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78 Section I: Diagnostics and Planning

A Intervertebral Disc Disease
The decision to use myelography to assess spinal cord disease
is now dependent on the availability of CT or MRI and the suspected
clinical problem [42]. Although CT and MRI are frequently
preferred, myelography remains a viable method of assessing spinal
cord disease in dogs and cats, with an accuracy of 86–97% and the
choice of imaging modality does not affect patient outcome
[13,43–45].
Intervertebral disc herniation can present with either an extra­
dural or intramedullary pattern depending on the degree of spinal
cord swelling. In the face of severe spinal cord swelling adequate
localization of the lesion can be difficult. Careful evaluation for any
deviation of the subarachnoid space is critical. Radiographs made
during or immediately following the contrast injection frequently
allow for improved localization as the contrast medium disperses
from the swollen area over time [13,22,46]. Circumferential locali­
B zation of herniated disc material is important if hemilaminectomy
or pediculectomy are planned. Clinical lateralization is not always
consistent with lateralization on imaging and cannot be relied upon
to plan surgery [2].
The accuracy of myelography for circumferential localization
ranges from 41 to 100% depending on whether or not oblique views
were obtained [1,2,13]. Ventrodorsal views alone resulted in accu­
rate lateralization in 41–70% of patients [1,2]. Various techniques
used to improve the accuracy of myelography in the circumferential
localization of extradural material include the paradoxical contrast
obstruction sign and oblique radiographs [1,2,47]. If deviation of
the contrast column is not seen on the ventrodorsal view, the
paradoxical contrast obstruction sign can be used to determine the
circumferential localization of the extradural lesion. When a lateralized
Figure 7.8 Sagittal MRI of an intervertebral disc protrusion (A) compared extradural lesion is present, the spinal cord and subarachnoid space
with an intervertebral disc extrusion (B). The protrusion has a broad base will be displaced away from the side of the lesion. The vertebral
along the dorsal aspect of the intervertebral disc and a fusiform shape while canal is a rigid structure and when the spinal cord is pushed against
the extrusion has a narrow base along the intervertebral disc and a rounded the canal the attenuation of the subarachnoid space will occur over
shape. a longer distance on the side opposite the extradural material
(Figure 7.10). This sign has been shown to allow for lateralization of
the extradural compression in 83% of dogs that had no clear devia­
An intradural–extramedullary lesion will cause a filling defect in tion of the contrast column on the ventrodorsal view [47]. Many
the subarachnoid space on at least one projection. This is classi­ authors advise that ventral 45° left‐dorsal right and ventral 45°
cally described as a “golf‐tee” sign where there is focal widening of right‐dorsal left oblique radiographs should be obtained in all cases
the subarachnoid space with a convex margin adjacent to the fill­ where the ventrodorsal view does not provide adequate lateraliza­
ing defect. On the orthogonal view the appearance is similar to tion as oblique radiographs facilitate the lateralization of lesions in
that seen with an intramedullary lesion. The primary differential 94–100% of cases (Figure 7.11) [1,2,13]. Oblique projections facili­
diagnosis for an intradural–extramedullary lesion is neoplasia, tate the imaging of the extradural lesion tangentially, allowing for
such as peripheral nerve sheath neoplasms and meningiomas. A detection of the medial deviation of the contrast column.
focal round dilation of the subarachnoid space can also be seen Splitting of the contrast column (double myelographic line sign)
(golf ball sign) and is associated with a subarachnoid cyst/diver­ occurs because the spinal cord is draped over the extradural mate­
ticulum (Figure 7.9). rial. This was originally reported to be associated with lateralization
of the extradural material but has subsequently been shown to also
occur with ventral extradural lesions and bilateral ventrolateral
extradural lesions, resulting in this sign only being useful in con­
firming that a lesion is extradural in location [48,49].
For extradural lesions CT allows more accurate circumferential
localization compared with myelography [28]. The major question
about spinal CT is whether or not to combine it with myelography.
The advantage of not performing a myelogram prior to CT is that it
is a less invasive study that requires less time to acquire. For the
evaluation of all spinal cord disease the sensitivities of myelography,
Figure 7.9 MRI T2 myelogram image showing focal dilation of the suba­ CT, and CT myelography are 79%, 66%, and 97%, respectively [29].
rachnoid space with a rounded margin (golf ball sign). This is consistent Noncontrast CT can be used to diagnose intervertebral disc when
with a subarachnoid cyst. there is mineralization or hemorrhage associated with the disc