Lumbar Puncture (Spinal Tap)

A lumbar puncture (LP), also called a spinal tap, involves inserting a needle into the lower back to measure the pressure of cerebrospinal fluid (CSF) and collect a sample for testing. It's essential for diagnosing conditions like IIH and ruling out infections.

Why It's Done

• Measure opening pressure for IIH or papilledema
• Rule out infection (meningitis, encephalitis)
• Diagnose inflammatory conditions (MS, NMO) - including oligoclonal bands and IgG index
• Therapeutic - removing fluid can temporarily lower pressure

When a Lumbar Puncture Should Not Be Done (or Should Be Delayed)

A lumbar puncture is generally avoided or delayed when:

• Brain imaging shows a mass lesion, midline shift, or signs of a posterior fossa lesion that could cause brain herniation if pressure is suddenly released. This is why brain imaging is usually done first.
• The patient has a bleeding disorder or significantly abnormal coagulation (e.g., INR >1.4, platelets <50,000) - these usually need to be corrected first.
• The patient is on anticoagulants (warfarin, DOACs, therapeutic heparin) - these are typically held and bridged per institutional protocol.
• There is infection at the puncture site (cellulitis, abscess) - pick a different site or defer.
• The patient is hemodynamically unstable or unable to be safely positioned.

What to Expect

Before the Procedure

• Brain imaging (CT or MRI) is usually done first to rule out a mass lesion
• Review of all medications, especially blood thinners (warfarin, apixaban, rivaroxaban, dabigatran, clopidogrel, therapeutic heparin) - many will need to be held
• Recent INR and platelet count if you are on anticoagulation or have liver disease
• You may be positioned sitting up or lying on your side

During the Procedure

• Lower back cleaned and numbed with local anesthetic
• Needle inserted between vertebrae
• Opening pressure measured
• Fluid collected (feels like pressure, not pain)
• Needle removed, bandage applied
• Takes about 30 minutes

After the Procedure

• Lie flat for 30-60 minutes if your procedural team asks you to for observation or comfort
• Maintain normal hydration
• Avoid strenuous activity for 24 hours
• Monitor for headache

Post-LP Headache

What It Is

• Headache worse when upright, better lying flat
• Can occur 1-5 days after procedure
• Due to CSF leak at puncture site

Prevention and Treatment

• A smaller, atraumatic needle and careful technique are the most evidence-based ways to reduce risk when available
• Routine prolonged bed rest has not been shown to reliably prevent post-LP headache
• Maintain normal hydration, but extra fluids have not clearly been shown to prevent headache
• Lying flat may temporarily improve symptoms once a post-LP headache occurs, but it is a comfort measure, not proven prevention
• Caffeine may be offered for symptomatic post-LP headache in the first 24 hours when appropriate; avoid stacking multiple caffeine sources
• Epidural blood patch is the definitive treatment when the headache is severe, persistent, or disabling

Results

Opening Pressure

• Normal: 10-25 cm H2O
• Elevated in IIH: usually >25 cm H2O in adults, interpreted in clinical context; 25-30 cm H2O can be a gray zone, and pediatric thresholds are higher in some settings, especially with sedation

CSF Analysis

• Cell count and differential (white and red cells)
• Protein and glucose (compared to a same-day blood glucose)
• Gram stain, culture, and PCR for suspected infection (meningitis, encephalitis)
• Oligoclonal bands and IgG index for multiple sclerosis workup
• Cytology and flow cytometry when malignancy or leptomeningeal disease is suspected
• AQP4 and MOG antibodies - serum is the preferred initial sample, but CSF testing is reserved for selected NMOSD/MOGAD workups when serum is negative but suspicion remains high (more useful for MOG, since MOG-IgG can be intrathecally produced)

Frequently Asked Questions

Why is imaging usually done before an LP?

Imaging checks for a mass, severe swelling, or other pressure pattern that could make removing spinal fluid unsafe. This is especially important when papilledema is part of the reason for the test.

Does a normal opening pressure rule out IIH?

Not always. Opening pressure is interpreted with the exam, symptoms, body position, sedation status, and the quality of the measurement. Borderline values may need repeat assessment when the clinical picture is unclear.

When should I call after a lumbar puncture?

Call urgently for fever, worsening back redness, new leg weakness or numbness, severe persistent headache, confusion, or a headache that is strongly worse upright and not improving with conservative measures.