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