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This is a clinician-written, evidence-based summary aligned to the 2026 MLA Content Map. It is intended for medical students and junior doctors preparing for the UKMLA. Always cross-reference with NICE guidance, local protocols, and clinical judgement.
Key points
- Cauda equina syndrome (CES) = compression of cauda equina nerve roots below the conus medullaris (L1/L2 level)
- Red flag features: bilateral leg weakness/sciatica, saddle anaesthesia (S2-S4), bladder dysfunction (retention or incontinence), loss of anal tone
- Most common cause: large central lumbar disc prolapse (usually L4/L5 or L5/S1)
- EMERGENCY MRI lumbar spine — ideally within hours
- Urgent surgical decompression within 24–48 hours of onset of bladder symptoms for best outcome
Overview
Cauda equina syndrome (CES) results from compression of the nerve roots forming the cauda equina (below the level of the conus medullaris at L1/L2). Unlike spinal cord compression, CES produces lower motor neurone signs because the nerve roots (not the cord) are compressed. The most common cause is a large central disc prolapse, typically at L4/L5 or L5/S1. Other causes include spinal canal stenosis, tumour (primary or metastatic), infection (epidural abscess, discitis), trauma, and haematoma. CES is classified as incomplete (CES-I, early — urinary difficulties without retention) or complete (CES-R, retention — painless urinary retention with overflow). Outcomes are significantly worse if decompression is delayed after onset of retention.
Epidemiology
CES occurs in approximately 2–6% of lumbar disc herniations. It is a rare but devastating condition if missed. Peak incidence is 30–50 years (age group for disc prolapse). Risk factors include heavy lifting, prior lumbar disc disease, and lumbar spine surgery. It is one of the most common causes of medicolegal claims in orthopaedic and neurosurgical practice due to delayed diagnosis.
Clinical Features
Symptoms
Bilateral leg pain (sciatica) or bilateral leg weakness
Saddle anaesthesia: numbness in the perineum, perianal area, inner thighs (S2-S4 dermatomes)
Bladder dysfunction: difficulty passing urine, urinary retention, or urinary incontinence
Bowel dysfunction: faecal incontinence, loss of awareness of rectal fullness
Sexual dysfunction: loss of genital sensation, erectile dysfunction
Progressive or severe low back pain
Signs
LMN signs in legs: reduced reflexes (ankle jerks), flaccid weakness, wasting (contrast with UMN in cord compression)
Reduced or absent perianal sensation (test with pinprick)
Reduced anal tone on digital rectal examination
Palpable bladder (painless urinary retention with overflow)
Bilateral straight leg raise positive
Investigations
First-line
Emergency MRI lumbar spineGold standard — shows the level and cause of compression. Must be performed URGENTLY if CES suspected. Include whole lumbar spine and conus
Post-void residual bladder volumeBladder scan — if >200 mL suggests urinary retention from CES. Catheterise
Second-line
BloodsFBC, CRP, ESR (infection/abscess), coagulation (epidural haematoma), PSA and myeloma screen if malignancy suspected
Specialist
CT myelogramIf MRI contraindicated (e.g. non-MRI-compatible pacemaker) — invasive but may demonstrate compression
1
Urgent surgical decompression
- Surgical discectomy/decompression ideally within 24–48 hours of onset of bladder symptoms
- CES-I (incomplete — urinary difficulties without retention): urgent surgery within 24 hours. Better prognosis
- CES-R (retention — painless overflow): surgery as soon as possible. Worse prognosis but still indicated urgently
- Contact on-call spinal surgery team immediately
2
Immediate management
- Catheterise if urinary retention — record residual volume
- Analgesia: avoid excessive opioids that may confuse assessment
- Document neurological examination in detail (leg power, sensation, reflexes, perianal sensation, anal tone)
- If MSCC suspected as cause: add dexamethasone 16 mg
3
Post-operative rehabilitation
- Physiotherapy for leg weakness
- Bladder management: may need long-term catheterisation if function does not recover
- Bowel management programme
- Sexual dysfunction counselling
- Psychology support — significant impact on quality of life
Complications
- Permanent bladder dysfunction: If decompression delayed — may require permanent catheterisation
- Persistent leg weakness: Some patients do not fully recover motor function
- Chronic pain: Neuropathic pain in legs and perineum
- Sexual dysfunction: Erectile dysfunction, loss of genital sensation — often permanent if CES-R
- Bowel dysfunction: Faecal incontinence or chronic constipation
- Medicolegal implications: Delayed diagnosis is a common cause of litigation — always document red flag assessment
UKMLA Exam Tips
- 1Bilateral leg symptoms + saddle anaesthesia + bladder dysfunction = CES until proven otherwise
- 2CES = LMN signs (reduced reflexes, flaccid weakness). Cord compression = UMN signs (hyperreflexia, upgoing plantars)
- 3Ask every patient with back pain about bladder function, bowel function, and saddle anaesthesia
- 4EMERGENCY MRI lumbar spine — do NOT delay
- 5CES-I (incomplete, early) has better prognosis than CES-R (retention) — hence urgency of early detection
- 6Always perform a digital rectal examination and document anal tone
- 7Most common cause: large central lumbar disc prolapse at L4/L5 or L5/S1
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