Neuraxial anaesthesia

Overview & definition

Neuraxial anaesthesia is the reversible blockade of the spinal nerve roots and the spinal cord by the injection of a local anaesthetic (with or without an adjuvant such as an opioid or clonidine) into the subarachnoid space (the spinal — a single injection into the CSF) or the epidural space (the epidural — a catheter placed in the fat-filled space outside the dura, for a continuous infusion or a titrated bolus). The combined spinal-epidural (CSE) combines the rapid onset of the spinal with the flexibility of the epidural catheter.[1]

The neuraxial techniques are among the safest and the most effective in anaesthesia — they avoid the general anaesthetic airway, the aspiration risk and the awareness, they provide excellent postoperative analgesia, and their major complications are rare. The NAP3 audit documented the incidence of the major complications (the epidural haematoma, the abscess, the nerve injury) as approximately 1 in 24,000 to 1 in 50,000 for the spinal and 1 in 2,500 to 1 in 5,000 for the epidural in the higher-risk groups (the elderly, the obstetric).[1]

Anatomy: the epidural space, the dura and the subarachnoid space

The neuraxial anatomy is layered from outside in:

• The skin and the supraspinous and interspinous ligaments — the needle passes through these to reach the epidural space.
• The ligamentum flavum — the tough, elastic ligament that marks the posterior border of the epidural space. The epidural needle is identified by the loss of resistance as it passes through the flavum into the epidural space.
• The epidural space — the fat-and-vein-filled space between the dura and the vertebral canal, extending from the foramen magnum to the sacral hiatus. The epidural catheter sits here.
• The dura mater — the tough outer meningeal layer, containing the CSF. The spinal needle pierces it to enter the subarachnoid space.
• The subarachnoid space — the CSF-filled space between the dura and the pia, containing the spinal nerve roots and the spinal cord (which ends at L1/L2 in the adult, L3 in the infant). The spinal injection is made below the cord (L3/L4 or below) to avoid injury. [1]

The spinal anaesthetic: technique, drugs and physiology

The spinal is a single-shot injection of a hyperbaric (heavy) or isobaric local anaesthetic into the subarachnoid space at L3/L4 or below, through a fine (25 to 27 G) pencil-point needle (the Whitaker or the Sprotte, designed to spread rather than cut the dural fibres, reducing the PDPH risk). The hyperbaric bupivacaine (0.5 per cent in 8 per cent dextrose) is the standard agent (about 2.5 to 3 mL for a lower-abdominal or a lower-limb block, reaching T6 to T10; more for a caesarean at T4). An opioid (fentanyl or diamorphine) is added for the prolonged analgesia.[1]

The onset is rapid (within 2 to 5 minutes), the surgical block within 10 to 15 minutes, and the duration about 2 to 3 hours (limited by the clearance of the drug from the CSF). The block is a differential block: the sympathetic fibres (small, myelinated) are blocked first (2 to 6 segments higher than the sensory), then the sensory (pain, then temperature, then touch), then the motor (large, heavily myelinated). This explains the sympathetic blockade that produces the hypotension.[1]

The epidural: technique and the catheter

The epidural is the identification of the epidural space (by the loss of resistance to saline technique — the needle is advanced through the ligamentum flavum with a syringe of saline; the sudden loss of resistance as the needle tip enters the epidural space confirms the position), and the placement of a catheter through the needle for the continuous infusion or the titrated bolus of a local anaesthetic (bupivacaine 0.1 to 0.25 per cent with fentanyl). The level of the block is determined by the volume injected (the more volume, the more segments spread) and the concentration (the higher the concentration, the denser the motor block). The onset is slower than the spinal (15 to 30 minutes) and the duration is determined by the infusion.[1]

The combined spinal-epidural

The CSE combines the spinal needle (for the rapid onset of the surgical block) with the epidural catheter (for the prolonged, titratable analgesia). A needle-through-needle technique is used: the epidural needle is placed in the epidural space, the spinal needle is passed through it to enter the subarachnoid space, the spinal drug is injected, the spinal needle is withdrawn, and the epidural catheter is threaded. It is used widely in obstetrics (the rapid onset for the emergency caesarean, the epidural catheter for the postoperative analgesia).[1]

