Page 38 - Atlas of Small Animal CT and MRI

28 Atlas of Small Animal CT and MRI

enhancing on both CT and MR images as a result of high of the petrous temporal bone may be seen, and meningeal
vascular density and increased vascular permeability and cranial nerve VII/VIII enhancement is often present
(Figure 1.2.6). Gravitational migration of inflammation on contrast‐enhanced images (Figures 1.2.9, 1.2.10, 1.2.11). 8
may result in development of fistulae that may be tracked
back to the external ear canal by conventional or CT Cholesteatoma
fistulography. Aural cholesteatomas are epidermoid cysts that form
expansile masses of keratin debris and keratinized squa­
Bulla effusion mous epithelium. They may be congenital or acquired;
While not necessarily inflammatory, bulla effusion must however, in dogs cholesteatomas appear to be acquired
be differentiated from otitis media. Unilateral or bilateral and are likely initiated by underlying otitis media.
sterile bulla effusion can occur secondary to obstruction Cholesteatomas are most often unilateral, but bilateral
of auditory canal flow (Figure 1.2.5). This entity has lesions can occur. Imaging findings include a combination
been described as a common sequela to either nasal of bulla expansion, reactive osteoproliferation, and bulla
disease, particularly in association with pharyngeal osteolysis (Figure 1.2.12). A soft‐tissue mass is present
5–7
masses or pharyngitis, or to brachycephalic syndrome. centrally in the region of the tympanic bulla, which usually
In affected dogs, the effusion is progressive but is not contrast enhances heterogeneously or peripherally. In
accompanied by clinical signs other than loss of hearing. some patients, osteolysis of the petrous and squamous
Bulla fluid accumulates from bulla lining secretions and parts of the temporal bone may occur, with resulting
therefore contains macromolecules and cellular debris intracranial extension of disease (Figure 1.2.13). In these
that appear fluid attenuating on CT images, hyperintense cases, neurologic signs associated with cranial nerves VII
on T2 images, and of variable intensity on T1 images. and VIII may be evident, and regional meningeal contrast
enhancement is sometimes present. Sclerosis and osteo­
9
Otitis media proliferation of the temporomandibular joint and para­
Bulla effusion may be the only abnormal imaging feature condylar process can be seen.
in early otitis media, although the disorder is often
present concurrent with otitis externa and may involve Neoplasia
the petrosal part of the temporal bone, depending on
chronicity and severity. Exudative effusion appears Neoplastic masses may arise within the external ear
soft‐tissue attenuating on CT images, hyperintense on canal. The World Health Organization (WHO) recog­
T2 images, and of intermediate intensity on T1 images nizes both ceruminous gland adenomas and ceruminous
(Figure 1.2.7). The bulla lining typically becomes gland adenocarcinomas in this category.
thickened and irregular and markedly contrast enhances Ceruminous gland adenomas expand into and may
on both CT and MR images. With increasing chronicity, occlude the external ear canal, leading to secondary otitis
the bulla wall may become thickened and irregularly externa; however, the integrity of the external ear canal
margined as a result of reactive osteitis, and the bulla wall is typically maintained (Figure 1.2.14). Adenomas
cavity volume may increase, presumably because of the appear similar to inflammatory polyps. As with polyps,
effect of hydrostatic pressure from the effusion adenomas contrast enhance on both CT and MR images,
(Figure 1.2.8). On CT images, one must use caution in increasing conspicuity.
assessing the thickness of the osseous bulla wall because Ceruminous adenocarcinomas are often well
replacement of air by fluid within the bulla cavity may advanced by the time of imaging evaluation, and the
artifactually increase apparent thickness. Thickening specific site of origin may not be easily determined.
and/or expansion of the osseous bulla may also be These tumors are aggressive and highly invasive, typi­
present without other abnormal imaging findings in cally obliterating the external ear canal and often extend­
patients with previous otitis media that has resolved. ing to the middle and inner ear. Adenocarcinomas are
also highly destructive, resulting in osteolysis of the
Otitis interna and intracranial extension osseous bulla and erosion of the petrous and squamous
Osteitis of the petrous temporal bone is commonly parts of the temporal bone (Figure 1.2.15). These tumors
associated with chronic otitis media, and progression are highly but heterogeneously contrast enhancing on
to otitis interna is suggested by the presence of cranial both CT and MR images. Depending on the size of the
nerve VII and VIII deficits. Infection may progress mass, adjacent structures, such as the pharynx, larynx,
through the internal acoustic meatus or by direct mandibular salivary gland, and temporal musculature,
extension through osteolysis of the petrous temporal may be involved. Intracranial extension can occur with
bone. Some combination of osteosclerosis and osteolysis advanced disease, resulting in intracranial mass effect

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