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Cauda equina syndrome
Compression of the nerve roots below the end of the spinal cord causes bilateral radicular pain, saddle sensory loss and bladder and bowel dysfunction; it is an emergency that needs immediate MRI and urgent surgical decompression, before complete urinary retention develops.
Why it matters in the exam
The typical stem describes a patient with known low back pain who has developed pain radiating to both legs over recent days, numbness of the perineum and buttocks, difficulty starting to pass urine or a feeling that the bladder does not empty, and sometimes reduced sexual function or bowel control. The usual cause is a large central disc herniation at L4–L5 or L5–S1; other causes are tumour, epidural abscess, haematoma and fracture. The key exam distinction is between two stages: incomplete syndrome, with altered sensation and bladder function but preserved voluntary control of voiding, and syndrome with retention, in which the bladder is full and painless, sometimes with overflow. The prognosis is much better when surgery is done at the incomplete stage, so any suspicion calls for urgent MRI, the same day and at any hour, not observation. On examination check perineal sensation, anal sphincter tone and post-void residual volume by bladder ultrasound; a high residual supports the diagnosis, but a normal residual does not exclude it. Early symptoms are often vague, so every patient with back pain and sciatica should receive written advice on the warning signs and the need to seek care at once if they appear. A patient with complete syndrome may be left with permanent bladder, bowel and sexual dysfunction even after surgery.
Key points
- Compression of the cauda equina roots — an emergency.
- Bilateral sciatica, saddle sensory loss and bladder disturbance.
- The usual cause: a large central disc herniation.
- Urgent MRI and surgical decompression, ideally before urinary retention.
- Written warning-sign advice for every patient with sciatica.
Summary table
| Stage | Findings | Prognosis | Action |
|---|---|---|---|
| Early suspicion | Bilateral radicular pain, vague change in sensation or voiding | Good if recognised in time | Urgent MRI the same day |
| Incomplete syndrome (CES-I) | Saddle sensory loss, difficulty voiding but control preserved | Relatively good | Urgent surgical decompression |
| Syndrome with retention (CES-R) | Painless urinary retention, overflow, reduced anal tone | Worse; permanent damage possible | Urgent surgical decompression |
| Non-disc causes | Tumour, epidural abscess, haematoma, fracture | Depends on the cause | MRI, treat the cause and decompress |
| After surgery | Residual bladder, bowel or sexual dysfunction | Variable | Rehabilitation, urology and follow-up |
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Sources
- NICE Guideline NG59, 2016 (updated 2020) — Low back pain and sciatica in over 16s
- GIRFT National Suspected Cauda Equina Syndrome Pathway (NHS England), 2023 — Suspected cauda equina syndrome
- Campbell's Operative Orthopaedics, 14th ed. (2021) — Lower Back Pain and Disorders of Intervertebral Discs
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.