Feline restrictive cardiomyopathy

Siobhan O’Sullivan

Animal Health Laboratory, University of Guelph, Guelph, ON. 

AHL Newsletter 2025;29(2):29.

A 5-year-old Siberian cat initially presented with an acute episode of ataxia, but MRI imaging and cerebrospinal fluid analysis were unremarkable. Clinical signs progressed to lethargy and laboured breathing, followed by cardiac arrest. At postmortem, there was cardiomegaly and enlargement of the left auricle. The muscular wall of the left ventricle was not grossly hypertrophied. Histopathology revealed pulmonary edema, and in the left cardiac ventricle, the endocardium was markedly expanded by mature fibrous tissue with adherent fibrin (Fig. 1). There was also extensive replacement fibrosis of the myocardium. These cardiac lesions are typical of restrictive cardiomyopathy (RCM). It was considered that the episode of ataxia may have been secondary to either thromboembolism, or reduced cardiac output. 

Feline hypertrophic cardiomyopathy (HCM) is a top differential for feline sudden death, while RCM is a less common postmortem finding. The cause of death in animals with RCM is left-sided heart failure, or fatal arrhythmia secondary to dysfunction of the cardiac conduction system if there is extensive involvement of the myocardium. There are subtypes of RCM which vary by gross appearance and nature of histologic lesions, but all involve fibrosis of the left ventricle. These subtypes include: endomyocardial fibrosis, myocardial fibrosis, endomyocarditis/endocardial fibrosis and endocardial fibroelastosis.

The cause of RCM is undetermined. Some cases have been reported secondary to Bartonella induced myocarditis, but RCM is also considered to be heritable to some degree, similar to HCM in Maine coon and ragdoll cats. As yet, a specific mutation for RCM has not been identified, and it is unclear whether Siberian cats may be genetically predisposed to the development of RCM.   

Figure 1. Feline, heart. A: The endocardium of the left ventricle (arrow) is markedly expanded by mature fibrous tissue, and there is adherent fibrin (star). B: The thickness of the endocardium of the right ventricle (arrowheads) from the same cat. H&E stain.

References

1. Kittleson MD, Côté E. The feline cardiomyopathies: 3. Cardiomyopathies other than HCM. J Feline Med Surg. 2021; 23(11):1053-1067. 

2. Robinson W, Robinson N. Cardiovascular system. In: Jubb, Kennedy, and Palmer's Pathology of Domestic Animals, 6th ed. Maxie MG, ed. Elsevier. 2016;3:46.