Page 78 - Zoo Animal Learning and Training

Chapter 7: Advanced Imaging: Spinal Surgery 73

A A

B

B

Figure 7.3 Obtaining an image while injecting contrast medium into the
subarachnoid space (A) can result in better definition of the lesion when
there is spinal cord swelling. When images are obtained after the injection is
made (B) the contrast medium dissipates from the lesion site and accurate
localization is more difficult.

Tips
Figure 7.2 The presence of contrast in the epidural space (arrows in A,B) Contrast medium is hyperosmolar relative to CSF and is subject to
creates an undulating margin that can be confused with an extradural com­ redistribution with gravity. On the ventrodorsal view of the cervical
pression on the lateral view (A) but can be seen extending out the interver­ vertebral column the contrast medium tends to accumulate
tebral foramen on the ventrodorsal view (B). cranially, resulting in poor opacification of the caudal cervical
region. Obtaining a dorsoventral view in these cases redistributes
made rapidly [17]. The normal central canal should be small the contrast medium to the caudal cervical region, thus facilitating
(<1 mm) and uniform in size. Slight iatrogenic hydromyelia can a diagnosis [20]. Similar phenomena can be seen on the lateral
occur if the injection is made under high pressure [9]. If preferential views. Elevating the front or hind end (or both) can help to
opacification of the central canal is identified during the injection, redistribute the contrast medium to the area of interest [9,12].
the procedure should be stopped and repeated from an alternative Rarely do we consider the patient’s state of recumbency during
site or with the needle in the dorsal subarachnoid space. myelography but this gravity effect can influence whether lesions
are detected. One case report indicates that a lateralized lesion was
Side Effects not evident when the images were acquired in right lateral recum­
The primary disadvantage of myelography is that it is an invasive bency but was clearly defined when the images were acquired in left
technique. Reported side effects include worsening of the myelopa­ lateral recumbency [21]. This lesion was also noted on the oblique
thy, apnea, cardiac arrhythmias, and seizures [10,11,18,19]. The ventrodorsal views but, if these are not routinely obtained, consid­
incidence of side effects is significantly reduced with newer contrast eration should be given to obtaining both lateral views when a
agents such as iohexol. Seizures remain the most common compli­ lesion is not clearly defined.
cation of myelography, with an incidence of 3–21% in dogs. Factors Inadequate characterization of the lesion can occur when there is
that significantly impacted post‐myelographic seizures include severe attenuation of the subarachnoid space. In some cases this can
iohexol dose (total volume), size of dog (large breeds are at higher be overcome by obtaining an image while injecting so the subarach­
risk), and injection site [10,11]. Many of these factors are interre­ noid space is maximally distended (Figure 7.3). This is particularly
lated and it was concluded that dogs receiving a higher total volume true in cats [22].
of contrast medium (despite the same mg/kg dose) and having a
cervical injection were more likely to have a seizure upon recovery. Advantages
Subsequent work suggested a maximum total volume of 8 mL of • Requires no special equipment so is readily available.
contrast medium for cervical injection in large breed dogs [11]. • Typically is less expensive than CT and MRI.