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Spinal and epidural anaesthesia
In spinal anaesthesia a small dose is injected into the cerebrospinal fluid and gives a fast, dense block; in epidural anaesthesia a large volume is given through a catheter into the epidural space and gives a gradual block that can be extended — and in both the sympathetic block lowers blood pressure.
Why it matters in the exam
The exam asks about the contraindications (refusal, infection at the site, coagulopathy, raised intracranial pressure, severe hypovolaemia), the times for stopping anticoagulants before puncture, and the recognition of complications: post-dural puncture headache (PDPH), high spinal block, epidural haematoma and LAST from an intravascular catheter. It also asks about the level needed for different operations and the dermatomes.
In adults the spinal cord ends at L1–L2, so spinal puncture is done below that level; the line between the two iliac crests crosses at about L4 or L4–L5. A block reaching T1–T4 paralyses the cardiac accelerator fibres and causes bradycardia. Key dermatomes: T4 at the nipple, T6 at the xiphoid, T10 at the umbilicus. Caesarean section needs a block up to T4.
In exam questions always think of two sides: does the block suit the operation (block height, length of surgery, position), and is it safe for the patient (clotting, infection, haemodynamic stability). A patient on anticoagulants is a constant distractor — the times are counted from the last dose, and the time from removal of the epidural catheter to the next dose also matters. After every neuraxial block the return of movement and sensation in the legs is checked, because early recognition of a haematoma decides the outcome.
Key points
- Spinal = fast and dense; epidural = gradual and controlled through a catheter.
- The sympathetic block causes hypotension; above T4 also bradycardia.
- Dermatomes: T4 nipple, T6 xiphoid, T10 umbilicus; caesarean section needs T4.
- PDPH is postural; conservative treatment, then a blood patch.
- Stopping anticoagulants: prophylactic enoxaparin 12 hours, therapeutic 24 hours, Xa inhibitors 72 hours.
Summary table
| Item | Spinal | Epidural |
|---|---|---|
| Injection site | Into the CSF, below L1–L2 | Into the epidural space, at any level |
| Onset and quality | Fast (minutes), dense | Gradual; can be adjusted and extended through a catheter |
| Volume and dose | Small | Large — risk of LAST |
| Typical complication | High block, hypotension, PDPH | Accidental dural puncture, intravascular injection |
| Test dose | Not needed | Lidocaine with adrenaline: rising heart rate = intravascular; rapid motor block = intrathecal |
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Sources
- ASRA Evidence-Based Guidelines: Regional Anesthesia in the Patient Receiving Antithrombotic or Thrombolytic Therapy, 4th edition (2018) — Neuraxial techniques and anticoagulants
- Miller's Anesthesia, 9th edition (2020) — Spinal, Epidural, and Caudal Anesthesia
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.