Medicine – Cauda Equina Lesions

Cauda equina lesions result from compression or damage to the bundle of lumbosacral nerve roots below the termination of the spinal cord. The cauda equina contains nerve roots that supply the lower limbs as well as sensory and autonomic fibres controlling the bladder, bowel, and sexual function.

Severe compression produces cauda equina syndrome (CES), which is a neurological and spinal surgical emergency because delayed decompression may result in permanent weakness, sensory loss, and bladder or bowel dysfunction.

1. Anatomy of the Cauda Equina

In adults, the spinal cord usually terminates around the L1–L2 vertebral level as the conus medullaris.

Below this level, the lumbar, sacral, and coccygeal nerve roots descend within the spinal canal before leaving through their respective foramina.

This collection of nerve roots resembles a horse’s tail and is therefore called the:

Cauda equina.

2. Nature of the Neurological Lesion

The cauda equina consists of peripheral nerve roots, so damage generally produces lower motor neurone-type abnormalities rather than the upper motor neurone signs expected from spinal cord compression.

Patients may therefore develop:

Flaccid weakness.

Reduced muscle tone.

Reduced or absent tendon reflexes.

Muscle weakness in a nerve-root distribution.

However, the exact findings depend on which roots are compressed.

3. Bilateral Leg Weakness

Cauda equina compression can produce weakness of both lower limbs.

The weakness may be asymmetric, especially early in the disease, because individual nerve roots can be affected to different degrees.

Therefore, although bilateral weakness is characteristic of extensive cauda equina compression:

Cauda equina weakness does not have to be perfectly symmetrical.

4. Weakness Most Marked at the Ankles

Weakness may be particularly prominent distally because the lower lumbar and sacral nerve roots supplying the ankle and foot can be heavily affected.

Patients may have difficulty with:

Ankle dorsiflexion.

Ankle plantarflexion.

Toe movements.

This may produce:

Foot drop.

Difficulty walking on the heels.

Difficulty walking on the toes.

The exact pattern depends on the affected nerve roots.

5. Sensory Loss and Numbness

Patients frequently develop numbness, tingling, or reduced sensation in the lower limbs.

Because multiple nerve roots may be involved, sensory abnormalities can occur in several dermatomes rather than following a single peripheral nerve.

One of the most important patterns is sensory loss in the sacral distribution.

6. Saddle Anaesthesia

Compression of the lower sacral roots can cause sensory loss around the:

Perineum.

Perianal region.

Genital region.

Inner thighs.

This distribution corresponds approximately to the area that would contact a saddle and is therefore called:

Saddle anaesthesia.

This is a major warning sign of cauda equina syndrome.

7. Sacral Sensory Loss

The original description that sensory loss is most marked in the sacral region refers particularly to involvement of the sacral nerve roots.

Patients should be asked specifically about:

Numbness around the anus.

Altered sensation when wiping after using the toilet.

Perineal or genital numbness.

These symptoms can be more clinically important than ordinary leg numbness.

8. Loss of Bladder Control

Damage to the sacral autonomic roots can interfere with bladder function.

A particularly concerning feature is:

Difficulty initiating urination or urinary retention.

The patient may lose the normal sensation of bladder filling.

As retention progresses, an overfilled bladder may eventually produce overflow urinary incontinence.

Therefore, urinary dysfunction in cauda equina syndrome is not simply incontinence.

A particularly important progression is:

Reduced bladder sensation → difficulty voiding → urinary retention → overflow incontinence.

9. Bowel Dysfunction

Sacral nerve-root compression can also interfere with bowel control.

Patients may develop:

Reduced sensation of rectal fullness.

Constipation.

Reduced anal sphincter control.

Faecal incontinence in severe disease.

These findings suggest significant sacral nerve-root dysfunction.

10. Sexual Dysfunction

The sacral nerve roots also participate in sexual function.

Cauda equina syndrome may therefore produce:

Erectile dysfunction.

Reduced genital sensation.

Other disturbances of sexual function.

This can provide another clue to sacral nerve-root involvement.

11. Lower Motor Neurone Signs

Because the cauda equina consists of nerve roots rather than spinal cord tissue, the affected legs may demonstrate:

Reduced tone.

Reduced or absent reflexes.

Flaccid weakness.

For example, involvement of the S1 nerve roots may reduce or abolish the ankle jerk.

12. Radicular Pain

Severe low-back pain with radicular leg pain commonly accompanies cauda equina compression.

Pain may radiate down one or both legs according to the affected nerve roots.

However, absence of severe pain does not completely exclude cauda equina syndrome.

13. Central Lumbar Disc Prolapse

A large central lumbar intervertebral disc prolapse is one of the most important causes of acute cauda equina syndrome.

A small posterolateral disc prolapse may compress only one nerve root and cause ordinary sciatica.

