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72 Section I: Diagnostics and Planning

A B

C D

Figure 7.1 Technique for lumbar puncture. For the median approach (A, B) the needle is placed perpendicular to the spine along the midline, cranial to the
spinous process of L6. For the paramedian approach (C, D) the needle is placed to the left or right of the spinous process and directed cranially approxi­
mately 45°.

survey radiographs the angle of the needle typically parallels the more common when multiple punctures are made through the dura
angle of the spinal articulation. The median approach involves and in obese animals where the landmarks are not easily palpable
placement of the needle immediately cranial to the spinous process [9]. Epidural contrast can mimic the signs of an extradural lesion so
on the midline and directing the needle ventrally (perpendicular to proper identification of this artifact is critical (Figure 7.2). Epidural
the vertebral canal). contrast medium is absorbed faster than contrast medium in the
For each approach the contrast medium can be injected into subarachnoid space. Repeating the radiographs 10–15 min after the
either the dorsal or the ventral subarachnoid space. Injecting into injection may allow for sufficient clearing of epidural contrast
the ventral subarachnoid space is technically easier and decreases medium to facilitate interpretation. Air bubbles in the subarachnoid
the risk of intramedullary injection but results in the needle travers­ space frequently change position between images so careful evalua­
ing the spinal cord or nerve roots with the potential to cause dam­ tion should prevent misinterpretation of air as a lesion [3].
age [3]. In a study by Tilmant et al. [15] injection into the dorsal
subarachnoid space results in compression of the spinal cord by 1–2 Normal Variations
mm as the dura is indented prior to needle penetration and this The dorsal subarachnoid space is widest at C2. The ventral suba­
resulted in needle penetration of the spinal cord once the dura was rachnoid space at C2–C3 is narrow and slightly dorsally deviated
punctured. Therefore regardless of technique some spinal cord pen­ [9]. In the caudal cervical region the ventral subarachnoid space is
etration is inevitable. Although clinical signs are often not evident, often wide, creating the false impression of spinal cord displace­
spinal cord puncture can be associated with hemorrhage, gliosis, ment [3]. Focal attenuation and slight undulation of the ventral
and axonal degeneration [16]. Multiple needle punctures should be contrast column is common over the intervertebral disc spaces.
avoided to reduce the risk of epidural leakage. Opacification of the central canal can occur if the bevel of the
needle is placed in the central canal, the central canal communi­
Artifacts cates with the subarachnoid space at the conus medullaris, or if
Artifacts, including air in the subarachnoid space and epidural injec­ there is disruption of the spinal cord parenchyma from neoplasia or
tions, can negatively affect the interpretation of the myelogram and severe trauma [17]. In normal dogs opacification of the central
care should be taken to avoid these occurring. Epidural leakage is canal is more likely to occur if the injection is cranial to L5–L6 and