Lumbar puncture for spinal anaesthesia is safely performed at the L3-L4 or L4-L5 interspace in adults because:

• A The conus medullaris terminates at approximately the lower border of the L1 vertebra ✓
• B The cauda equina roots are displaced laterally by the advancing needle
• C The dura mater ends at the S2 vertebral level
• D The ligamentum flavum is thickest below L3, deflecting the needle

Explanation

In the adult the spinal cord ends as the conus medullaris at about the L1-L2 disc level, so needles inserted below L2 enter a dural sac containing only the cauda equina, where individual rootlets move aside rather than being transected. That the dural sac extends to S2 is true but irrelevant to cord injury, and the ligamentum flavum offers no protective deflection. In neonates the cord reaches L3, so lower interspaces are chosen.

Reference: Gray's Anatomy for Students, 4th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.