Equipment
The necessary equipment for a lumbar puncture is
usually available as a packaged, sterilized set.
Antiseptic—povidone-iodine
Local anesthetic—lidocaine hydrochloride
Sterile syringe and plunger—a 10-ml syringe is used
to administer the anesthetic.
Disposable needles—25- and 20-gauge needles are
usually used when administering the anesthetic
Spinal needle and stylet - 20-gauge or smaller; a
26-gauge needle is best
Three-way stopcock
Manometer
Specimen containers
Sterile adhesive bandage
Procedure
1. Monitor the patient's cardiorespiratory status
during and following the procedure.
2. Place the patient in the lateral recumbent position
with the craniospinal axis parallel to the floor
and the flat of the back perpendicular to the
procedure table.
3. Have the patient assume the flexed knee-chest
position with the back at the edge of the proce-
dure table. An assistant is often needed to aid
the patient in maintaining this position.
4. Identify by the palpation the spinal processes and
interspaces. The line connecting the tops of the
two iliac crests usually crosses the L3-L4 inter-
space. Use interspace L3-L4, L4-L5, or L5-S1.
5. Cleanse the skin over the puncture site using the
antiseptic. Allow it to air dry. The remainder
of the procedure is performed using sterilized
equipment and sterile technique.
6. Local anesthesia is usually employed. Infiltrate
the skin and soft tissue at the puncture site with
2-3 ml of the anesthetic. Use the 25-gauge
needle for the skin and the 20-gauge needle for
the soft tissue.
7. Insert the spinal needle with stylet in the midsag-
gital line of the prepared interspace. Hold the
needle perpendicular to the plane of the back.
Advance the needle through the longitudinal
ligament into the subarachnoid space. A slight
give is usually felt when the needle penetrates
the dura.
8. Remove the stylet. If cerebrospinal fluid (CSF)
appears, the space has been entered. If no fluid
appears, replace the stylet and rotate the needle
90
º
. Again remove the stylet and check for
CSF. If there is still no fluid, replace the stylet
and advance the needle a few more millimeters.
Feel for the give of the dura and check for fluid.
If this fails, replace the stylet and withdraw the
needle until the tip is subcutaneous, then redirect
it along a new midline path.
9. When fluid appears at the needle hub, quickly
attach the three-way stopcock and manometer.
Orient the manometer in the true vertical. CSF
should flow freely into the manometer. If the
CSF flow is sluggish, rotate the needle or, if
necessary, reposition it.
10. Record the "opening pressure" (mm CSF) once
it has become steady. The patient should be
relaxed with legs extended during the
measurement.
11. If the "opening pressure" is elevated (greater
than 200 mm CSF) or if the pressure quickly
falls, only 1-2 ml of CSF should be removed. If
the opening pressure is less than 200 mm CSF,
withdraw adequate fluid to perform the desired
studies. (If more than 20-30 ml is removed
rapidly, a mild transient postural headache is
likely.)
12. After the CSF sample has been removed, record
the volume of CSF obtained and the "closing
pressure" (mm CSF).
13. Replace the stylet and remove the needle.
14. Apply a sterile adhesive bandage.
Sources of Variability
1. The CSF pressure is raised in patients who are
straining. The patient should be relaxed and
quiet during the determination of the opening
pressure.
2. Incision of a vessel in the ventral vertebral venous
plexus can lead to contamination of the CSF
specimen with blood. This is referred to as a
traumatic tap. In order to distinguish a
Specimen Collection Procedures A-8
CEREBROSPINAL FLUID: LUMBAR SUBARACHNOID SPACE
