Residency · Residency · Internal Medicine

Lumbar Puncture: Indications, Technique, and CSF Interpretation

Introduction

Lumbar puncture (LP) is a fundamental diagnostic and therapeutic procedure in internal medicine. Mastery of its indications, contraindications, technique, and cerebrospinal fluid (CSF) interpretation is essential for the evaluation of meningitis, encephalitis, subarachnoid hemorrhage, and numerous neurological conditions.

Indications

Diagnostic

• Suspected meningitis or encephalitis: bacterial, viral, fungal, tuberculous
• Subarachnoid hemorrhage (SAH): when CT head is negative but clinical suspicion remains high
• Suspected CNS malignancy: leptomeningeal carcinomatosis, CNS lymphoma
• Demyelinating diseases: multiple sclerosis (oligoclonal bands, IgG index)
• Normal pressure hydrocephalus: large-volume LP as both diagnostic and therapeutic trial
• Guillain-Barre syndrome: albuminocytologic dissociation (elevated protein, normal cell count)

Therapeutic

• Idiopathic intracranial hypertension (pseudotumor cerebri): reduce elevated opening pressure
• Intrathecal medication administration: chemotherapy, antibiotics

Contraindications

• Absolute: skin infection at the puncture site, spinal epidural abscess at LP level
• Relative: elevated intracranial pressure with mass lesion (risk of herniation), severe coagulopathy (INR > 1.5, platelets < 50,000), anticoagulation therapy
• CT head before LP is indicated if: focal neurological deficits, papilledema, altered mental status, immunocompromised state, seizure within 1 week, or age > 60
• In suspected bacterial meningitis, do not delay antibiotics for imaging or LP

Technique

Patient Positioning

• Lateral decubitus (fetal position): preferred for accurate opening pressure measurement; knees to chest, chin to chest
• Seated upright: easier landmark identification in obese patients but opening pressure is unreliable
• Identify the L3-L4 or L4-L5 interspace using the intercristal line (Tuffier's line)

Procedure Steps

1. Obtain informed consent and perform a timeout
2. Sterilize the area and apply sterile drape
3. Anesthetize the skin and subcutaneous tissue with 1% lidocaine
4. Insert a 20-22 gauge atraumatic (Sprotte or Whitacre) needle with the bevel parallel to the longitudinal dural fibers
5. Advance through skin, subcutaneous tissue, supraspinous ligament, interspinous ligament, ligamentum flavum, and dura
6. A pop or give is often felt upon entering the subarachnoid space
7. Measure opening pressure with the patient relaxed and legs extended
8. Collect CSF into 4 sequential tubes (typically 1-2 mL per tube)
9. Replace the stylet before withdrawing the needle

Ultrasound Guidance

• POCUS can identify the midline, interspace, and depth to the posterior complex
• Particularly useful in patients with obesity or difficult landmarks
• Reduces number of attempts and improves success rate

Opening Pressure

• Normal: 6-20 cm H2O in lateral decubitus
• Elevated (> 25 cm H2O): bacterial or fungal meningitis, idiopathic intracranial hypertension, cerebral venous sinus thrombosis
• Low (< 6 cm H2O): CSF leak, dehydration, post-LP state
• Ensure patient is relaxed with legs extended; Valsalva or anxiety falsely elevates pressure

CSF Interpretation

Normal Values

• WBC: 0-5 cells/mcL (all mononuclear)
• Protein: 15-45 mg/dL
• Glucose: 50-80 mg/dL (or > 60% of serum glucose)
• Appearance: clear and colorless

Patterns by Disease

Traumatic Tap vs. True SAH

• Traumatic tap: RBC count decreases from tube 1 to tube 4; CSF clears with sequential tubes
• True SAH: RBC count remains constant across tubes; xanthochromia (yellow supernatant) is present after centrifugation
• Xanthochromia develops 2-12 hours after SAH onset and can persist for weeks

Complications

• Post-LP headache: occurs in 10-30% of patients; positional (worse upright, better supine); use atraumatic needles to reduce risk
• Epidural blood patch: definitive treatment for persistent post-LP headache; 85-90% effective
• Infection and bleeding: rare with proper technique
• Cerebral herniation: prevented by appropriate pre-LP imaging when indicated

