Cardiomyopathy in Cats and Dogs: HCM, DCM and What They Mean

Cardiomyopathy in Cats and Dogs: HCM, DCM and What They Mean

Cardiomyopathy means disease of the heart muscle. Cardio for heart, myo for muscle, pathy for disease. It refers to a problem with the muscular wall of the heart itself, as distinct from a problem with the valves inside it, which is a separate category of disease.

The abbreviations get used constantly in breed discussions and almost never explained. They are worth getting straight, because the two main ones describe close to opposite problems.

HCM and DCM Are Opposites

In HCM the muscle of the left ventricle, the chamber that pumps blood to the whole body, becomes thick. Hypertrophied is the medical word and it simply means enlarged through growth of the muscle. That thickening reduces how much blood can enter the ventricle, and it makes the muscle relax abnormally. Less blood in means less blood out, and the left atrium behind it dilates with blood that cannot flow into the stiffened chamber.

DCM works the other way. The heart muscle degenerates, becoming thinner and weaker, particularly the normally thick wall of the left ventricle. As it weakens it loses the ability to pump effectively, blood congests inside the heart, pressure rises, and the thin walls stretch. That stretching is what produces the large, dilated appearance the name refers to.

Cats: HCM Is the Common One

HCM is the most common heart disease of cats. There are five recognized categories of feline cardiomyopathy, and it is worth knowing the others exist: RCM, restrictive, where the heart stiffens with scar tissue; DCM, which cats do get though rarely; ARVC, arrhythmogenic right ventricular cardiomyopathy, where the right ventricle deteriorates; and NCM, nonspecific, for changes that fit none of the other categories.

Predisposed breeds: Maine Coon, Ragdoll, British Shorthair, American Shorthair, Sphynx, Bengal, Chartreux, Norwegian Forest, Siberian and Persian. Mutations in several cardiac genes have been identified in some affected cats, and heritable mutations are recognized in breeds including the Maine Coon, Ragdoll and Sphynx.

An important caveat: not all cats carrying an identified mutation develop HCM, so genetic screening results need veterinary interpretation rather than being read as a straight yes or no.

What the Research Actually Shows About Progression

A UK study followed 107 cats from rehoming centers over a median of 5.6 years. It is one of the more useful pieces of evidence available, because it watched ordinary cats over time rather than cats already referred for heart problems.

Those events broke down as congestive heart failure in 5.9%, arterial thromboembolism in 5.9% and sudden death in 5.9%.

Signs, and Why They Arrive Late

Early HCM often produces no signs at all. When they do appear, they include breathlessness and fatigue, fluid accumulation in the abdomen, paralysis of the back legs from blood clots, and sudden death.

That list is unusually front-loaded with catastrophe, which is the honest problem with feline heart disease: for many cats the first visible sign is already an emergency. It is the reason breed screening exists at all.

Saddle Thrombus, the Complication Owners Fear

Aortic thromboembolism (ATE), commonly called a saddle thrombus, happens when a blood clot dislodges, travels through the aorta and lodges at a distant point, cutting off blood and oxygen to whatever is downstream. It is typically associated with underlying heart disease, particularly HCM, though cancer and sepsis can also cause it.

It is more common in cats than dogs, affects males more often than females, and typically occurs between 8 and 12 years of age. Reported breeds include mixed breed, Abyssinian, Ragdoll and Birman. Signs are sudden paralysis and pain usually in the rear legs, weakness, rapid breathing, pale nail beds, and vocalizing from pain.

The prognosis in general is very poor, with most cats surviving only a few months even with treatment. Leg function may return over days to weeks, but permanent nerve or muscle damage is common.

Dogs: DCM and the Large Breeds

DCM is the most common cause of heart failure in certain large breeds. Smaller dogs are more prone to valve disease instead, which is a different problem covered by its own literature.

DCM may seem to arrive suddenly, but the disease has been developing slowly and subtly for a long time before that. Some dogs develop severe congestive heart failure in only a few hours, and rapid heavy breathing, a blue tongue, excessive drooling or collapse may be the first thing an owner notices.

Dogs that begin cardiac medication in the early stages have a better prognosis than those already showing signs of heart failure, which is the argument for screening in predisposed breeds rather than waiting.

Diagnosis and Screening

An echocardiogram, an ultrasound of the heart, is required for a specific diagnosis in both species. It is the only test that shows chamber size and wall thickness directly. Supporting tests include chest X-rays, bloodwork and blood pressure measurement.

In cats there is a specific diagnostic requirement worth knowing: high blood pressure and hyperthyroidism both cause thickening that mimics HCM, so both must be ruled out before the diagnosis is made.

For the cat breeds where annual screening from age 2 is standard practice, see our Maine Coon guide, Ragdoll guide, Persian guide and Bengal guide.

Treatment

There is no cure for HCM. Medications used include diuretics, beta-blockers, calcium channel blockers and clot preventatives such as clopidogrel. Cats with idiopathic HCM, meaning HCM with no identified underlying cause, may remain stable for several years. Prognosis worsens once heart failure or clots appear.

Sources & Further Reading