Cauda equina and conus medullaris syndromes

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

The adult spinal cord usually ends around the L1 vertebral level as the conus, below which lumbar and sacral roots descend in the thecal sac. Compression of these mobile roots creates cauda equina syndrome, often with severe radicular pain, asymmetric lower-motor-neurone weakness, reduced reflexes and patchy saddle or leg sensory loss. A lesion of the conus is more compact, so bilateral sacral dysfunction, saddle anaesthesia and mixed long-tract or root signs may occur together.

CES is a clinical suspicion followed by MRI, not a checklist diagnosis. The patient may initially retain the ability to void but lose normal sensation or need to strain. Progression from incomplete CES to painless retention worsens prognosis. Conversely, pain, medicines, anxiety and constipation commonly alter bladder function without compression. Because neither symptoms nor examination are perfectly discriminating, a credible new sacral syndrome deserves emergency imaging rather than reassurance based on one negative sign.

Assessment should be respectful and specific. Ask about urinary sensation and control, bowel fullness and continence, genital sensation, erection or ejaculation and bilateral motor change. Test lower-limb myotomes, reflexes and sensation plus perianal sensation when it will inform the pathway, with consent and a chaperone. Digital rectal tone has limited discriminatory value and must not be used as the sole reason to avoid MRI.

Key points

Cauda equina syndrome is dysfunction from compression of multiple lumbosacral nerve roots below the cord; a large central disc is common, but tumour, abscess, haematoma, stenosis and trauma also cause it.
Conus medullaris syndrome affects the terminal spinal cord around L1 and often produces more symmetrical deficits, early severe bladder or bowel dysfunction, saddle loss and mixed upper- and lower-motor-neurone signs.
Early urinary clues are difficulty initiating micturition, impaired awareness of flow or bladder filling, reduced urge and altered urethral sensation; painless overflow retention is a late feature.
Saddle, perianal or genital sensory change, loss of rectal fullness and new erectile, ejaculatory or genital sensory dysfunction are important even when not volunteered.
Leg symptoms may be bilateral or progressive and include sciatica, weakness and reduced reflexes, but an asymmetric presentation does not exclude cauda equina compression.
No individual history item, rectal examination finding or post-void residual has sufficient sensitivity to rule out CES; the decision for MRI integrates the whole evolving syndrome.
Bladder scanning is a useful adjunct for retention and catheter planning, not a gatekeeper that cancels imaging when red flags persist.
MRI is the investigation that confirms or excludes compressive CES and should be available as an emergency through the local or regional GIRFT-aligned pathway.
Decompression timing is an urgent spinal surgical decision based on symptoms, imaging and progression; arbitrary cut-offs must not justify avoidable delay.
Document exact onset and progression of bladder, bowel, sexual, saddle and leg symptoms, the informed intimate examination and all escalation times because clinical change drives management.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Central disc prolapse and stenosis

A large lumbar disc or severe degenerative canal narrowing can compress several lumbosacral roots, sometimes after preceding back pain or sciatica.

Tumour and vertebral disease

Metastasis, primary spinal tumour or pathological collapse can compress the terminal cord or roots, often with progressive, nocturnal or mechanical pain.

Infection and inflammatory collections

Spinal epidural abscess or inflammatory tissue can narrow the canal, particularly after bacteraemia, spinal procedures, injected drug use or immunosuppression.

Haemorrhage and trauma

Epidural haematoma, fracture or dislocation may produce abrupt compression, with anticoagulation, coagulopathy and recent neuraxial intervention increasing haemorrhagic risk.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Root or conus compression
Disc, bone, tumour, pus or blood occupies the canal and mechanically deforms mobile cauda-equina roots or the compact terminal spinal cord.
2
Venous and arterial compromise
Continuing pressure impairs venous drainage and microvascular perfusion, producing oedema, conduction failure and eventually irreversible axonal or cord injury.
3
Sacral pathway failure
Damage to sacral sensory, motor and autonomic fibres disrupts saddle sensation, bladder awareness and emptying, bowel control and sexual function.
4
Lower-limb dysfunction
Involvement of lumbar and sacral roots causes radicular pain, patchy weakness, sensory loss and reduced reflexes; conus injury may add upper motor-neurone signs.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Incomplete CESRed flag

Back or bilateral radicular pain accompanies altered urinary sensation, impaired initiation, need to strain or reduced stream awareness while voluntary voiding remains possible, often with saddle change.

CES with retentionRed flag

Painless urinary retention and overflow occur with marked saddle or genital sensory loss and possible faecal or sexual dysfunction, indicating advanced sacral-root compromise.

Conus phenotypeRed flag

Early symmetrical bladder, bowel and sexual dysfunction with saddle anaesthesia and relatively modest leg weakness combines absent ankle reflexes with possible brisk knees or extensor plantars.

