Cauda Equina Syndrome
Compression of the lumbosacral nerve roots below the spinal cord causes bilateral sciatica, saddle anaesthesia and bladder or bowel dysfunction together, because they share one anatomical compartment, and this is a time-critical surgical emergency.
In a nutshell
Cauda equina syndrome is compression of the lumbosacral nerve roots below the spinal cord, usually by a large central disc prolapse. Because sacral roots serving bladder, bowel and saddle sensation share the same compartment as leg roots, compression causes them all together, and because root ischaemia becomes irreversible with time, this is a surgical emergency needing same-day MRI and decompression.
Classic presentation
Bilateral sciatica with new saddle numbness, urinary retention or altered bladder sensation, and reduced anal tone.
Key points
- The spinal cord ends at L1/L2; below this the canal contains a bundle of nerve roots (the cauda equina), so compression gives a lower motor neurone, multi-root pattern, not an upper motor neurone one.
- Sacral roots (S2-S4) carry bladder, bowel and saddle sensation and run alongside the leg roots, which is why bladder or bowel dysfunction and saddle anaesthesia accompany bilateral leg symptoms.
- Time to decompression drives outcome: sustained compression causes root ischaemia that becomes irreversible, so this is treated as a same-day surgical emergency.
- Digital rectal examination and a post-void bladder scan give objective evidence of sacral root function when the history is ambiguous.
- Early cauda equina syndrome can present with bilateral sciatica alone before bladder symptoms appear: a low threshold for MRI is essential.
First-line investigation
Emergency MRI of the whole spine, obtained same-day, to confirm and localise the compressive lesion.
First-line management
Immediate emergency referral to the on-call spinal or neurosurgical team for urgent MRI and, if confirmed, emergency surgical decompression: do not wait for a routine outpatient pathway.
Exam traps
- Do not wait for bladder or bowel symptoms to appear before acting: bilateral sciatica with saddle numbness alone is enough to trigger emergency imaging.
- A normal urge to void does not exclude retention; check a post-void residual.
- This is never an outpatient or 'urgent but routine' referral: any red flag mandates same-day MRI.
- Unilateral sciatica with normal perianal sensation and no sphincter disturbance is not cauda equina syndrome; do not over-call it.
Educational content pending clinical review. Not medical advice.