Small Animal Review

Hypertrophic cardiomyopathy (HCM) is a common cause of sudden death in cats that is characterised by thickening of the ventricular walls. At postmortem heart weight is a reliable proxy of cardiac size with heart:body weight (BW) ratios in excess of 4.2 g/kg BW indicating a large heart; most cases of HCM show ratios in excess of 5 g/kg BW. Hypertrophic cardiomyopathy may present in various ways including congestive heart failure, aortic thromboembolism or sudden death with no premonitory signs. Reports suggest a familial predisposition in Persian, American Shorthair, Maine Coon and Ragdoll cats with specific gene mutations identified in Maine Coon and Ragdolls. Male cats are overrepresented and typically the idiopathic form of HCM is seen in young to middle aged adults. The pathology of HCM is characterised by a generalised or focal thickening of the myocardium offen with dilatation of the left atrium, cardiomyocyte hypertrophy, some interstitial fibrosis and mixed inflammatory cell aggregates. The pathophysiology of the myocardial injury is unclear, with developmental issues (gene mutations), toxicities, inflammatory processes and myocardial infarction suggested. There is an association between HCM in the older cat and hyperthyroidism. Mediators of inflammation and remodelling are present in the cats myocardium, and there is a recognised shift from a pro-inflammatory state in younger cats to a pro-fibrotic state as cats age. Their role in causing morphological changes in feline hearts was studied by Kitz et al (2019). They compared the hearts of 18 cats with HCM, with those from 18 control cats. Histological features of HCM (cardiomyocyte disarray and degeneration, leukocyte infiltration, and interstitial fibrosis) were observed in 83% of the HCM cases with patchy to disseminated multifocal loss and/or degeneration of cardiomyocytes, collagen deposition with moderate numbers of fibrocytes, increased interstitial cellularity associated with the presence macrophage and blood vessel proliferation.