Perineal Tears & OASI
Summary
Perineal trauma affects 85% of women having a vaginal birth. While most tears are minor (1st/2nd degree), severe tears involving the anal sphincter complex (Obstetric Anal Sphincter Injuries - OASI) occur in 3-6% of Primiparous women. OASI (3rd/4th degree tears) are the leading cause of anal incontinence in women and represent a major medicolegal risk if missed or poorly repaired. [1,2]
Classification (RCOG)
- 1st Degree: Injury to perineal skin and vaginal mucosa only.
- 2nd Degree: Injury to perineal muscles (but NOT anal sphincter). Includes Episiotomy.
- 3rd Degree: Injury to anal sphincter complex:
- 3a: less than 50% of External Anal Sphincter (EAS) torn.
- 3b: >50% of EAS torn.
- 3c: Internal Anal Sphincter (IAS) torn.
- 4th Degree: Injury to anal sphincter complex AND anal epithelium (rectal mucosa).
Clinical Pearls
The "Buttonhole" Tear: Sometimes the rectal mucosa tears (4th degree) but the sphincter remains intact (or appears so). This is an isolated rectal buttonhole. Always PR to check for mucosal defects.
Episiotomy: A Right Mediolateral Episiotomy (60 degree angle) is protective against OASI in operative deliveries (forceps). Midline episiotomies (US practice) are a huge risk factor for OASI and are avoided in the UK.
The PR Exam: A rectal examination is MANDATORY after every vaginal delivery to exclude OASI. Missing a 3rd degree tear is negligent.
Incidence of OASI
- Primiparous: 6% (Forceps), 3% (Spontaneous).
- Multiparous: less than 2%.
Risk Factors for OASI
- Primiparity.
- Large Baby (>4kg).
- Shoulder Dystocia.
- Operative Vaginal Delivery: Forceps > Ventouse.
- Midline Episiotomy.
- Persistent Occipito-Posterior (OP) position.
Anatomy
- External Anal Sphincter (EAS): Striated muscle. Voluntary control. "Squeeze" pressure.
- Internal Anal Sphincter (IAS): Smooth muscle. Involuntary control. "Resting" tone (keeps you clean at rest).
- Damage to IAS is more strongly associated with incontinence (soiling) than EAS damage.
Identification
Systematic Assessment
- Inspection: Adequate light. Part labia.
- Palpation: Palpate perineal body thickness.
- PR Examination (Digital Rectal Exam):
- Feel for sphincter tone.
- Feel for mucosal defect.
- "Pill-rolling" motion to feel integrity of sphincter ring.
Intrapartum
- Clinical Examination is the diagnostic tool.
Post-Partum (Follow up)
- Endoanal Ultrasound: Gold standard to assess sphincter defects in women with symptoms.
- Manometry: Assess resting/squeeze pressures.
Management Algorithm
VAGINAL DELIVERY COMPLETE
↓
SYSTEMATIC PERINEAL & PR EXAM
↓
TEAR IDENTIFIED?
┌─────────┴─────────┐
NO YES
↓ ↓
ROUTINE CARE CLASSIFY
┌───┴───┐
1st/2nd 3rd/4th (OASI)
↓ ↓
MIDWIFE OBSTETRICIAN
REPAIR IN THEATRE
(Room) (Regional/GA)
Repair Techniques (RCOG Guideline)
- 1st/2nd Degree:
- Repaired in room under local infiltration.
- Continuous suture (Vicryl Rapide) is better than interrupted (less pain).
- 3rd/4th Degree (OASI):
- Setting: Operating Theatre. Good light. Regional (Spinal) or GA.
- Antibiotics: Broad spectrum IV intra-op.
- EAS Repair: Overlapping or End-to-End approximation (using PDS or Vicryl).
- IAS Repair: End-to-End (mattress suture).
- Mucosa: Repair anal mucosa first (Vicryl).
Post-Operative Care (The "OASI Bundle")
- Broad Spectrum Antibiotics (5-7 days).
- Laxatives: (Lactulose + Fybogel) to keep stools soft. Avoid constipation (straining rips stitches).
- Analgesia: (Paracetamol/NSAIDs). Avoid Codeine (causes constipation!).
- Physiotherapy: Pelvic floor exercises after healing.
- Pain: Perineal pain / Dyspareunia (painful sex).
- Infection: Wound breakdown.
- Incontinence:
- Flatus (Gas) incontinence (most common).
- Urgency (can't hold on).
- Pasive soiling.
- Rectovaginal Fistula: Hole between rectum and vagina. Faeces leaking from vagina. Needs surgical repair.
- Vaginal Birth After OASI:
- If asymptomatic + good repair: Can have vaginal birth (risk of recurrence 5-7%).
- If symptomatic (incontinent) or psychological trauma: Offer Elective Caesarean Section.
Key Guidelines
| Guideline | Organisation | Key Recommendations |
|---|---|---|
| GTG 29 | RCOG | "Management of Third- and Fourth-Degree Perineal Tears". (The Bible for this topic). |
| OASI Bundle | RCOG/RCM | Care bundle to reduce OASI rates (Hands-on protection, Episiotomy angle). |
Landmark Evidence
1. The Hands-On vs Hands-Off Trial
- Conflicting evidence, but RCOG suggests "Hands-on" (supporting the perineum during crowning) reduces severe tears.
What is a tear?
During birth, the skin and muscle between the vagina and back passage (perineum) stretches. Sometimes it tears.
- Minor tears (1st/2nd): Like a graze or muscle cut. Heals well with stitches.
- Severe tears (3rd/4th): The tear extends into the muscle ring that controls your bowel (sphincter).
Will I be incontinent?
Most women (60-80%) recover full function after proper repair. Some may have trouble holding wind. We give you physio to help strengthen the muscle.
Why do I need laxatives?
We don't want you straining on the toilet, as that pulls on the stitches. We want your poo to be soft like toothpaste so it passes easily.
Can I have another baby naturally?
If you heal well and have no leakage, yes. If you have problems, a Caesarean might be safer next time.
Primary Sources
- RCOG Green-top Guideline No. 29. The Management of Third- and Fourth-Degree Perineal Tears. 2015.
- RCM/RCOG. The OASI Care Bundle. 2018.
Common Exam Questions
- Classification: "Tear involving Internal Anal Sphincter?"
- Answer: 3c.
- Management: "Analgesia to AVOID?"
- Answer: Codeine/Opiates (Constipation risk). Use NSAIDs.
- Technique: "Correct angle for Episiotomy?"
- Answer: 60 degrees from midline (mediolateral).
- Next Preg: "Symptomatic woman post-OASI?"
- Answer: Elective C-Section.
Viva Points
- Overlap vs End-to-End: For full EAS tears, overlap repair has historically shown slightly better results, but guidelines say both are acceptable. For partial tears, only end-to-end is possible.
- Vicryl Rapide vs PDS: Rapide dissolves in 2 weeks (good for skin). PDS lasts months (good for sphincter muscle which heals slowly).
Medical Disclaimer: MedVellum content is for educational purposes and clinical reference. Clinical decisions should account for individual patient circumstances. Always consult appropriate specialists.