Conus Medullaris Syndrome

Conus medullaris syndrome is a lesion of the lowest part of the spinal cord (spinal segments S3 to Co1, ending at about L1 to L2), causing early sphincter and sexual dysfunction, saddle sensory loss and variable lower limb signs. It overlaps with cauda equina syndrome and is a surgical emergency when compressive.

It is uncommon, and accounts for a minority of lumbar spinal cord and root emergencies. It occurs at any age, with disc herniation and tumours the commonest causes.

Causes. Large central disc herniation at T12 to L2, burst fracture, tumour (ependymoma, metastasis, lymphoma), epidural abscess or haematoma, spinal arteriovenous malformation, infarction, inflammatory myelitis, tethered cord and spinal anaesthesia complications.

Course. Bladder, bowel and sexual function are often permanently affected if decompression is delayed; the outcome depends on the time to surgery and the cause.

Pathophysiology.

Compression or injury of the conus damages the sacral spinal cord segments and the lumbosacral roots that arise next to it, giving a mixed upper and lower motor neuron picture.

Early involvement of S2 to S4 centres explains bladder and bowel failure before significant weakness.

• Early urinary retention with overflow incontinence, faecal incontinence and loss of anal tone and the bulbocavernosus reflex.
• Symmetric saddle (perianal) anaesthesia, sexual dysfunction (erectile failure).
• Mild or no leg weakness at first, with variable reflexes (ankle reflexes may be absent while knee reflexes are present; upper motor neuron signs possible).
• Low back pain, less severe radicular pain than in cauda equina syndrome.
• Possible fasciculations and mixed signs.

Red flags.

• New urinary retention or incontinence, saddle numbness or faecal incontinence (emergency).
• Progressive leg weakness or bilateral sciatica.
• Fever, cancer history, anticoagulation or recent spinal procedure.

• Emergency MRI of the whole spine, including the lumbar spine and conus, with contrast.
• Post-void residual bladder volume by scan (over 200 mL is concerning); rectal examination for tone and perianal sensation.
• CT for bony injury; spinal angiography for vascular malformation.
• Blood tests: full blood count, inflammatory markers, cultures if infection is suspected, clotting, glucose and tumour markers.
• Lumbar puncture only after compression has been excluded.

Differential diagnosis. Cauda equina syndrome, transverse myelitis, spinal cord infarction, Guillain-Barré syndrome, diabetic neuropathy, tethered cord, tumour and functional urinary retention.

• Same day emergency spinal surgical decompression for compressive lesions, ideally within 24 hours of the onset of sphincter dysfunction.
• Malignant compression: dexamethasone 16 mg orally or intravenously once daily with a proton pump inhibitor, started at once, with urgent radiotherapy or surgery. Maximum 16 mg daily for this use, reduced after treatment. Monitor glucose, blood pressure, mood and infection. Avoid with untreated systemic infection; use caution in diabetes and peptic ulcer.
• Epidural abscess: drainage and prolonged intravenous antibiotics directed by cultures; stop anticoagulants and reverse for haematoma, with specialist advice.
• Inflammatory myelitis: methylprednisolone 1000 mg intravenously once daily for 3 to 5 days. Maximum 5 g in a course. Monitor blood pressure, glucose, mood and infection. Avoid with active untreated infection.
• Bladder care: catheterisation for retention, then intermittent self-catheterisation; bowel programme with suppositories and regular evacuation.
• Thromboprophylaxis: enoxaparin 40 mg subcutaneously once daily when bleeding risk allows. Maximum 40 mg daily (20 mg daily when creatinine clearance is under 30 mL/min). Monitor platelets, renal function and bleeding.
• Neuropathic pain: gabapentin 300 mg orally at night on day 1, 300 mg twice daily on day 2, then 300 mg three times daily on day 3, increased every 3 to 7 days to 900 to 2400 mg daily. Maximum 3600 mg daily. Monitor sedation, dizziness and oedema. Adjust for renal function: creatinine clearance 50 to 79 mL/min, 600 to 1800 mg daily; 30 to 49 mL/min, 300 to 900 mg daily; 15 to 29 mL/min, 150 to 600 mg daily; under 15 mL/min, 150 to 300 mg daily.
• Rehabilitation, sexual health and fertility counselling, urology follow up and psychological support.

Monitoring. Neurological and bladder observation every 1 to 4 hours before and after surgery; review at 4 to 6 weeks, 3 months and yearly with urodynamics.

Referral and escalation. Emergency spinal surgery or oncology the same day; urology and spinal rehabilitation.

Special groups. Pregnancy: urgent decompression is not delayed. Children: tethered cord and tumour more likely.