In contrast, a sufficiently large central disc prolapse can compress multiple cauda equina roots simultaneously.

Therefore:

Large central lumbar disc prolapse → multiple root compression → cauda equina syndrome.

14. Degenerative Spondylolisthesis

The original note uses the term spondylolithiasis, but the appropriate term here is usually spondylolisthesis.

Spondylolisthesis means displacement of one vertebra relative to another.

Degenerative changes can narrow the spinal canal and compress the cauda equina, particularly when associated with spinal stenosis.

15. Tumours

Tumours can cause cauda equina syndrome by compressing the lumbosacral nerve roots.

Compression may result from:

Metastatic disease.

Primary spinal tumours.

Tumours involving vertebral structures.

Epidural masses.

Tumours arising around the nerve roots.

Therefore, the important concept is mechanical compression of the cauda equina, rather than assuming that all tumours are necessarily external to the spinal canal.

16. Spinal Stenosis

Lumbar spinal stenosis is narrowing of the spinal canal, commonly due to degenerative changes.

Causes of narrowing may include:

Facet-joint hypertrophy.

Ligamentous thickening.

Disc degeneration or bulging.

Spondylolisthesis.

Severe stenosis can compress multiple cauda equina nerve roots.

Chronic lumbar stenosis more commonly causes neurogenic claudication, but severe compression can occasionally produce cauda equina syndrome.

17. Other Important Causes

Although not included in the original list, other clinically important causes include:

Spinal epidural abscess.

Spinal epidural haematoma.

Severe spinal trauma.

Postoperative or procedural complications.

These are important because some can produce rapidly progressive compression requiring emergency treatment.

18. Red-Flag Symptoms

The combination of back or radicular pain with new neurological abnormalities should raise concern for cauda equina syndrome.

Particularly important red flags are:

New urinary retention or impaired bladder sensation.

Saddle or perineal sensory loss.

New bowel dysfunction.

Bilateral or progressive leg weakness.

Sexual dysfunction.

These findings require urgent assessment.

19. Investigation

When cauda equina syndrome is suspected, the key investigation is generally an urgent MRI of the lumbosacral spine.

MRI can identify:

Large disc prolapse.

Spinal stenosis.

Tumour.

Epidural abscess or haematoma.

Other compressive lesions.

Bladder assessment, including measurement of post-void residual volume, can provide useful additional information but does not replace appropriate neurological assessment and imaging.

20. Treatment

Management depends on the underlying cause, but compressive cauda equina syndrome generally requires urgent specialist spinal assessment and decompression when indicated.

Examples include:

Surgical decompression of a large disc prolapse.

Treatment of spinal tumour compression.

Drainage and antimicrobial therapy for an epidural abscess.

Management of an epidural haematoma.

The aim is to relieve nerve-root compression before irreversible neurological damage develops.

21. Cauda Equina Lesions – Note Form

Site: lumbosacral nerve roots below the spinal cord.

Motor: bilateral or asymmetric lower-limb weakness.

Type of weakness: lower motor neurone pattern.

Distal weakness: may be prominent at the ankles and feet.

Reflexes: reduced or absent depending on the affected roots.

Sensation: lower-limb numbness with important sacral/perineal sensory loss.

Classic sensory sign: saddle anaesthesia.

Bladder: impaired bladder sensation and urinary retention are particularly important.

Late bladder manifestation: overflow incontinence may occur.

Bowel: impaired bowel sensation/control ± faecal incontinence.

Sexual function: may be impaired.

Disc cause: large central lumbar disc prolapse.

Degenerative cause: lumbar spinal stenosis ± degenerative spondylolisthesis.

Tumour: may compress the cauda equina.

Other emergencies: epidural abscess, epidural haematoma and major trauma.

22. Cauda Equina Syndrome versus Simple Sciatica

Simple radiculopathy/sciatica usually involves one or a small number of nerve roots and commonly produces unilateral radicular pain ± focal weakness or sensory loss.

In contrast, cauda equina syndrome involves multiple lumbosacral roots and may produce:

Saddle anaesthesia + bladder/bowel dysfunction + bilateral or progressive neurological deficits.

These autonomic and sacral sensory findings make cauda equina syndrome much more concerning.

Key Clinical Pattern

Think of cauda equina syndrome as:

Multiple lumbosacral nerve-root compression → LMN leg weakness + saddle anaesthesia + sphincter/autonomic dysfunction.

The classic high-yield combination is:

Back/radicular pain + bilateral or progressive leg weakness + saddle anaesthesia + urinary dysfunction.

Important causes are:

Large central lumbar disc prolapse + severe spinal stenosis/spondylolisthesis + tumour + epidural abscess or haematoma.

Most importantly:

New urinary retention or impaired bladder sensation with saddle anaesthesia is an emergency pattern requiring urgent assessment for cauda equina compression.