Cauda Equina Syndrome: A High-Yield PLAB 2 & MSRA Guide
- Ann Augustin
- 10 hours ago
- 4 min read
What is Cauda Equina Syndrome?
Cauda Equina Syndrome (CES) is a spinal emergency caused by compression of the cauda equina nerve roots, which lie below the end of the spinal cord.
The spinal cord ends at approximately the L1-L2 vertebral level as the conus medullaris. Below this level, the spinal canal contains a bundle of nerve roots known as the cauda equina ("horse's tail").
These nerve roots control:
Bladder function
Bowel function
Sexual function
Sensation around the perineum and genitals
Movement and sensation of the lower limbs
Failure to recognize CES promptly can result in permanent neurological disability.
Relevant Anatomy
Intervertebral Disc Structure
Each intervertebral disc consists of:
Annulus Fibrosus
Tough outer fibrous ring
Nucleus Pulposus
Soft gel-like centre
In a disc prolapse, the annulus fibrosus tears, allowing part of the nucleus pulposus to protrude backwards into the spinal canal.
The prolapsed disc material may compress:
Individual nerve roots (causing sciatica)
Multiple cauda equina nerve roots (causing CES)
Clinical Correlation
The spinal cord ends at approximately L1-L2 as the conus medullaris. Below this level, the spinal canal contains only the cauda equina nerve roots.
Therefore:
Cervical and thoracic disc prolapses may compress the spinal cord.
Lumbar disc prolapses usually compress nerve roots or the cauda equina.
A large central lumbar disc prolapse can compress multiple cauda equina nerve roots simultaneously, resulting in Cauda Equina Syndrome.
Common Causes of Cauda Equina Syndrome
Most Common Cause
Large central lumbar disc herniation
Commonly at:
L4-L5
L5-S1
Other Causes
Spinal trauma
Epidural abscess
Epidural haematoma
Metastatic spinal cord compression
Primary spinal tumours
Severe lumbar spinal stenosis
Post-operative complications
How Does Cauda Equina Syndrome Present?
Acute Presentation (Hours)
Example:
Lifting a heavy object
Sudden severe back pain
Rapid development of urinary symptoms
Subacute Presentation (Days)
Most common examination scenario.
Example timeline:
Day 1:
Severe lower back pain
Day 2–3:
Unilateral or bilateral sciatica
Day 4–5:
Numbness and weakness
Day 5–7:
Saddle anaesthesia
Bladder dysfunction
Bowel dysfunction
Chronic Presentation (Weeks to Months)
More suggestive of:
Tumours
Severe spinal stenosis
Epidural abscess
SOCRATES Assessment
Site
Lower back
Onset
Sudden or gradual
Triggered by lifting, twisting, or trauma
Character
Sharp
Shooting
Burning
Electric shock-like
Radiation
Buttock
Posterior thigh
Calf
Foot
Associated Symptoms
Bladder Symptoms
Difficulty initiating urination
Urinary retention
Weak stream
Loss of sensation during urination
Overflow incontinence
Bowel Symptoms
Faecal incontinence
Reduced awareness of bowel movements
Saddle Symptoms
Numbness around genitals
Numbness around buttocks
Altered sensation when wiping
Leg Symptoms
Weakness
Numbness
Bilateral sciatica
Difficulty walking
Sexual Dysfunction
Reduced genital sensation
Erectile dysfunction
Timing
Progressive worsening
Exacerbating Factors
Movement
Coughing
Sneezing
Straining
Severity
Often severe (8–10/10)
Important Red Flags
The key red flags for CES are:
✅ Saddle anaesthesia
✅ Urinary retention
✅ Difficulty initiating urination
✅ Reduced sensation of urinary flow
✅ Faecal incontinence
✅ Bilateral sciatica
✅ Bilateral leg weakness
✅ Sexual dysfunction
✅ Progressive neurological deficit
Typical Patient Phrases
Bladder Symptoms
"I feel like I need to pee but nothing comes out."
"I have to strain to pass urine."
Saddle Anaesthesia
"The area around my private parts feels numb."
"I can't feel properly when I wipe."
Leg Weakness
"My legs feel heavy."
"I keep tripping."
Bowel Symptoms
"I nearly had an accident."
Straight Leg Raise Test
What is it?
The examiner raises the patient's straight leg while they lie supine.
Positive Test
Reproduction of radicular pain radiating below the knee between 30° and 70°.
Significance
Suggests:
Sciatica
Lumbar nerve root irritation
Lumbar disc herniation
The SLR primarily tests:
L4
L5
S1 nerve roots
Important Limitation
A positive SLR does NOT diagnose cauda equina syndrome.
It simply suggests nerve root irritation.
Upper Lumbar Radiculopathy
Disc prolapses at:
L1-L2
L2-L3
L3-L4
may not produce a positive Straight Leg Raise (SLR) test.
Instead, patients may present with:
Groin pain
Anterior thigh pain
Medial knee pain
Femoral Nerve Stretch Test (Reverse SLR)
More useful for assessing:
L2
L3
L4 nerve root irritation
A positive test reproduces groin or anterior thigh pain.
Quick Localization
Clinical Finding | Likely Nerve Roots |
Posterior leg pain + Positive SLR | L4/L5/S1 |
Groin/anterior thigh pain + Positive Femoral Stretch Test | L2/L3/L4 |
Saddle anaesthesia + bladder dysfunction | Cauda Equina Syndrome |
Investigation
Gold Standard
MRI Spine
MRI is the investigation of choice in both acute and chronic presentations.
Why MRI?
MRI remains the gold standard investigation even in acute presentations.
It accurately identifies:
Disc prolapse
Cauda equina compression
Epidural abscess
Epidural haematoma
Tumours
Degree and level of nerve compression
Clinical suspicion → Urgent MRI → Surgical decompression (if confirmed).
A normal CT scan cannot reliably exclude Cauda Equina Syndrome.
Why Not CT?
CT may detect:
Fractures
Severe disc prolapse
However, CT is less sensitive for:
Nerve roots
Cauda equina
Soft tissues
Abscesses
A normal CT does not reliably exclude CES.
Management
Immediate Actions
Cauda Equina Syndrome is a surgical emergency.
If suspected:
Urgent senior review
Emergency referral
Same-day MRI
Spinal surgery consultation
Do NOT delay referral while awaiting symptom progression.
Definitive Treatment
Disc Herniation
Emergency decompression surgery
Microdiscectomy
Epidural Abscess
IV antibiotics
Surgical drainage if required
Epidural Haematoma
Emergency decompression
Reversal of anticoagulation where appropriate
Tumour
Neurosurgical and oncology input
Symptomatic Management
While awaiting definitive treatment:
Appropriate analgesia
Bladder scan
Catheterisation if urinary retention is present
Complications of Delayed Diagnosis
Permanent:
Urinary incontinence
Urinary retention
Faecal incontinence
Sexual dysfunction
Lower limb weakness
Chronic neuropathic pain
PLAB 2 Examination Approach
When assessing a patient with back pain:
Take a SOCRATES history.
Ask specifically about sciatica.
Screen for leg weakness and numbness.
Ask about saddle anaesthesia.
Ask about bladder symptoms.
Ask about bowel symptoms.
Ask about sexual dysfunction.
Escalate urgently if any red flags are present.
Exam Pearl
Back pain + Bilateral Sciatica + Saddle Anaesthesia + Bladder Dysfunction = Cauda Equina Syndrome until proven otherwise.
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