Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

638 Small Animal Radiolo g y and Ultrasono graphy
Occipital Dysplasia. Occipital dysplasia is a morphologic variation in which the margin
of the foramen magnum is extended dorsally. 87 This produces a foramen magnum that is
enlarged, elongated, or pear shaped. In severe cases, this may result in a foramen magnum
large enough to allow cerebellar herniation. The radiographic appearance is observed readily
using the modified occipital view (Fig. 5-34). A potential pitfall is misinterpreting the
impression of the vermis in the occipital bone as evidence of occipital dysplasia. Even when
a foramen magnum malformation is present, usually there is a fibrous band covering the
bony defect preventing cerebellar herniation. 88 A flexed lateral view of a myelogram may
be useful in demonstrating herniation of the cerebellum through a patent defect. 89 The
clinical significance of occipital dysplasia is not established. In a study of 80 Pekingese
skulls, the shape of the foramen magnum varied from ovoid to rectangular, and all but two
had a dorsal notch. 88 Many dogs with occipital dysplasia are also hydrocephalic, and the
clinical signs probably are related to the hydrocephalus rather than to the foramen magnum
conformation. Some dogs may also have syringomyelia or hydromyelia. 89
Acromegaly. Acromegaly has been reported in cats in association with diabetes mellitus
and pituitary adenoma. 90 Thickening of the bony ridges of the calvarium, mandibular
prognathism, and spondylosis deformans have been described in these cats. Osteoarthritis
of the shoulder, elbow, carpus, stifle, and digits also is present. CT scans were performed on
six cats, and the pituitary tumor was demonstrated in five. Mild to moderate cardiomegaly
was observed on thoracic radiographs, and thickening of the left ventricular free wall and
interventricular septum was demonstrated by echocardiography. Pulmonary edema,
ascites, and pleural effusion were observed in those cats that progressed to heart failure. In
acromegalic dogs, no abnormalities were noted in survey radiographs of the appendicular
skeleton. 91 Soft-tissue swelling was observed ventral to the pharynx. This abnormality was
reversible and disappeared posttreatment.
Mucopolysaccharidosis. An exaggerated brachycephalic appearance to the skull and a
widened maxilla have been reported as features of mucopolysaccharidosis in cats. 92-95 The
Fig. 5-34 A 5-year-old Maltese cat that had cervical pain. On
cervical radiographs, a suspicious appearance to the occiput
was noted. The modified occipital view revealed a large bony
defect extending dorsally from where the normal roof of the
foramen would be expected (white arrows). Diagnosis:
Asymptomatic occipital dysplasia. A cervical disc extrusion
was responsible for the clinical signs.