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88 Section I: Diagnostics and Planning

to perform a lumbar myelogram but a CSF sample for analysis is not
essential for diagnostic work‐up, a small amount of contrast media
(0.2–0.3 mL) can be injected as a trial after confirming radiographi-
cally the correct positioning of the spinal needle. Adequate injection
into the subarachnoid space can be confirmed radiographically by
the presence of myelogram and differentiated from epidurographic,
subdural, or nonvertebral canal injections as solid contrast lines
delineating dorsally and ventrally the lumbar cistern and even the
nerve roots inside the cistern (Figure 8.5). If no myelogram is
obtained after that, repositioning a second needle to L4–L5 should
Figure 8.4 Lumbar puncture at L5–L6. Spinal needle inserted in the caudal
aspect of L6 spinous process, mildly lateral, pointing cranioventrally at 45° be tried without removing the L5–L6 needle to avoid leakage of con-
angle aiming for the midline, and advanced into the vertebral canal through trast into the epidural canal through the previous L5–L6 dural hole
the L5–L6 interarcuate space. Source: Courtesy of Dr. Andrea Sanchez. (Figure 8.5). When injecting contrast, the bevel of the spinal needle
should be oriented in the direction of desired flow of contrast.
Approximately, 1 mL of CSF per 5 kg body weight can be safely
Technique
The lumbar puncture is performed through a midline approach. removed at one time for analysis, although only 1–1.5 mL is usually col-
Bending over the patient, the appropriate L5–L6 intervertebral lected [5]. The rate of CSF flow is usually slower than from the cerebel-
space is identified by palpating the cranial border of the wings of lomedullary cistern, and the fluid quantity retrieved is less. Compression
the ilium (iliac crests) using the nondominant hand, and putting of the jugular veins by an assistant will greatly increase the flow of CSF.
the fingers of that hand on each to maintain the relationship while The fluid should be collected in a sterile glass tube, preferably without
proceeding (Figure 8.3). The spinous process located in the midline EDTA, since it can cause falsely elevated protein concentrations and
and level with the iliac crests is L7, while immediately cranial to the falsely low cell concentrations in small samples, and it is bactericidal,
iliac crests is L6. Assessing the caudal lumbar spine with survey interfering with CSF culture if needed. During collection, CSF should
radiographs or fluoroscopy also assists in locating the various not be aspirated by using negative pressure applied with a syringe
spinous processes, especially in obese animals. While palpating the attached directly to the needle hub. Aspiration can cause a rapid decrease
spinous process of L6 with the nondominant hand, the dominant in CSF pressure, which may trigger hemorrhage or herniation.
hand is used to insert the spinal needle just mildly lateral to the As previously discussed, lumbar myelography is safer and
caudal aspect of the L6 spinous process. The spinal needle is typically results in better‐quality images as the contrast material
inserted pointing cranially at about 45° (range 30–60°) perpendicu- flows forward under pressure to outline lesions, particularly in the
larly to the long axis of the spine , and advanced cranioventrally thoracic and lumbar spine. Compared with cerebellomedullary
1
toward the midline of the spine in order to reach the L5–L6 interar- injections, the lumbar needle often penetrates the spinal cord and is
cuate space (Figure 8.4). The most medial epaxial muscles (multifi- more likely to deposit some contrast epidurally. The calculated dose
dus pars lumborum and interspinales) and the interspinous of contrast (0.3–0.5 mL/kg body weight) is slowly injected. It is very
ligament are penetrated, and if bone is contacted the needle should important to only used aqueous‐based nonionic contrast material
be slowly moved a few millimeters cranially or caudally over the that is approved for intrathecal use (i.e., Isovue‐M®). The author
roof of the vertebra until a soft consistency is felt, corresponding to prefers to always administer a test injection (0.2–0.3 mL) to ensure
the ligamentum flavum. Resistance suddenly decreases after pene- the contrast is in the subarachnoid space before administering the
trating this ligament and accessing the vertebral canal; frequently, remainder of the calculated contrast dose.
tail or pelvic limb twitching can be observed as the needle irritates When the procedure is completed, the lumbar puncture needle
2
the cauda equina nerve roots. The reported “popping” as the needle can be removed without reinserting the stylet , and the area can be
penetrates the flavum ligament is uncommonly felt. Even though compressed to prevent local bleeding.
the spinal needle penetrates the spinal cord during a lumbar CSF An ultrasound‐guided lumbar puncture technique has been
tap, this does not appear to cause any clinical problems. The needle reported in the dog to facilitate the introduction of the needle while
is inserted until it contacts the bone of the ventral aspect of the spi- avoiding exposure of the operator to ionizing radiation if fluoro-
nal canal to reach the ventral subarachnoid space; then, the stylet is scopic guidance is used [13].
removed to assess presence or absence of CSF fluid and collect it
(Figure 8.3). If CSF is not flowing, the needle is slowly readjusted,
rotating it and changing the orientation of the bevel. If CSF flow is Contraindications and Complications
still not observed, the needle is slowly withdrawn and rotated with- Lumbar puncture is usually a simple and safe technique, but there
out abandoning the vertebral canal until CSF is visible. are some risks that can be minimized by employing proper anes-
If L5–L6 puncture is not successful, a second needle can be thetic and collection techniques, and by excluding patients that
inserted at L4–L5. It is relatively frequent in acute compressive mye- have an increased risk of complications.
lopathies, such as intervertebral disc extrusion, that even if the nee-
dle is adequately positioned in the L5–L6 subarachnoid space, no Contraindications for Lumbar CSF Tap
CSF flows due to blockage at a more cranial level. If the main goal is 1 Elevated intracranial pressure: potentially leads to fatal brain
herniation, typically cerebellar through the foramen magnum,
rapidly precipitated by the removal of CSF and resulting in apnea
1 Editors’ note: Another placement technique involves insertion of the needle perpendicular
to the axis of the spinal cord at a point just barely medial to the spinous process of the
L6 vertebra in an L5–L6 puncture. The needle is advanced very slightly medially and 2 Editors’ note: In cases of lumbar puncture and myelography, the stylet is generally
ventrally through the interarcuate space. Using this technique, a smaller cross‐sectional area replaced before needle removal to prevent any contrast material from being deposited
of the spinal cord is traversed by the needle. into the spinal cord parenchyma.