Case 170526 -3

Two-year-old, male, castrated, mini-rex rabbit.

This rabbit was presented to a veterinary clinic for lethargy, inappetance and constipation for a period of twelve hours. The physical exam findings included dehydration, “muddy” mucous membranes, mild dental disease, and hypothermia. Radiographs revealed a gas distended stomach and a loop of gas distended intestine. These findings suggested a gastrointestinal obstruction. An abdominal focused assessment with sonography for trauma (A-FAST scan) was performed, and the findings were thought be consistent with a gastrointestinal perforation. Euthanasia was elected due to the rapidly declining condition and poor prognosis.

The stomach was moderately distended with viscous ingesta containing fibrous plant material. There was no evidence of gastrointestinal perforation.

This slide contains a transverse section through the heart at the level of the ventricles. The section includes a left ventricular papillary muscle, in which a thick band of dense, mature fibrous connective tissue circumferentially thickens the endocardium and separates, isolates or replaces subendocardial cardiomyocytes predominantly of the left ventricle. Similar fibrous connective tissue extends into the adjacent myocardium, affecting approximately 50% of the myocardial tissue. The connective tissue dissects between myofibers, surrounds vessels, and entraps individual myocytes or small clusters of myocytes. Isolated cardiomyocytes have moderate anisocytosis and anisokaryosis. Sarcoplasmic vacuolation is common but variable in these cells. Some are multinucleate with rowing of up to five nuclei. Rarely the entrapped myocardial fibers have pyknotic nuclei with fragmented sarcoplasm. Some cardiomyocytes have mild to moderate, intra-sarcoplasmic amphophilic to basophilic pigment (basophilic degeneration) or brown, granular pigment. At the periphery of heavily affected areas, thin streams of collagen dissect into the myocardium and separate cardiomyocytes without the severe changes seen in the entrapped, isolated muscle fibers. Some of the fibrotic areas are mildly hypercellular with suspected lymphocyte infiltration. There are also several clusters of adipocytes that multifocally replace subendocardial cardiomyocytes (steatosis).

No special stains.

Heart: Left ventricle: severe, chronic, circumferential endomyocardial fibrosis with cardiomyocyte atrophy, vacuolar degeneration, regeneration and rare, myocyte necrosis

This pattern of endomyocardial fibrosis that creates a band-like restriction is consistent with changes reported to result in restrictive cardiomyopathy (RCM). Unlike other types of cardiomyopathy, such as hypertrophic cardiomyopathy (HCM), which is associated with a consistent morphologic change, a myriad of pathologic processes may result in a clinical presentation of RCM with wide phenotypic heterogeneity. Restrictive cardiomyopathy refers to a number of conditions, which ultimately result in reduced diastolic relaxation (diastolic dysfunction) of either the left or both ventricles, with subsequent impaired diastolic filling, increased ventricular filling pressure and normal (or near normal), systolic function. Often there is no change in ventricular wall thicknesses but atrial enlargement may or may not occur. Restrictive cardiomyopathy may result from a primarily endocardial process or a more generalized infiltrative disease. Examples of endocardial causes of RCM include endocardial or endomyocardial fibrosis in either a diffuse or trabecular pattern, as well as endocardial fibroelastosis. Examples of infiltrative disease resulting in RCM include amyloidosis, storage diseases, or hemachromatosis.

Both the endocardial or infiltrative myocardial forms essentially act as a “straight jacket” for the affected heart, in which contraction is unaffected but relaxation is compromised.

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