The physiological effects of neuraxial blockade

The neuraxial block produces a sympathetic block (the vasodilation below the level of the sensory block, reducing the SVR and the venous return), a sensory block (the loss of pain, temperature and touch), and a motor block (the loss of voluntary movement). The physiological consequences: [1]

• Hypotension — from the sympathetic block, the commonest and most important acute effect.
• Bradycardia — if the block reaches T1 to T4 (the cardiac sympathetic fibres), the unopposed vagal tone slows the heart.
• Respiratory — the intercostal muscle paralysis from a high thoracic block reduces the vital capacity and the cough effort; but the diaphragm (C3 to C5) is spared unless the block reaches the cervical levels.
• Gastrointestinal — the unopposed parasympathetic tone produces a contracted, hyperactive gut and nausea. [1]

Hypotension after spinal anaesthesia: prevention and management

The hypotension from the sympathetic block is common (in the majority of obstetric spinals and in many elderly patients). It is dangerous to the foetus (the uteroplacental perfusion falls with the maternal blood pressure) and to the patient with a fixed cardiac output (the aortic stenosis).[1]

The prevention and management: the left lateral tilt (for the obstetric patient), the phenylephrine infusion (the preferred vasopressor — a pure alpha-agonist), the intravenous coloading (crystalloid at the time of the spinal), and the close monitoring of the blood pressure (every 2 to 3 minutes for the first 15 minutes). For the refractory case, the adrenaline infusion may be needed. [1]

Complications: post-dural puncture headache

The post-dural puncture headache (PDPH) is a postural headache (worse on sitting or standing, relieved by lying flat) from the leak of CSF from the dural puncture, causing a low CSF pressure and the traction on the meningeal structures. It is more common with the larger needle (the Tuohy for the epidural) and the cutting (Quincke) than the pencil-point needle. The conservative treatment is the bed rest, the hydration, the simple analgesia and the caffeine; the epidural blood patch (15 to 20 mL of the patient's own venous blood injected into the epidural space to seal the dural leak) is the definitive treatment for the severe or persistent case.[1]

The epidural haematoma and abscess

The epidural haematoma (the bleeding into the epidural space, compressing the spinal cord) and the epidural abscess (the infection in the epidural space) are the two most feared complications of the neuraxial technique. Both present with a new, severe back pain after the neuraxial technique, followed by a progressive neurological deficit (the weakness, the sensory loss, the bowel and bladder dysfunction). The diagnosis is by an urgent MRI and the treatment is the urgent surgical decompression — the delay beyond 8 hours from the onset of the deficit dramatically worsens the outcome. The anticoagulated patient is at greatly increased risk of the haematoma (below).[1][4]

The NAP3 audit documented the incidence as approximately 1 in 24,000 for the spinal and 1 in 2,500 for the epidural in the higher-risk groups. The obstetric study confirmed a very low but real incidence of the epidural haematoma and abscess in obstetric epidural practice.[1][4]

The anticoagulated patient: the ASRA guidelines

The ASRA guidelines (the Third Edition, 2010, and the 2018 update) set out the timing of the neuraxial needle and catheter relative to the anticoagulant:[2][3]

• LMWH (enoxaparin): wait 12 hours after a prophylactic dose, 24 hours after a therapeutic dose, before the neuraxial technique. Remove the epidural catheter 2 to 4 hours before the next LMWH dose.
• Warfarin: wait until the INR is normal (under 1.4) before the needle or the catheter removal.
• Clopidogrel: wait 7 days.
• Aspirin/NSAIDs: not a contraindication on their own.
• Direct oral anticoagulants (rivaroxaban, apixaban, dabigatran): the timing varies (typically 2 to 3 days for the factor Xa inhibitors, 4 to 5 days for dabigatran, or the normalisation of the specific assay). [1]