Root-predominant caudaRed flag

Severe bilateral or asymmetric sciatica, flaccid myotomal weakness, reduced knee or ankle reflexes and dermatomal loss accompanies evolving sacral autonomic symptoms.

Infective or malignant causeRed flag

Fever, immunosuppression, injecting drug use, cancer, weight loss or unremitting night pain broadens the emergency beyond disc prolapse and affects contrast imaging and treatment.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Emergency MRI lumbosacral spineFirst step
Why
Confirm or exclude compression of the cauda equina or conus and define disc, tumour, infection, blood or stenosis for surgery.
Interpretation and limitations
Use the national and local emergency pathway; include contrast or wider spinal coverage when abscess, malignancy or another level is plausible. Imaging and clinical syndrome are interpreted together.
02
Focused sacral and leg neurological examination
Why
Localise deficits, establish severity and detect progression while imaging is arranged.
Interpretation and limitations
Document hip, knee, ankle and toe power, reflexes, dermatomes, gait if safe and consented perianal sensation. Normal rectal tone does not rule out CES.
03
Post-void bladder scan
Why
Identify retention, guide safe drainage and add objective evidence of autonomic dysfunction.
Interpretation and limitations
A high residual increases concern, but thresholds vary and a low volume cannot exclude incomplete CES. Do not require the patient to overfill the bladder merely to perform the test.
04
Cause-directed blood tests
Why
Prepare for surgery and investigate infection, bleeding or malignancy without delaying MRI.
Interpretation and limitations
Use blood count, renal profile, inflammatory markers, coagulation, group and save, cultures and pregnancy testing as context requires. Normal markers do not exclude an epidural abscess.
05
CT when MRI cannot be performed
Why
Provide alternative anatomical assessment and identify fracture or severe canal compromise when MRI is contraindicated.
Interpretation and limitations
Discuss CT myelography or the best alternative with radiology and spinal surgery; ordinary CT has less soft-tissue sensitivity and should not become a reason for discharge.
06
Bladder and bowel complication assessment
Why
Prevent overdistension, renal injury, constipation and pressure harm while definitive care proceeds.
Interpretation and limitations
Record urine output, renal function, catheter timing, bowel history, skin and analgesic exposure; these measures support rather than replace the neurological diagnosis.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Uncomplicated radiculopathy

Single-root pain and weakness without new saddle, bladder, bowel or sexual dysfunction supports radiculopathy, but evolving bilateral symptoms lower the threshold for emergency MRI.

Spinal cord myelopathy

A clear truncal sensory level, brisk legs and extensor plantar responses favour cord disease above the conus rather than isolated mobile-root compression.

Guillain–Barré syndrome

Symmetrical ascending weakness and areflexia without a structural sensory level favour polyradiculoneuropathy; early bladder paralysis and severe local pain suggest an alternative.

Functional or pain-related dysfunction

Pain, medicines and anxiety can alter bladder emptying and effort, but positive inconsistency cannot safely exclude compression when a credible new sacral syndrome is evolving.

Additional chapter-specific clues

Non-compressive mimic

Pain-related voiding difficulty, medication effects, urinary infection, pelvic disease, diabetic neuropathy and functional symptoms can resemble elements, but are concluded only after proportionate emergency exclusion of compression.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01Suspected CESAsk, examine and imageFirst stepNew back or radicular pain accompanies any credible bladder, bowel, sexual, saddle or severe bilateral motor red flag.

1Record the onset and evolution of urinary initiation, flow sensation, filling awareness, saddle and genital sensation, rectal fullness, sexual function and leg symptoms using clear non-leading language.
2Perform a focused neurological and consented sacral examination, scan the bladder after voiding when feasible and provide analgesia without allowing normal tone or low residual to overrule the syndrome.
3Activate the local GIRFT-aligned emergency MRI pathway and discuss with the spinal service; keep the patient appropriately monitored and nil by mouth if operative compression is plausible.
4EscalationEscalate progression immediately, documenting MRI request, reporting, referral and transfer times and communicating any change directly to the receiving surgeon.

02Confirmed compressionDecompress without artificial delayMRI shows cauda-equina or conus compression consistent with the clinical presentation.

1The spinal surgeon reviews imaging, neurological stage, onset, cause, medical fitness and progression urgently and agrees decompression, stabilisation or cause-specific treatment.
2Correct sepsis, coagulopathy or haemodynamic instability in parallel with theatre preparation rather than waiting for complete normalisation when neural function is deteriorating.
3Drain significant retention, prevent skin and thrombosis complications and explain that surgery aims to prevent further injury while recovery of established sphincter loss may be incomplete.
4After intervention, document sacral and limb function and arrange bladder, bowel, sexual, pain and rehabilitation follow-up.

