At what vertebral level does the spinal cord terminate (conus medullaris) in an adult, and what are the clinical implications of this for lumbar puncture?

• A L3–L4 in adults; lumbar puncture must be above L3
• B L1–L2 in adults; lumbar puncture is safe at L3–L4 or L4–L5 to avoid cord injury ✓
• C T12 in adults; lumbar puncture must be in the sacral region
• D L4–L5 in adults; lumbar puncture must be at L2–L3

Explanation

In adults, the conus medullaris (the tapered lower end of the spinal cord) lies at the L1–L2 level. At birth, the conus is at L3 and ascends with differential growth. Below the conus lies the cauda equina (nerve roots floating in CSF within the subarachnoid space), which can be displaced rather than injured by a needle. This is why lumbar puncture is safely performed at L3–L4 or L4–L5 in adults (below the conus), with the L3–L4 interspace identified by Tuffier's intercristal line. In neonates, the L3 level must be used cautiously as the conus is at L3.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.