Special Considerations

• Anticoagulation: hold warfarin until INR < 1.5; hold LMWH for 12 hours; hold DOACs for 24-72 hours depending on agent
• Send appropriate studies: cell count (tubes 1 and 4), protein, glucose, Gram stain, culture; additional studies as indicated (HSV PCR, cryptococcal antigen, cytology, oligoclonal bands)

Key Clinical Pearls

• Never delay empiric antibiotics for LP or imaging in suspected bacterial meningitis.
• Use atraumatic (Sprotte/Whitacre) needles to significantly reduce post-LP headache rates.
• Opening pressure must be measured in the lateral decubitus position with legs extended for accuracy.
• Xanthochromia distinguishes true SAH from a traumatic tap and requires 2-12 hours to develop.
• Always send cell count on both tube 1 and tube 4 to help differentiate traumatic tap from true hemorrhage.

References

1. Engelborghs S, Niemantsverdriet E, Struyfs H, et al. Consensus Guidelines for Lumbar Puncture in Patients with Neurological Diseases. Alzheimer's & Dementia: Diagnosis, Assessment & Disease Monitoring. 2017;8:111-126.
2. Nath S, Koziarz A, Badhiwala JH, et al. Atraumatic versus Conventional Lumbar Puncture Needles: A Systematic Review and Meta-analysis. The Lancet. 2018;391(10126):1197-1204.
3. Tunkel AR, Hartman BJ, Kaplan SL, et al. Practice Guidelines for the Management of Bacterial Meningitis. Clinical Infectious Diseases. 2004;39(9):1267-1284.
4. Doherty CM, Forbes RB. Diagnostic Lumbar Puncture. Ulster Medical Journal. 2014;83(2):93-102.

For the intern

When you're called at 3am for suspected meningitis, start empiric antibiotics immediately—do not wait for imaging or LP results. If the patient has altered mental status, focal deficits, immunocompromise, or papilledema, order a CT head first, but don't delay antibiotics. Position the patient in lateral decubitus, use an atraumatic needle, and send tube 1 and 4 for cell counts to distinguish traumatic tap from true hemorrhage. Opening pressure measurement, xanthochromia testing for suspected subarachnoid hemorrhage (SAH), and special CSF studies can be obtained during the procedure, but antibiotics come first in bacterial meningitis. Most CSF interpretation and non-urgent therapeutic LPs can wait until morning rounds.

Common pitfalls

• Delaying antibiotics for LP or imaging in suspected bacterial meningitis—give ceftriaxone and vancomycin immediately; you can still culture CSF up to several hours after antibiotics start
• Forgetting to measure opening pressure in lateral decubitus with legs extended—seated position yields unreliable pressures and you'll miss elevated intracranial pressure
• Misinterpreting a traumatic tap as SAH—compare red blood cell counts in tubes 1 and 4; true SAH has consistent counts and xanthochromia (which takes 2-12 hours to develop)
• Using cutting needles instead of atraumatic (Sprotte/Whitacre) needles—this triples the risk of post-LP headache

Make it stick

Imagine it's 3 AM, and your patient in the ED presents with sudden-onset severe headache, fever, and neck stiffness. Your attending says, "Suspected bacterial meningitis – let's get an LP." While you're getting ready, you remember a crucial point from this lecture: always give empiric antibiotics first, before the LP, especially if there are red flag symptoms like altered mental status or focal neurological deficits. You immediately order ceftriaxone and vancomycin. You then proceed with the LP, carefully positioning the patient and using an atraumatic needle. You collect four tubes, ensuring to send both tube 1 and tube 4 for cell count, because you know this helps differentiate a traumatic tap from a true subarachnoid hemorrhage. You also make sure to obtain an accurate opening pressure in the lateral decubitus position. This sequence of actions not only saves crucial time but also ensures patient safety while maximizing diagnostic yield.

If you remember one thing

In suspected bacterial meningitis, prioritize giving empiric antibiotics immediately; do not delay for imaging or lumbar puncture. Always use an atraumatic needle to reduce the risk of post-LP headache and remember to send cell counts on both tube 1 and tube 4 to help distinguish a traumatic tap from a true hemorrhage.

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