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A lumbar puncture or spinal tap is usually performed to obtain cerebrospinal fluid (CSF) and thereby diagnose various central nervous system infectious and inflammatory diseases (ie, meningitis, encephalitis, Guillain-Barré syndrome), to evaluate CSF pressure (pseudotumor cerebri), or to administer medications (ie, antibiotics, chemotherapeutic agents).

Definitions and Terms

  • ▪  Interspace: The space between two spinous processes of adjacent vertebral bodies, which are referred to by a letter corresponding to which portion (cervical, thoracic, and lumbar) of the spine and which number vertebral body of that portion—L4 is the fourth lumbar vertebra, and the L4-5 interspace lies between the fourth and fifth lumbar vertebral bodies as well as being the typical location for lumbar puncture (Figure 13-1).
  • ▪  Spinous process: Posteriorly protruding bony portion of the vertebra, which is typically palpable through the skin of the back in the midline.
  • ▪  Laminae: The two struts that join to form the spinous process and thereby form the “roof” of the spinal canal.
  • ▪  Cauda equina: The lower end of the spinal cord, which typically ends between L2 and L3 (Figure 13-2).
  • ▪  Dura: The leathery membrane that is the outer layer of the meninges, envelopes the brain and spinal cord and contains the CSF.
  • ▪  Spinal needle: A long needle typically between 25 and 22 gauge and used to sample CSF percutaneously.
  • ▪  Introducer needle: A larger needle through which a small gauge spinal needle may be passed to prevent bending during performance of the procedure.
  • ▪  Obturator needle: A solid needle used to occlude the bore of the spinal needle and thereby prevent blockage with a plug of skin or tissue while the spinal needle is advanced—the obturator needle is removed to determine whether there is CSF flow through the central bore of the spinal needle.

Figure 13-1.

The lumbar spine in a patient in the right lateral decubitus position, showing the location of the L4-5 interspace and the posterior iliac crests.

Figure 13-2.

Diagram showing lumbar puncture needle entering CSF below cauda equina.


  • ▪  Consent, wash hands, prepare, and drape patient and perform universal protocol.
  • ▪  Position patient for procedure: Lateral decubitus or sitting positions are both acceptable.
  • ▪  Prepare and drape area around L4-5 interspace (typically at the level of the posterior superior iliac crests)—this interspace is below the distal end (cauda equina) of the spinal cord in adults and at the level of the posterior iliac crests (Figure 13-3).
  • ▪  Palpate L4-5 interspace and infiltrate skin and subcutaneous tissue with local anesthetic agent.
  • ▪  Insert spinal needle (directly or through introducer needle) in midline (Figures 13-4, 13-5, and 13-6) aiming in a slightly cephalad direction (ie, toward the body of ...

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