for significant abdominal pain or persistent bowel difficulty.

    Reduces postoperative infection and wound-dehiscence risk after repair of a contaminated perineal injury.

    Broad-spectrum antibiotics

    Administer the locally specified OASI regimen at repair and postoperatively as directed; drug, route, dose and duration depend on allergy, operative findings, renal function and the protocol.

    A national recommendation for broad-spectrum cover does not identify one mandatory UK regimen. Document doses and stop date, and reassess clinically if infection develops rather than simply extending prophylaxis.

    Provides baseline pain relief to support movement, rest and comfortable bowel function.

    Paracetamol for post-repair pain

    For a usual adult of at least 50 kg, give 500 mg–1 g orally every 4–6 hours as required, with at least 4 hours between doses and a maximum of 4 g in 24 hours.

    Use a lower individual maximum when low weight, liver disease, malnutrition or other risk factors require it. Avoid duplicate paracetamol-containing medicines, reassess ongoing need and investigate worsening pain rather than only increasing analgesia.