Obstetric Anal Sphincter Injury (OASI): Education and General Advice

What is an OASI?

An Obstetric Anal Sphincter Injury (OASI) refers to a third- or fourth-degree perineal tear sustained during vaginal birth that involves one or both of the anal sphincter muscles. These muscles are essential for bowel continence.

OASI can occur in any vaginal birth, but risk increases with:

  • Instrumental birth (forceps or vacuum)

  • Prolonged or difficult second stage of labour

  • Episiotomy, particularly with instruments

  • Larger babies or less-optimal fetal positioning

Understanding the anatomy and why OASI matters

Between the vagina and anus lies the perineum. Bowel control depends on two sphincter muscles:

Internal Anal Sphincter (IAS)

  • Involuntary muscle

  • Provides approximately 70–80% of resting continence

  • Works automatically to keep the anus closed

  • If injured, can result in passive soiling (leakage without warning)

External Anal Sphincter (EAS)

  • Voluntary muscle

  • Provides approximately 20–30% of resting tone

  • Becomes critical when stool enters the rectum and continence is consciously required

  • Can be retrained and strengthened with rehabilitation

Damage to either muscle can affect continence, urgency, and confidence.

Classification of OASI

  • 3A: Less than 50% of the external anal sphincter involved

  • 3B: More than 50% of the external anal sphincter involved

  • 3C: External and internal anal sphincters involved

  • 4th degree: Anal sphincters and anal lining (mucosa) involved

The extent of muscle involvement influences recovery needs and long-term management.

What typically happens after birth

  • Surgical repair is performed soon after birth, often in theatre.

  • Initial tissue healing occurs over 6–8 weeks, though neuromuscular recovery takes longer.

  • Early symptoms may include soreness, stinging, tightness, heaviness, or altered bowel sensations.

  • Emotional distress, shock, or grief about the birth experience is common and valid.

Early follow-up with a pelvic health physiotherapist is recommended for all OASI cases.

Bowel management after OASI

Stool consistency

Maintaining optimal stool consistency is critical.

  • Aim for Bristol Stool Type 4

    • Soft, formed, easy to pass

  • Avoid:

    • Hard stools (increase straining and pressure)

    • Loose stools (increase urgency and continence demand)

This may require adjustment of:

  • Fibre intake

  • Stool softeners or laxatives

  • Hydration

Consistency is more important than frequency.

Toileting habits

  • Limit time on the toilet to no more than 5 minutes

  • Support your perineum with a small amount of toilet paper around your hand and gentle upward pressure

  • If unable to empty, stand up and try again later

  • Avoid straining or prolonged sitting

  • Respond to natural bowel urges rather than delaying excessively

Poor toileting habits can increase pelvic floor load and delay recovery.

Common symptoms to monitor

Symptoms may improve with time and rehabilitation but should be monitored:

  • Urgency to open bowels

  • Difficulty holding wind or stool

  • Sensation of incomplete emptying

  • Perineal, anal, or pelvic heaviness

  • Pain, stinging, or tightness

Early assessment allows intervention before symptoms become persistent.

Pelvic floor rehabilitation principles

Recovery after OASI is not only about strengthening.

Early focus

  • Restore coordination between contraction and full relaxation

  • Reduce fear-based muscle guarding

  • Improve awareness and confidence in bowel control

Exercise approach

  • Begin with gentle anal sphincter contractions followed by complete relaxation

  • Avoid long or maximal holds early

  • Incorporate diaphragmatic breathing to support nervous system regulation

  • Progress strength work gradually and symptom-guided

Over-gripping or excessive strengthening too early can worsen pain and heaviness.

Sexual function after OASI

  • Fear of pain or re-injury is common

  • Return to intercourse should be gradual and pressure-free

  • Scar sensitivity and muscle tension are treatable with physiotherapy

  • There is no timeline that must be met

Emotional and psychological recovery

OASI can be physically and emotionally traumatic.

Common experiences include:

  • Anxiety around bowel control

  • Hyper-vigilance to bodily sensations

  • Loss of trust in the body

Support from GPs, pelvic health clinicians, and mental health professionals can be an important part of recovery.

Key take-home messages

  • OASI affects muscles critical for bowel continence, but recovery is possible

  • Stool management and toileting habits are foundational

  • Pelvic floor rehab should prioritise coordination and relaxation before strength

  • Emotional recovery is as important as physical healing

  • Early, specialised follow-up improves long-term outcomes

Kathryn WarrOASI, pelvic floor physiotherapy, Obstetric Anal Sphincter Injury