The principle is that the neuraxial technique must not be performed while the anticoagulant effect is present, and the catheter must not be removed during the anticoagulant peak.[2][3]

Total spinal and systemic toxicity

The total spinal (the excessive cephalad spread of the spinal local anaesthetic, or the accidental intrathecal injection of the epidural dose) produces a high block with hypotension, bradycardia, respiratory depression (the intercostal paralysis) and unconsciousness (the brainstem blockade). The management is the airway (intubation and ventilation), the cardiovascular support (the vasopressor, the fluid, the atropine for the bradycardia), and the waiting for the block to recede as the drug is cleared from the CSF. [1]

The local anaesthetic systemic toxicity (LAST) can occur from the accidental intravascular injection through the epidural catheter (the epidural veins are prominent). The prevention is the aspiration test, the test dose (the adrenaline), and the incremental injection; the management is the lipid emulsion as described in the local anaesthetic pharmacology topic. [1]

The NAP3 audit: the incidence of the major complications

The Third National Audit Project (NAP3) of the Royal College of Anaesthetists is the definitive national audit of the major complications of the central neuraxial block in the UK. It documented:[1]

• An overall incidence of the permanent injury or death of approximately 1 in 24,000 to 1 in 50,000 for the spinal, and 1 in 2,500 to 1 in 5,000 for the epidural in the higher-risk groups.
• The epidural haematoma and the epidural abscess the commonest of the serious complications.
• The perioperative epidural (in the surgical patient) carried a higher risk than the obstetric epidural (possibly from the comorbidity and the anticoagulation).
• The overall conclusion: the central neuraxial block is safe in the appropriate patient and setting, but the complications, when they occur, are devastating, and the vigilance for the back pain and the neurological deficit is essential. [1]

Ultrasound-assisted neuraxial techniques

The ultrasound is increasingly used to assist the neuraxial technique — to identify the interspinous and the interlaminar spaces, to estimate the depth to the epidural space, and to guide the needle in the difficult spine (the obesity, the scoliosis, the previous spinal surgery). It does not replace the loss-of-resistance technique, but it improves the first-pass success rate and reduces the complications in the predicted-difficult spine.[1]

The indications and contraindications

Indications: the lower-abdominal, the perineal, the lower-limb and the obstetric surgery; the labour analgesia; the postoperative analgesia (the thoracic epidural for the major abdominal and the thoracic surgery); the chronic pain management. [1]

Contraindications: the patient refusal; the coagulopathy (the ASRA guidelines); the local infection at the injection site; the raised intracranial pressure (the risk of the coning); the severe aortic stenosis (the fixed cardiac output tolerates the vasodilation poorly); the hypovolaemia (the cardiac output collapses with the vasodilation). [1]

[1] [1]

Red flags

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References

1. [1]Cook TM, Counsell D, Wildsmith JAW. Major complications of central neuraxial block: report on the Third National Audit Project of the Royal College of Anaesthetists Br J Anaesth, 2009.PMID 19139027
2. [2]Horlocker TT, Wedel DJ, Rowlingson JC, et al. Regional anesthesia in the patient receiving antithrombotic or thrombolytic therapy: American Society of Regional Anesthesia and Pain Medicine Evidence-Based Guidelines (Third Edition) Reg Anesth Pain Med, 2010.PMID 20052816
3. [3]Narouze S, Benzon HT, Provenzano D, et al. Interventional Spine and Pain Procedures in Patients on Antiplatelet and Anticoagulant Medications (Second Edition): Guidelines From the American Society of Regional Anesthesia and Pain Medicine, the European Society of Regional Anaesthesia and Pain Therapy, the American Academy of Pain Medicine, the International Neuromodulation Society, the North American Neuromodulation Society, and the World Institute of Pain Reg Anesth Pain Med, 2018.PMID 29278603
4. [4]Tunn R, Ramakrishnan R, Hartopp R, et al. Neurological complications following obstetric neuraxial anaesthesia: a four-year United Kingdom population-based study of epidural haematoma and epidural abscess (2014-2017) Int J Obstet Anesth, 2025.PMID 40505292