A 34-year-old parturient receives an epidural block for labour analgesia. The anaesthetist withdraws the epidural catheter 3 cm and is surprised to find that sensory testing reveals a unilateral block. What is the MOST likely explanation?
- A The catheter has migrated into the subdural space
- B The catheter has entered an epidural vein
- C Insufficient volume of local anaesthetic was injected
- D The catheter tip lies in a lateral paravertebral position or tracks along a nerve root sheath ✓
Explanation
Unilateral epidural block most commonly results from the catheter tip migrating laterally within the epidural space, lying alongside a single nerve root sheath, or threading into a paravertebral location through an intervertebral foramen. This produces asymmetric spread of local anaesthetic. Subdural placement typically produces an unexpectedly extensive or patchy block, not simply unilateral block. Intravascular placement produces no block or systemic toxicity. Pulling the catheter back 1–2 cm and injecting additional volume is a standard manoeuvre to correct the unilateral block.
Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.
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Written and medically reviewed by the StethoPrep medical team.