Cauda Equina Syndrome After Spinal Anaesthesia

Cauda equina syndrome after spinal anaesthesia is injury to the lumbosacral nerve roots under the conus, causing saddle anaesthesia, sphincter dysfunction and leg weakness. It usually follows neurotoxic concentrations of local anaesthetic.

Epidemiology. It was linked to continuous spinal anaesthesia through microcatheters and to 5% lidocaine, with maldistribution causing sacral pooling. It is now rare (about 1 in 100,000 or less) with single shot spinals.

Risk factors. Hyperbaric 5% lidocaine, microcatheters (28 to 32 gauge), repeated doses, large total dose, pooled sacral injection, lithotomy position, spinal stenosis and chloroprocaine with bisulphite preservative (historical).

Pathophysiology.

Prolonged high concentrations of local anaesthetic in the sacral subarachnoid space cause direct neurotoxicity with demyelination and axon loss of cauda equina roots. Mechanical damage and ischaemia contribute.

Perineal and saddle numbness, bilateral leg weakness and pain, urinary retention (or overflow incontinence), faecal incontinence, loss of anal tone and sexual dysfunction. Onset follows resolution of the block or persists after it.

Red flags.

• Urinary retention or incontinence.
• Loss of anal tone or perianal sensation.
• Progressive bilateral leg weakness.

Emergency MRI to exclude haematoma, abscess or disc compression. Bladder scan and anal tone examination. Nerve conduction studies and electromyography at 3 weeks help assess severity. Cerebrospinal fluid is normal or shows raised protein.

Differential diagnosis.

• Epidural haematoma.
• Epidural abscess.
• Disc prolapse.
• Transient neurological symptoms.
• Arachnoiditis.
• Conus medullaris syndrome.

Exclude and treat compressive causes surgically at once. Supportive care follows for neurotoxic injury: catheterisation or intermittent self-catheterisation, bowel regimen, pain control, physiotherapy and rehabilitation.

• Gabapentin (neuropathic pain): 300 mg orally on day 1, then 300 mg twice daily, then increase every 3 days to a usual 900 to 1,800 mg daily in three divided doses; maximum 3.6 g daily; monitor sedation and renal function; reduce in creatinine clearance under 60 mL/min.
• Amitriptyline: 10 to 25 mg orally at night, increased weekly to 75 mg; maximum 150 mg daily; monitor ECG, anticholinergic effects and sedation; avoid in recent myocardial infarction, arrhythmia and urinary retention; reduce in older adults.
• Co-trimoxazole prophylaxis is not routinely needed; treat urinary infection by culture.

Recovery is variable, with many patients left with permanent deficit.

Prevention. Avoid 5% lidocaine and continuous spinal microcatheters, cap total intrathecal dose, and use the lowest effective dose.

Monitoring. Neurological examination, post-void residual and bowel function; urodynamics at 3 months.

Referral and escalation. Neurosurgery and neurology urgently. Urology and rehabilitation. Disclosure and reporting.