Lumbar Puncture — Landmarks, Layers Pierced, Contraindications and Post-Dural Puncture Headache

The spinal cord ends as the conus medullaris, usually at the lower border of L1 (sometimes L2). In the adult the mean position is the lower third of L1, with a range from the middle third of T12 to the upper third of L3. Below the conus the subarachnoid space widens into the lumbar cistern, which contains CSF, the filum terminale and the nerve roots of the cauda equina, but no spinal cord. A needle placed below the conus can therefore reach CSF without entering the cord.

The dural sac (the dura and arachnoid together, also called the thecal sac) extends down to about S2–S3 in the adult. The pia mater is the only meningeal layer that clings to the cord; it continues beyond the conus as the filum terminale. In children the cord ends lower, around L3, so the safe interspace is correspondingly lower.

Tuffier's line (the intercristal line) joins the highest points of the two iliac crests. In the adult it crosses roughly the L4 spinous process at the midline, so the L3–L4 interspace lies just above the line and L4–L5 just below. Palpate the crests, slide the thumbs medially to the line, then feel the spinous processes and choose the space.

• Position: left lateral decubitus with the spine flexed (knees to chest), or sitting and leaning forward — both open the interspinous gap.
• Needle bevel: parallel to the longitudinal fibres of the dura (facing up or down in the lateral position, to either side when seated), which separates rather than cuts the fibres and spares the cauda equina.
• Direction: midline, angled slightly cephalad.
• Needle type: a cutting (Quincke) needle is traumatic; pencil-point (Sprotte, Whitacre) needles are atraumatic and give less post-lumbar-puncture headache but are harder to handle.

From skin to CSF the needle crosses the following structures. The sequence is a favourite MCQ, especially the ligamentum flavum (a distinct 'give' or 'pop') and the epidural space that lies just beyond it.

Lumbar puncture is done to sample CSF for infection, malignancy and inflammatory or demyelinating disease, to measure CSF pressure in suspected raised intracranial pressure or idiopathic intracranial hypertension, to give intrathecal drugs (chemotherapy, baclofen) or contrast for myelography, and to place a lumbar shunt or an epidural blood patch. For how CSF results look in each type of meningitis, see CSF findings in meningitis.

Post-dural puncture headache (PDPH) follows CSF leakage from the dural hole faster than it is produced, causing intracranial hypotension. The brain sags when upright, tugging on pain-sensitive meninges, vessels and cranial nerves. It is the commonest complication — it occurs in about a third of cases overall, ranging from about 10–40% with ordinary needles to as low as about 2% with a small-gauge (24G or finer) non-cutting needle.

• Conservative: lie flat, hydration, simple analgesics; over two-thirds resolve spontaneously in 1–2 weeks.
• Caffeine: oral 300 mg or intravenous 300–500 mg in 1 L of fluid over 1 hour — often a transient benefit.
• Epidural blood patch: definitive treatment when conservative measures fail; 10–30 mL of autologous venous blood injected into the epidural space, with success of about 75–90%, especially if repeated.

The same landmarks serve all three procedures: Tuffier's line and the L3–L4 or L4–L5 interspace. They differ in how deep the needle goes. For spinal anaesthesia, local anaesthetic is injected into the CSF after the dura and arachnoid are pierced. For epidural anaesthesia, the needle stops in the epidural space just beyond the ligamentum flavum — recognised by loss of resistance to saline or air.