Page 630 - Atlas of Small Animal CT and MRI

620 Atlas of Small Animal CT and MRI

MR features include intercondylar STIR and T1 Secondary hyperparathyroidism
heterogeneity corresponding to the hypoattenuating The two most common causes of secondary hyper­
linear regions seen on CT. Central T1 hyperintensity is parathyroidism are chronic hyperphosphatemic renal
surrounded by signal void due to adjacent sclerosis. disease and diets deficient in calcium or with low calcium
Normal condyles are described as having uniform inter­ to phosphorus ratio. Low serum calcium levels cause an
condylar STIR and T1 intensity. 48 increase in parathyroid hormone production. This in turn
CT appears to be more sensitive than radiography for leads to bone calcium mobilization that results in fibrous
detection of incomplete humeral condylar ossification, osteodystrophy. Affected animals have clinical signs of
particularly when fissures are incomplete. Numbers systemic disease, but the imaging manifestations are
reported for MR diagnosis are not yet sufficient to deter­ reduced bone density and pathologic fractures.
mine the merit of this modality. The skeletal manifestations of renal secondary hyper­
parathyroidism are not uniform, with bones of the
Disruptions of endochondral ossification skull being more affected early in the course of the
Disorders of enchondral ossification, such as multiple disease. Nutritional secondary hyperparathyroidism,
cartilaginous exostoses and retained ulnar cartilage particularly in skeletally immature animals, appears to
core, are due to interruption or alteration of normal affect the bones more uniformly because of the overall
endochondral ossification. These disorders are readily higher bone metabolic activity in young animals.
recognized on survey radiographic examinations and CT features include reduced bone density and cortical
are therefore most likely to be seen as secondary find­ thinning, which can be striking in some patients
ings on CT or MR examinations. (Figure 6.1.17). Pathologic fractures are a common
sequela in both the appendicular and axial skeleton. 53
Metabolic disorders
Soft‐tissue mineralization due
Hypertrophic osteopathy to hyperadrenocorticism
Hypertrophic osteopathy most often results from neo­ Elevated cortisol levels are thought to a have catabolic
plastic or inflammatory masses in the thoracic, abdomi­ effect on collagen, elastin, and other proteins, resulting in
nal, or pelvic cavities or from direct alterations in vascular increased calcium binding. Calcinosis cutis, mineraliza­
perfusion. 49–52 The disorder manifests radiographically as tion within the skin, occurs in about 30% of dogs with
dense, but often irregularly margined, periosteal new hyperadrenocorticism, but other soft tissues, such as skel­
bone formation of the diaphysis and metaphysis of long etal muscle, lung, and stomach, can also mineralize. 54
bones, which seems to preferentially involve the distal CT features include plaque‐like hyperattenuation in
extremities although all long bones are sometimes the skin (Figure 6.1.18). Similar changes also seem to
affected. New bone is often, but not always, symmetrically occur frequently in muscle and along fascial planes.
distributed, and thoracic and pelvic limbs may be affected
to different degrees. Computed tomographic features Other metabolic disorders
of hypertrophic osteopathy have not been previously Although there is a plethora of metabolic disorders that
reported but generally parallel survey radiographic affect the skeletal system, they are uncommon, and there
findings (Figure 6.1.16). are few reports of the use of CT or MR for diagnosis.

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