Lumbar Puncture (LP)

What it is

Indications

• Suspected CNS infection (meningitis, encephalitis) - most urgent indication
• Suspected subarachnoid hemorrhage after a normal CT scan
• Evaluation of unexplained neurologic symptoms - headache, seizure, cognitive decline, polyneuropathy - especially when malignancy, demyelinating disease (e.g., multiple sclerosis), or vasculitis is suspected
• Diagnosis of idiopathic intracranial hypertension or normal-pressure hydrocephalus (via opening pressure and CSF dynamics)
• Meningeal carcinomatosis workup
• Therapeutic/procedural use: intrathecal antibiotics, chemotherapy, spinal anesthesia, myelography/cisternography contrast injection, or therapeutic removal of CSF in idiopathic intracranial hypertension

Contraindications

• Local skin/soft-tissue infection at the puncture site
• Raised intracranial pressure with signs of a mass lesion or impending herniation (papilledema, suspected posterior fossa mass, midline shift, loss of basilar/suprachiasmatic cisterns on CT) - CT should be obtained first if any high-risk clinical features are present (age >60, altered consciousness, focal neurologic deficit, immunocompromise, history of CNS disease, recent seizure, etc.)

• Coagulopathy or thrombocytopenia (e.g., platelets <20,000/mm3, INR >1.4) - correct if possible, use smallest-gauge needle, most experienced operator
• Anticoagulant/antiplatelet therapy - agent-specific timing matters (e.g., hold clopidogrel/ticlopidine 5-10 days; heparin significantly raises spinal epidural hematoma risk, especially with a traumatic tap)
• Hemodynamic or respiratory instability - stabilize first
• Note: aspirin/NSAID monotherapy is generally not considered a contraindication

Technique

1. Position the patient in the lateral decubitus position (knees drawn to chest, neck flexed) or seated upright/leaning forward - lateral decubitus is required if an accurate opening pressure is needed.
2. Identify the L3-L4 or L4-L5 interspace (a line connecting the iliac crests, Tuffier's line, roughly marks L4).
3. Prep and drape, infiltrate with local anesthetic.
4. Advance the spinal needle in the midline, bevel oriented parallel to the long spinal fibers (to reduce post-LP headache), until a "pop" or flow of CSF is felt/seen.
5. Attach a manometer immediately to measure opening pressure.
6. Collect 0.5-5 mL of CSF sequentially into 3-4 sterile tubes without additives: tube 1 for chemistry (protein/glucose), tube 2 for Gram stain/culture, tube 3 for cell count and differential, and a 4th tube if cytology or fungal/AFB studies are needed. Tube 1 should never be used for microbiology (risk of skin-flora contamination).
7. Total volume: about 8 mL for routine studies, up to 40 mL if cytology/fungal/mycobacterial cultures are also needed. Fluoroscopic guidance is reserved for difficult anatomy or repeated failed attempts.

Opening pressure interpretation

• Normal adult opening pressure (lateral decubitus, legs/neck neutral): 90-180 mm H2O (up to 250 mm H2O in obese patients is sometimes accepted; Harriet Lane Handbook cites <28 cm H2O, i.e., ~280 mm H2O, as a pediatric/general cutoff)
• Normal pediatric range: 10-100 mm H2O, reaching adult values by age 6-8

• 250 mm H2O is generally diagnostic of intracranial hypertension (meningitis, hemorrhage, tumor, idiopathic intracranial hypertension)

• If pressure is markedly elevated, remove no more than ~2 mL of fluid and stop if pressure falls to 50% of opening value
• A sharp pressure drop after removing only 1-2 mL suggests spinal block or impending herniation - stop immediately

Complications

• Post-dural puncture headache (PDPH) - the most common complication (incidence 3-36%, depends heavily on needle type/gauge; atraumatic/pencil-point needles lower the risk compared with cutting-tip needles). Positional (worse upright, better lying flat), managed with hydration, caffeine, and if refractory, an epidural blood patch.
• Spinal/epidural hematoma - rare but catastrophic, particularly with traumatic taps in anticoagulated or thrombocytopenic patients; presents with worsening back pain and neurologic deficit; requires emergent MRI and possible laminectomy.
• Infection (meningitis, epidural abscess) - risk <0.01% with sterile technique.
• Cerebral herniation - if performed in the setting of an undiagnosed mass lesion or severely raised ICP.
• Bleeding, nerve root irritation/radicular pain, CSF leak.

CSF interpretation pearls

Recent evidence flag