How to perform LP

Summary

Lumbar puncture (LP) is performed to obtain cerebrospinal fluid for diagnostic testing (e.g., meningitis, subarachnoid hemorrhage), measure opening pressure, or administer medications. Ultrasound assistance—with either a high‐frequency linear probe for pediatrics/thin patients or a curvilinear probe for obese patients—may improve first‐pass success by helping to identify midline anatomy and the optimal intervertebral space.

Indications
  • Evaluation for meningitis, encephalitis, or subarachnoid hemorrhage
  • Measurement of opening pressure in suspected idiopathic intracranial hypertension
  • Diagnosis of demyelinating or inflammatory CNS processes
Contraindications
  • Local infection or cellulitis at the needle entry site
  • Evidence of increased intracranial pressure with mass effect (e.g., midline shift, herniation risk)
Complications
  • Post-dural puncture headache
  • Traumatic taps (blood contaminating CSF)
  • Bleeding, spinal hematoma, or local infection

Equipment

  • Essential: LP needle set (with stylet and collecting tubes), sterile gloves and drapes, antiseptic solution, local anesthetic (e.g., lidocaine), manometer (if measuring opening pressure)
  • Optional: Ultrasound machine with high-frequency linear probe (or curvilinear for obese patients) and marking pen

Positioning and Landmarks

  • Position: Lateral decubitus (preferred when measuring opening pressure) or sitting with maximal lumbar flexion
  • Landmarks/US: Identify Tuffier’s line by palpating the iliac crests to locate the L4 spinous process; mark the L3-L4 or L4-L5 interspace. If using ultrasound, start in the transverse view to locate the spinous processes and then switch to the longitudinal view to confirm the interspinous space and measure skin-to-dura distance ^1^2

Analgesia/Sedation

OptionWhenKey note
Local anesthetic (lidocaine)Prior to skin punctureEnsure adequate anesthesia of skin and soft tissues
IV sedationIn anxious or non-cooperative patientsUse judiciously; balance sedation with patient cooperation

Steps

  1. Palpate for the iliac crests to identify the L4 level and select an interspace (typically L3-L4 or L4-L5).
  2. If available, perform a pre-procedure ultrasound: use a high-frequency probe in transverse view to locate the midline (spinous processes) and then in longitudinal view to identify the intervertebral space; mark the target site with a surgical pen ^1.
  3. Prepare the patient by cleaning the skin with antiseptic solution, and ensure strict sterile technique with draping and wearing sterile gloves.
  4. Administer local anesthetic to numb the skin and deeper tissues along the planned needle path.
  5. Insert the LP needle in the midline with the bevel parallel to the long axis of the spine, advancing slowly until a “pop” is felt as the needle traverses the ligamentum flavum.
  6. Remove the stylet briefly to check for free-flowing CSF; if CSF is obtained, collect the fluid into the prescribed sterile tubes, or measure opening pressure with a manometer if indicated.
  7. Reinsert the stylet, then carefully withdraw the needle and secure a sterile dressing over the puncture site.

Post-procedure

  • Confirm: Verify the quality of the CSF sample and, if applicable, record the opening pressure.
  • Aftercare: Monitor the patient for signs of headache, bleeding, or neurological deficits and advise on post-LP care (e.g., lying flat, hydration).
  • Re-check: Reassess the puncture site for any bleeding or signs of infection.

^1

References

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