Lumbar puncture - anatomy and clinically important implications
This section summarises the anatomy required for safe lumbar puncture (LP), age‑related variation important for site selection, the tissue layers traversed, and key diagnostic implications for cerebrospinal fluid (CSF) analysis in suspected subarachnoid haemorrhage (SAH).
Practical points for emergency practice and examination‑style pitfalls are highlighted. Where relevant, UK guideline recommendations (NICE, RCEM) are referenced.
Anatomical principles and site selection
- The goal of LP is to enter the subarachnoid space below the termination of the spinal cord (conus medullaris) to obtain CSF, measure opening pressure or deliver drugs.
- In adults the conus usually ends at about L1-L2; below this level the thecal sac contains only the cauda equina (mobile nerve roots). Perform LP below the conus to minimise cord injury (RCEM).
- Typical safe interspaces in older children and adults are L3-4 or L4-5.
- In neonates and young infants the conus is lower at birth (≈L3) so a lower interspace (commonly L4-5) is chosen - co‑ordinate with paediatrics/neonatology for local practice (RCEM).
Vertebral levels and surface landmark
- The highest points of the iliac crests (Tuffier’s line) generally cross the spine at the L3-4 level in adults and is a convenient landmark.
- Landmark accuracy decreases in obesity, pregnancy and spinal deformity; consider ultrasound guidance when surface anatomy is unreliable (RCEM).