Page 368 - Atlas of Small Animal CT and MRI

358 Atlas of Small Animal CT and MRI

intervertebral disks can be narrowed, and disk material vertebral articulation. Synovial cysts appear as T1 hypoin­
can be mineralized. MR features are structurally similar tense, T2 hyperintense thin‐walled cystic masses on MR
to those seen with CT. New bone has variable T1 and images (Figure 3.5.16). Cyst walls will variably enhance. 38,41,42
T2 signal intensity depending on bone density. If
degenerate, associated intervertebral disks can be nar­ Spinal meningeal cysts
rowed and have reduced T2 signal intensity. Meningeal cysts are diverticula of the arachnoid or dura
mater or of a spinal nerve root sheath. In people, cysts
Disseminated idiopathic skeletal hyperostosis can occur at any level of the spinal cord but are often
As the name implies, the underlying cause for dissemi­ located at the level of the lower lumbar cord or sacrum
nated idiopathic skeletal hyperostosis (DISH) is unknown. and are classified as type I (an extradural meningeal cyst
Diagnosis in people is based on several radio graphic that does not contain neural tissue), type II (an extra­
criteria: dural cyst that contains neural tissue), and type III (an
43,44
• the presence of flowing ossification on the anterolateral intradural arachnoid cyst). Although type I and type
(ventrolateral) margins of at least four adjacent verte­ II cysts have not been reported in veterinary medicine,
brae with or without associated localized pointed type III cysts seem to closely resemble spinal arachnoid
45
excrescences at the intervening vertebral body/interver­ diverticula described in dogs. In recent retrospective
tebral disk junctions; reports, diverticula typically occur in the dorsal sub­
• relative preservation of intervertebral disk height arachnoid space at the first to third cervical vertebrae of
(width) and absence of degenerative changes; young, large‐breed dogs or the caudal thoracic vertebrae
• absence of other associated signs of spinal degenerative of older, small‐breed dogs and are most often located
disease. 33 dorsal to the spinal cord. 45,46 Although the etiology of
There are only sporadic reports of DISH in the veterinary arachnoid diverticula is unclear, it appears that some
literature, and there appears to be no consensus of what are likely developmental, while others are acquired. 45,46
imaging features constitute the syndrome in dogs and Regardless of type, meningeal cysts are thin‐walled and
cats. 34–37 Using the criteria defined in the human litera­ contain cerebrospinal fluid, and because of their similar­
ture, new bone formation seen on CT and MR images ity in appearance to other types of spinal cysts, they are
will appear similar to that described for mature spondy­ included in this chapter.
losis but will span a minimum of four adjacent vertebrae On CT images in people, type I and II meningeal cysts
(Figure 3.5.15). are well‐defined, ovoid, fluid‐attenuating masses that are
well delineated by surrounding extradural fat. Cysts will
Extrinsic cysts uniformly enhance on CT myelographic studies because
of direct communication with the subarachnoid space.
Facet synovial cysts MR features include a uniform, thin‐walled T1 hypoin­
Synovial cysts occasionally arise from the articular facet tense and T2 hyperintense mass. No enhancement
joints and can extend into the extradural space of the would be expected following intravenous contrast
vertebral canal. Cyst formation appears to be a sequela administration with either imaging modality.
of degenerative joint disease of the vertebral facet articu­ Myelography of spinal arachnoid diverticula, similar
lations. Although sometimes seen as an incidental to type III cysts in people, results in a teardrop‐shaped
38
finding, synovial cysts have been implicated as a cause or widening of the dorsal contrast column in those cysts
exacerbating factor in spinal cord compression in dogs that communicate with the subarachnoid space (25/36
with cervical spondylomyelopathy. 38–40 They have also in one review). Similar focal fluid collections are seen
46
been reported as a cause of spinal nerve compression in on CT images associated with varying degrees of spinal
dogs with cauda equina signs. 41,42 cord compression. MR features include focal T1 hypoin­
CT features include a well‐defined, usually ovoid, fluid‐ tense, T2 hyperintense fluid collections, most of which
attenuating mass in the extradural space associated with a signal attenuate on FLAIR sequences (Figure 3.5.17). 46

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