To perform a safe lumbar puncture, the needle is inserted in the midline at the level of the iliac crests. This level corresponds to which interspace and why is it safe?
- A L2–L3 interspace; the spinal cord ends at S1 in adults
- B L5–S1 interspace; only sacral nerve roots present
- C L4–L5 interspace; the spinal cord ends at L1–L2 so only cauda equina floats here ✓
- D L3–L4 interspace; the filum terminale attaches here
Correct answer: C. L4–L5 interspace; the spinal cord ends at L1–L2 so only cauda equina floats here
Explanation
The intercristal (Tuffier's) line connecting the highest points of both iliac crests passes through the L4 spinous process or the L4–L5 interspace. The spinal cord (conus medullaris) ends at the level of L1–L2 in adults, so inserting the needle at L4–L5 or L3–L4 ensures the cord is not encountered. The cauda equina nerve roots float in CSF and, being mobile, are pushed aside rather than transfixed by the needle. In neonates and infants the cord extends lower (to L3), so LP must be done at L4–L5 or lower.
Reference: BD Chaurasia's Human Anatomy, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.