Cauda equina syndrome: saddle numbness and bladder changes
Cauda equina syndrome is compression of the cauda equina (the bundle of nerve roots below the spinal cord), typically causing numbness around the anus with bladder or bowel changes.
Remember
Numbness around the anus and genitals (saddle numbness) with new bladder or bowel changes is the warning picture of cauda equina syndrome, and it needs emergency assessment and urgent MRI without waiting for leg weakness.
The cauda equina (Latin for horse's tail) is the bundle of nerve roots that fans out below the end of the spinal cord. The roots are most often compressed by a large lumbar disc herniation (a prolapsed disc in the lower back), but also by tumours, fractures, infection or a haematoma (a collection of blood).
The lowest roots supply the bladder, the bowel, the anal sphincter (the ring of muscle that keeps the anus closed) and the skin around the anus and genitals. Compression therefore causes saddle anaesthesia, numbness over the parts of the body that would touch a saddle, together with bladder dysfunction, such as urinary retention (being unable to pass urine) or incontinence (urine leaking out). Faecal incontinence (leakage of stool) and leg symptoms on both sides are often present as well.
Painless retention with overflow incontinence is a later bladder sign: once the bladder is overfull, urine leaks out in small amounts. Before that, altered bladder sensation, a poor stream or difficulty starting to pass urine, with saddle numbness, is already incomplete cauda equina syndrome.
Because the damage is to nerve roots, the legs show lower motor neurone signs (damage to the nerve fibres that run from the spinal cord to the muscle): reduced reflexes, not brisk ones.
It is an absolute emergency. Act on the early features with emergency assessment and urgent MRI, before retention develops.
Key facts
- Classic sign
- numbness around the genitals, anus or perineum (the skin between the genitals and the anus).
- Early bladder warning
- altered sensation, a poor stream or difficulty starting to pass urine, with saddle numbness.
- Late bladder sign
- painless retention with overflow incontinence.
- Legs
- reflexes are reduced, not brisk.
- Anal tone
- reduced anal tone (the resting tightness of the anal muscle, judged by a finger in the rectum) can support the diagnosis, but normal tone does not rule the syndrome out.
- Action
- emergency assessment and urgent MRI, without waiting for retention.
Exam traps
- Pain in one leg that eases with rest or sitting fits sciatica (pain down the back of the leg from an irritated nerve root), mechanical back pain (back pain from muscle, ligament or joint strain) or spinal stenosis (narrowing of the spinal canal) better than cauda equina syndrome, which typically affects both legs and brings saddle numbness and bladder or bowel changes.
- Brisk reflexes in both legs point to a problem in the brain or spinal cord, not to cauda equina syndrome, because damaged nerve roots reduce the reflexes.
- Fever with spinal tenderness suggests spinal infection, which can compress the roots, but it is not the characteristic feature of cauda equina syndrome itself.
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