Lumbar puncture

What it is

A lumbar puncture (LP) samples cerebrospinal fluid (CSF) from the subarachnoid space below the end of the spinal cord. In adults the cord ends at about L1–L2 (lower in newborns), so the needle goes in at the L3–L4 or L4–L5 interspace, where it meets only the floating nerve roots of the cauda equina. Tuffier's line, joining the tops of the iliac crests, usually crosses the L4 spinous process or the L4–L5 interspace. On its way the needle passes skin, subcutaneous fat, the supraspinous and interspinous ligaments, the ligamentum flavum (often felt as a give), the epidural space, the dura and the arachnoid. The patient lies on the side curled up, which allows the opening pressure to be measured, or sits leaning forward, which makes the midline easier to find. A fine spinal needle with its stylet is used; atraumatic (pencil-point) needles cause fewer post-puncture headaches than cutting ones. Before the LP, decide whether a CT is needed to rule out a mass that could make it unsafe, and never let that delay antibiotics in suspected bacterial meningitis. Fluoroscopic or ultrasound guidance helps when the landmarks cannot be felt.

Indications

• Suspected meningitis or encephalitis, the most important indication.
• Suspected subarachnoid haemorrhage with a normal CT, done after an interval so xanthochromia can form.
• Idiopathic intracranial hypertension: to measure the opening pressure and to relieve it.
• Neurological disease where CSF helps: Guillain–Barré syndrome, multiple sclerosis, leptomeningeal malignancy.
• Giving drugs or contrast intrathecally, and spinal anaesthesia.

Contraindications and precautions

• Raised intracranial pressure with mass effect or obstructed CSF flow: risk of brain herniation; image first.
• Skin infection over the puncture site, or a suspected spinal epidural abscess.
• Coagulopathy, low platelets or anticoagulant or antiplatelet drugs: risk of spinal haematoma; follow local timing rules.
• Cardiorespiratory instability or a patient who cannot keep still or be positioned.
• Spinal deformity, obesity or previous lumbar surgery: consider fluoroscopic or ultrasound guidance.

Equipment

• Spinal needles of suitable length (often 20 to 22 G), preferably atraumatic, with stylets.
• Manometer with a three-way tap for the opening pressure.
• Sterile gloves, gown, drape, antiseptic, gauze and dressing.
• Local anaesthetic, syringes and needles for skin and deeper infiltration.
• Numbered sterile collection tubes, labels, and a blood tube for paired glucose (and other paired tests).

Step by step (and why)

1. Check indication, imaging need, clotting and drugs; consent — this finds the patients in whom an LP is dangerous.
2. Position on the side, back at the bed edge, knees to chest and chin down — flexion opens the spaces between the spines.
3. Feel the iliac crests, find L4 and mark the L3–L4 or L4–L5 space in the midline — this stays below the spinal cord.
4. Wash, wear sterile gloves, clean widely and drape — CSF infection is a serious, preventable complication.
5. Infiltrate local anaesthetic into the skin and along the track — a comfortable patient keeps still.
6. Advance the needle in the midline, aimed towards the umbilicus, bevel facing the side — this follows the gap between the spines.
7. Remove the stylet every few millimetres after a give — CSF appears once the needle is in the subarachnoid space.
8. Attach the manometer with the legs extended and read the opening pressure — flexion and straining raise it falsely.
9. Collect CSF drop by drop into the numbered tubes — free dripping avoids collapsing the space; order matters for blood.
10. Replace the stylet, withdraw, apply a dressing and record the procedure — the stylet lowers the risk of headache.

Complications

• Post-dural puncture headache from CSF leakage: postural, often with neck stiffness, nausea or tinnitus.
• Back pain at the puncture site and transient shooting pain into a leg from touching a nerve root.
• Traumatic tap: blood from an epidural vein that clears from the first to the last tube.
• Spinal epidural or subdural haematoma, especially with coagulopathy: new back pain with leg weakness or numbness.
• Infection (meningitis, epidural abscess) and, rarely, brain herniation when raised pressure was missed.

Next step

• Send the numbered tubes: cell count (first and last), glucose and protein with paired blood glucose, Gram stain and culture.
• Add tests by the question: viral PCR, cytology, oligoclonal bands with paired serum, xanthochromia; then interpret the CSF.
• Do not delay treatment for results: in suspected meningitis antibiotics are already running.
• Tell the patient to report a headache that is worse when upright; analgesia, fluids and caffeine first, then a blood patch.
• New back pain, leg weakness, numbness or sphincter problems after an LP need urgent senior review and imaging.

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Elsewhere in DurgiPedia

• Meningeal signs: neck stiffness, Kernig and Brudzinski
• Intracranial hypertension syndrome
• CSF and the blood–brain barrier
• Informed consent: elements and how to obtain it

Connections

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Educational content; it does not replace supervised hands-on training, your institution's protocols or a prescriber's order. A procedure is learnt with a teacher and done only once you are trained to do it.

Sources: Fluoroscopy-Guided Lumbar Puncture (StatPearls (NCBI Bookshelf), 2023); Postdural Puncture Headache (StatPearls (NCBI Bookshelf), 2025).

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Representative model, for educational purposes only.