What are the differences between spinal anaesthesia and epidural anaesthesia?

Spinal vs. Epidural Anaesthesia: Key Differences

Spinal anaesthesia involves a single injection directly into the subarachnoid space (intrathecal), while epidural anaesthesia involves catheter placement in the epidural space outside the dura mater, allowing for continuous drug administration. 1

Anatomical Site of Injection

• Spinal anaesthesia: Local anaesthetic is injected into the cerebrospinal fluid in the subarachnoid space, directly bathing the spinal cord and nerve roots 2, 3
• Epidural anaesthesia: Local anaesthetic is deposited in the epidural space, outside the dura mater, requiring diffusion across tissue layers to reach nerve roots 3

Onset and Duration

• Spinal anaesthesia provides rapid, reliable onset (typically 3-5 minutes) with predictable block height, making it ideal when quick surgical anaesthesia is needed 1, 4
• Epidural anaesthesia has slower onset (15-20 minutes) but allows for prolonged analgesia through continuous infusion via catheter 1
• Duration: Single-shot spinal provides time-limited anaesthesia (90-180 minutes depending on agent), while epidural catheters can provide analgesia for days 1, 4

Drug Dosing

• Spinal anaesthesia requires much smaller doses (e.g., 10-15 mg bupivacaine) due to direct CSF contact 5, 6
• Epidural anaesthesia requires 10-20 times larger doses (e.g., 100-150 mg bupivacaine) because drug must diffuse through tissue 1
• Critical safety concern: Inadvertent administration of an epidural dose through an intrathecal catheter can cause high or total spinal block, respiratory arrest, and cardiovascular collapse 1, 2

Block Characteristics

Motor Block

• Spinal anaesthesia typically produces dense motor block affecting ambulation, which may be undesirable in some settings 4, 7
• Epidural anaesthesia allows titration of motor block by adjusting local anaesthetic concentration; dilute solutions (e.g., 0.125% bupivacaine) with opioids provide analgesia with minimal motor impairment 1, 8

Sensory Block

• Spinal anaesthesia produces predictable, symmetric sensory block with hyperbaric solutions spreading reliably based on patient position 4, 6
• Epidural block may be patchy or asymmetric, with failure rates of 3.5-32% reported in obstetric populations 1

Clinical Applications

When to Choose Spinal

• Elective surgery with anticipated duration under 3 hours (orthopedic, urologic, gynecologic, cesarean delivery) 1, 5, 4
• When rapid, dense surgical anaesthesia is required 1, 4
• Ambulatory surgery where faster recovery is desired (ropivacaine or mepivacaine produce shorter motor block than bupivacaine) 4, 7

When to Choose Epidural

• Labor analgesia where duration is unpredictable and ability to extend block for operative delivery is needed 1
• Postoperative analgesia lasting multiple days (thoracic, major abdominal, or orthopedic surgery) 1
• When motor-sparing analgesia is priority (using dilute local anaesthetic with opioid combinations) 1, 8
• Complicated parturients (twin gestation, preeclampsia, anticipated difficult airway, obesity) where early catheter insertion reduces need for general anaesthesia if emergency arises 1

Safety Profile

Spinal Anaesthesia Risks

• High or total spinal block occurs in approximately 1 in 4,400 cases, requiring cardiovascular and respiratory support 1, 2
• Hypotension is the most frequent complication, managed with vasopressors and IV fluids 2
• Post-dural puncture headache risk is 1-3% with cutting needles but reduced to <1% with pencil-point needles 1, 2
• Block height must be assessed every 5 minutes until stabilization to detect unintended cephalad spread 1, 2

Epidural Anaesthesia Risks

• Catheter failure rate 3.5-32%, requiring replacement or alternative technique 1
• Inadvertent dural puncture occurs in 0.5-2% of epidural placements 1
• Hypotension from sympathetic blockade, particularly with thoracic epidurals 1
• Motor block may delay mobilization and increase fall risk 8
• Urinary retention from autonomic blockade 8

Combined Spinal-Epidural (CSE) Technique

• CSE provides rapid spinal onset with epidural catheter flexibility for extension if needed 1, 3
• Meta-analysis shows CSE improves analgesia and speeds onset compared to epidural alone, but increases motor block frequency 1
• American Society of Anesthesiologists recommends CSE when labor may exceed spinal drug duration or operative delivery is possible 1

Drug Selection Considerations

• Hyperbaric bupivacaine produces more predictable blocks with fewer excessively high blocks than isobaric solutions 4, 6
• Ropivacaine produces less intense, shorter motor block than bupivacaine, advantageous for earlier mobilization and obstetrics 4
• Adding opioids to epidural local anaesthetics reduces supplemental analgesic consumption by 10.9 mg (p<0.00001) and prolongs time to first rescue request 1, 8
• Combination therapy permits lower local anaesthetic concentrations, minimizing motor block while preserving analgesia 1, 8

Critical Safety Measures

• Use pencil-point spinal needles instead of cutting-bevel needles to minimize post-dural puncture headache 1, 2
• Clear labeling of intrathecal catheters and structured handover communication are essential to prevent catastrophic dosing errors 1, 2
• Absence of CSF on aspiration does not rule out intrathecal placement; always give test dose before full epidural bolus 2
• Warning signs of high spinal: agitation, marked hypotension, bradycardia, upper-limb weakness, dyspnea, or speech difficulty require immediate evaluation and intervention 1, 2