Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

Chapter Six The Spine 663
In lumbar punctures a common problem is perforation of the dura mater. One study
revealed that it is virtually impossible to place the dorsal subarachnoid space without penetrating
the terminal bundle of nerve roots (filum terminale) and placement of the needle
bevel in the ventral subarachnoid was more likely. 7 Furthermore, the length of the bevel
was frequently larger than the subarachnoid space, resulting in simultaneous injection of
the contrast medium into both the subarachnoid and the extradural spaces. Contrast
medium in this space will appear on the lateral view to arch over the intervertebral disc
space and on the ventrodorsal view will appear to displace laterally over the intervertebral
disc space. 9 This appearance must be evaluated closely to prevent the errant diagnosis of a
ventral extradural mass. In many cases, close evaluation will reveal a small column of contrast
in the subarachnoid space crossing the disc space in a normal alignment. However, a
lack of apparent normal subarachnoid opacification may be due to lack of filling, and a
mixed extradural and subarachnoid injection may present a diagnostic dilemma.
A less common problem with lumbar punctures is injection into the central canal. This
usually is an incidental finding; however, if the rate and quantity of contrast injected result
in distention of the central canal, a transient exacerbation of the patient’s clinical signs may
occur. 7 Central canal filling is more likely to occur when the contrast is injected cranial to
L5-6 and when the contrast is injected into the ventral rather than the dorsal subarachnoid
space.
If CSF puncture is successful, there are other risks and complications to consider.
Bradycardia, arrhythmias, and apnea may occur during the contrast injection. These are
unrelated to the amount of contrast or site of injection and are usually transient. However,
careful monitoring of the patient during the contrast injection is essential. A major source
of morbidity is the development of seizures in the patient when awakened from anesthesia.
This is much less common when using nonionic contrast material. Seizures are
observed most often following cervical myelograms in large dogs and when the duration of
anesthesia that follows the myelogram is short. 3 Most of the postmyelographic seizures can
be controlled with diazepam, but deaths have occurred after myelograms. Other postmyelographic
changes that can occur include hyperthermia, depression, and worsening of the
original neurologic problem. Exacerbation of the neurologic signs may be due to the myelogram.
However, the effects of positioning for the CSF puncture and for the radiographs
have been blamed also.
Because of the possible myelographic complications the technique should be used cautiously.
We recommend that it not be performed unless surgery or another definitive treatment
is being considered. In those cases in which a diagnosis might allow an owner to make
a decision concerning the future for the pet, myelography is worth the risks.
LU M B A R S I N U S V E N O G R A P H Y
Lumbar sinus venography is performed occasionally to evaluate structures within the lumbosacral
spinal canal in patients suspected of having cauda equina compression. Several
methods have been described. 11-14 The easiest method is performed by placing a bone marrow
needle into the body of L7 or one of the rostral caudal vertebrae. 12-14 When a caudal
vertebra is used, a belly band may be placed tightly to occlude the caudal vena cava and to
increase filling of the venous sinus. Contrast medium injected through the needle then
flows through the ventral venous sinuses, allowing evaluation of their shape and integrity.
This procedure requires careful technique and frequently is difficult to interpret.
Extravasation of contrast into the epidural space or into the surrounding soft tissue may
occur. This produces no adverse effect but makes interpretation of the study more
difficult. 13
E P I D U R O G R A P H Y
Epidurography has been advocated for evaluation of the cauda equina in animals suspected
of having cauda equina compression. Nonionic contrast is injected directly into
the epidural space, usually at the level of the caudal vertebra. 15,16 Irregular filling of the
epidural space occurs, which makes it difficult to evaluate the radiographs. 17 Some individuals
have had a lot of experience performing and interpreting these studies, and in their
hands the technique is useful. Similar information can be obtained by myelography,