Key Facts

Spinal (subarachnoid/intrathecal) anaesthesia provides rapid, dense surgical anaesthesia for procedures below the umbilicus. It is the technique of choice for caesarean section, lower limb orthopaedic surgery, and many urological procedures.

Epidemiology

Approximately 700,000 spinal anaesthetics are performed annually in the UK. The most common indication is caesarean section (~150,000/year). NAP3 found the risk of permanent neurological injury from spinal anaesthesia to be approximately 1 in 25,000.

Aetiology

Spinal anaesthesia works by depositing LA directly into the CSF in the subarachnoid space. The LA blocks nerve roots as they traverse the CSF, producing motor, sensory, and autonomic blockade. The spread of block depends on dose, baricity (heavy/plain/light), patient position, and spinal curvature.

Pathophysiology

Autonomic effects:

• Sympathetic blockade extends 2-6 segments higher than sensory level
• Vasodilation below the block → venous pooling → reduced preload → hypotension
• Block above T4: Cardio-accelerator fibres blocked → bradycardia (Bezold-Jarisch reflex)
• Block above C3-5: Phrenic nerve involvement → respiratory arrest (total spinal)

Factors affecting block height:

• Dose and volume of LA (most important)
• Baricity: Hyperbaric settles in dependent areas; isobaric spreads less predictably
• Patient position during and immediately after injection
• Speed of injection, patient height, CSF volume (reduced in obesity, pregnancy)

Expected Effects

• Dense motor and sensory block below the level of injection
• Onset within 2-5 minutes, surgical anaesthesia within 5-10 minutes
• Duration 1.5-3 hours (bupivacaine); can be extended with intrathecal opioids

Common Side Effects

• Hypotension (30-50%) — vasodilation and reduced venous return
• Bradycardia (10-15%)
• Shivering
• Pruritus (especially with intrathecal opioids)
• Urinary retention

Complications

• Post-dural puncture headache: Postural headache, worse sitting/standing, relieved lying flat
• High/total spinal: Respiratory arrest, hypotension, loss of consciousness
• Epidural haematoma: Back pain, progressive neurological deficit (especially in anticoagulated patients)
• Epidural abscess: Fever, back pain, neurological deficit
• Cauda equina syndrome: Saddle anaesthesia, urinary retention, bilateral leg weakness
• Transient neurological symptoms (TNS): Buttock/leg pain post-spinal (more common with lidocaine)

Red Flags

• Respiratory distress/apnoea after spinal — total spinal (intubate, vasopressors)
• Severe headache worse on sitting — PDPH (epidural blood patch)
• Progressive weakness/numbness post-operatively — epidural haematoma (emergency MRI and decompression)
• Fever + back pain + neurological deficit — epidural abscess (emergency MRI)

Bedside

• Pre-procedure: Coagulation status, consent, spinal examination, IV access
• Intra-procedure: AAGBI standard monitoring — SpO2, ECG, BP every 2 min initially
• Post-procedure: Hourly motor/sensory assessment, pain score, urinary output

Bloods

• Coagulation screen: Before procedure in patients on anticoagulants
• Platelet count: >75 × 10⁹/L for neuraxial block
• FBC: Baseline

Imaging

• Urgent MRI spine: If epidural haematoma or abscess suspected (within hours)
• CT myelogram: If MRI contraindicated

Special Tests

• Assessment of block height: Cold sensation (ethyl chloride spray) to determine sensory level
• Bromage scale: Assess motor block (0=no block, 3=complete motor block)
• PDPH diagnosis: Clinical — postural headache relieved by lying flat, associated with neck stiffness, photophobia, tinnitus

Non-pharmacological

• Positioning: Left lateral or sitting for injection; supine with left tilt for caesarean section
• Pre-loading/co-loading: Crystalloid 500mL-1L or colloid to reduce hypotension
• Equipment: 25G or 27G pencil-point (Whitacre/Sprotte) needle — lower PDPH rate than cutting needles

Pharmacological

• Hyperbaric bupivacaine 0.5% (heavy): 2-3.5mL (10-17.5mg) — standard agent
• Intrathecal opioids: Fentanyl 15-25mcg (improves block quality, extends duration) or diamorphine 300mcg (for caesarean section — 12-24h analgesia)
• Hypotension management: Phenylephrine 50-100mcg IV boluses (first-line) or infusion 25-50mcg/min; ephedrine 6-12mg IV if bradycardia coexists
• PDPH treatment: Conservative (bed rest, fluids, caffeine 300mg PO, paracetamol); epidural blood patch (15-20mL autologous blood) if persistent >24-48h

Surgical/Interventional

• Epidural blood patch: Definitive treatment for PDPH — success rate >85% with first patch
• Emergency decompressive surgery: For epidural haematoma (within 6-12 hours) or abscess

Referral Criteria

• Neurological deficit post-spinal — emergency MRI and neurosurgical/orthopaedic spinal referral
• Persistent PDPH >48 hours — anaesthetic review for blood patch
• Suspected infection — microbiological and surgical input

• Permanent neurological injury: ~1 in 25,000 neuraxial blocks (NAP3)
• PDPH: 1-2% with pencil-point needles; 70% resolve within 1 week; blood patch effective in >85%
• Epidural haematoma: Neurological outcome depends on speed of decompression — >80% good outcome if operated within 6-12 hours
• Cauda equina syndrome: Permanent bladder dysfunction in up to 50% if not rapidly treated
• Overall safety: Spinal anaesthesia has lower morbidity and mortality than GA for many procedures (especially caesarean section and hip fracture surgery)

Intrathecal Drug Doses

Contraindications to Spinal Anaesthesia

Anticoagulant Timing for Neuraxial Block (AAGBI/ESRA)