What is hypertrophic cardiomyopathy? Hypertrophic cardiomyopathy (HCM) is the most common heart disease of cats, whether they are random bred or pedigreed. It is a heart muscle disease in which the papillary muscles (the muscles in the left ventricle that anchor the mitral valve) and the walls of the left ventricle become abnormally thickened. HCM is often a progressive disease, and a proportion of affected cats develop heart failure if the muscle hypertrophy and subsequent scarring of the heart muscle significantly affects heart function. Cats with the disease may die suddenly and may develop a blood clot in the chamber above the left ventricle (i.e., the left atrium) that often then gets carried into the systemic arterial system, most commonly lodging in the terminal aorta, stopping blood flow to the rear legs.
What causes HCM in cats? This is currently unknown in most cats although familial (hereditary) HCM has been observed in several breeds, such as the Maine Coon and American Short hair. Anecdotal information suggests there is familial HCM in many other breeds. Heart muscle hypertrophy in cats can be caused by other diseases, such as systemic hypertension (high blood pressure) and hyperthyroidism. HCM is a primary disease of the heart muscle. Hypertension and hyperthyroidism cause secondary thickening of the left ventricle and so are not causes of HCM (although it is possible that they may exacerbate the disease if they become present in a cat with mild to moderate HCM). HCM is diagnosed when these other causes are ruled out.
Is HCM genetic? In Maine Coons and American Shorthairs, HCM has been confirmed as an autosomal dominant inherited trait, as it is in humans where over 200 gene mutations in 10 genes have been found to cause the disease. The disease has variable expression; meaning some cats are severely affected, others are only mildly to moderately affected, and some cats may not have evidence of the disease yet produce affected offspring. Recently, a mutation in the cardiac myosin binding protein C (cMyBP-C) gene causing HCM in the Maine Coon cat has been identified. Undoubtedly, other mutations responsible for HCM in cats remain to be discovered. However, since few veterinary cardiologists and geneticists have the expertise to study genes, it may be some time before the responsible gene or genes for each affected breed will be found. The mutation identified as a cause of HCM in Maine Coon cats may not be the same mutation or even on the same gene in other breeds. The genetics of HCM in each breed will require investigation of each individual breed.
Can HCM have a nutritional cause? There is no evidence in cats, humans or other species of animals that HCM can have a nutritional cause.
How is HCM diagnosed? HCM is diagnosed using ultrasound of the heart – an echocardiogram. Echocardiography is a good way to detect moderate to severely affected cats. However, it may not always detect mildly affected cats where changes in the heart can be minimal. Ideally, an echocardiogram to test cats for HCM should be performed by a board-certified cardiologist or radiologist. In addition to an echocardiogram, other tests may also be useful in assessing cats with HCM. For example, a chest x-ray is necessary to detect heart failure in cats with severe HCM. An electrocardiogram is useful in cats that have abnormal heart rhythms. Blood pressure measurement and blood testing for hyperthyroidism are indicated to rule out other diseases that mimic HCM, especially mild to moderate HCM. A genetic test is now available for the known cMyBP-C mutation causing HCM in Maine Coon cats. The test is available from the Veterinary Cardiac Genetics Lab of Dr. Kathryn Meurs at the College of Veterinary Medicine, Washington State University (http://www.vetmed.wsu.edu/deptsvcgl/). The test can identify which cats have the mutation. If a cat is identified as having the mutation, the test can also determine whether the cat carries one copy of the gene (a heterozygote) or two copies of the gene (a homozygote).
What does "HCM free cattery" mean? There is no universally agreed upon definition of an HCM free cattery. The terminology is currently unclear, as different breeders mean different things when they use this term. Ideally, each breed should develop a specific definition and guidelines for use of this designation for catteries.
This article is written by the Winn Feline Foundation Mark Kittleson, DVM, PhD, DACVIM (Cardiology) School of Veterinary Medicine University of California, Davis Rebecca Gompf, DVM, MS, DACVIM (Cardiology) College of Veterinary Medicine, University of Tennessee Susan Little, DVM, DABVP (Feline) President, Winn Feline Foundation