03MRI negativeTreat the actual syndromeEmergency imaging shows no compressive lesion despite concerning symptoms.

1AlternativeReassess for an alternative spinal level, inflammatory myelitis, infection, neuropathy, urinary or pelvic disease, medication effect, severe pain and functional neurological symptoms.
2Treat retention or other complications and obtain neurology, urology, gynaecology or pain input according to objective findings rather than dismissing symptoms as anxiety.
3Provide explicit return advice for progression and arrange follow-up, reconsidering imaging quality and coverage if new objective deficits emerge.

Key medicines and prescribing safety4 treatments · regimens, roles and cautions

Relieves severe nociceptive and radicular pain during emergency imaging and transfer.

Analgesia

Titrate a local acute back-pain regimen while preserving consciousness and the ability to repeat examination.

Opioids can cause retention, constipation and sedation that complicate assessment; analgesic response neither confirms nor excludes CES.

Prevents painful overdistension and renal or detrusor injury while decompression is arranged.

Urinary catheterisation

Use aseptic drainage for clinically significant retention with volume and timing documented in the record.

A catheter can conceal subsequent voiding function and introduce infection; it treats retention but does not treat neural compression or replace MRI.

Controls infection alongside surgical drainage and decompression when indicated.

Antimicrobial therapy

Use urgent local spinal-infection treatment after cultures when an epidural abscess causes the syndrome.

Do not give a generic regimen divorced from allergy, renal function, source and local resistance; sampling must not cause harmful delay.

Limits further bleeding and facilitates safer decompression.

Anticoagulant reversal

Stop further anticoagulant and apply the current local, haematology-selected reversal strategy for a compressive spinal haematoma.

Balance thrombosis, last dose and renal clearance; licensed and commissioned reversal options differ by agent and bleed site. Document when and how the original anticoagulant indication will be reconsidered.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Permanent bladder and bowel dysfunction

Prolonged sacral injury may leave retention, overflow, incontinence, recurrent urinary infection, constipation and the need for lifelong catheter or bowel programmes.

Sexual and sensory loss

Genital numbness, erectile or ejaculatory dysfunction and loss of sexual sensation may persist and can profoundly affect identity, relationships and fertility planning.

Weakness and chronic neuropathic pain

Irreversible root damage can cause foot drop, gait disability, muscle wasting and persistent radicular pain despite technically adequate decompression.

Immobility and psychosocial harm

Loss of mobility and continence increases pressure injury, thrombosis, dependence, depression and occupational disruption, requiring coordinated rehabilitation and support.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Repeat urinary sensation, ability to initiate, saddle and genital sensation, leg power and reflexes whenever symptoms change and before and after transfer or surgery.
Record bladder scan, catheter residual and urine output without converting any single volume into a rule-out test.
Monitor pain, sedation, constipation, renal function, skin and thrombosis risk while waiting, especially when mobility is restricted.
After decompression, follow voiding trials, residual volumes, bowel programme, sexual symptoms, neuropathic pain and motor rehabilitation over months.
Ensure MRI and surgical decisions, delays and safety-net advice are explicitly documented and communicated at handover.
Return urgently for new flow difficulty, saddle change, bilateral weakness, loss of rectal fullness or sexual dysfunction after an initially negative assessment.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Retention is late

Waiting for painless overflow misses the earlier incomplete stage when altered urinary sensation or initiation may be the only autonomic clue.

Bladder scan assists

Residual volume helps manage retention and probability but cannot safely veto emergency MRI in a convincing evolving sacral syndrome.

Rectal tone is limited

Technique and subjective interpretation reduce sensitivity, so a normal digital examination must never be used as the sole exclusion.

Conus mixes signs

Terminal cord injury can combine brisk knees or plantar responses with absent ankles and sacral lower-motor-neurone dysfunction.

CES can be asymmetric

Mobile roots are compressed unevenly, so unilateral-dominant sciatica or weakness does not remove concern when sacral symptoms evolve.

MRI-negative symptoms matter

Most suspected cases lack compression, but they still require explanation, complication treatment and safety-netting rather than invalidation.

11Common pitfallsFrequent interpretation and management errors.

01
Asking only about incontinence and missing altered flow sensation, initiation, filling awareness and sexual function.
02
Using normal rectal tone or a low post-void residual as a standalone reason not to image.
03
Delaying MRI until weakness becomes bilateral or painless retention develops.
04
Assuming every sacral syndrome is a disc and overlooking abscess, tumour, haematoma or trauma.
05
Quoting an arbitrary decompression time threshold as permission to postpone urgent surgical discussion.
06
Discharging an MRI-negative patient without treating retention, considering alternatives or giving progression advice.