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What is it?

HCM, or Hypertrophic Cardiomyopathy, is the commonest heart disease in cats.

What causes it?

HCM occurs when the muscle in the cat's heart becomes excessively thickened. This might sound a good thing - a thick strong heart should mean a more efficient heart - but in HCM this process proceeds so far that the heart, although very powerful, is unable to fill with blood properly, resulting in abnormal blood flow, blood clot formation, and ultimately heart failure. There are two well recognised underlying causes of HCM - a genetic mutation and hyperthyroidism.

What cats are at risk?

Hyperthyroidism may occur in any older cat, and often results in HCM. The genetic disease is most common in Maine Coon cats, although it has also been recognised in some Shorthaired breeds and Persians, among others. The genetic condition usually presents in young to middle-aged adult cats, and toms are thought to be at an increased risk.

What are the symptoms?

In most cases, the initial symptom will sudden-onset heart failure or thromboembolic (blood clot) disease. Symptoms of heart failure in cats include exercise intolerance, difficulty breathing, collapse, pale gums, weak pulses, and even sudden death. Blood clots are more common, and may lead to sudden onset acutely painful paralysis (usually of the hindlimbs), a stroke (causing abnormal behaviour, blindness or paralysis), or a pulmonary embolism (rare, usually causing severe distress, difficulty in breathing and sudden death).

How is it diagnosed?

There is a blood test that is a useful screening test for heart disease in cats, but the only way to diagnose HCM is with an ultrasound scan of the heart (echocardiography). A heart scan like this can also assess how likely blood clots are in the near future, as micro-clots are visible as "smoke" in the left atrium of the heart.

How can it be treated or managed?

There is a licensed medication (diltiazem) for HCM, which is designed to allow the heart muscle to relax, so the chambers can fill more effectively with blood. In addition, it is increasingly seen as good practice to prescribe certain blood-thinners to cats with HCM, to reduce the risk of blood clots forming,

Can it be prevented?

Unfortunately, there is no way to prevent the genetic disease except by not breeding from known carriers. Good control of thyroxine levels in cats with hyperthyroidism will, however, usually prevent them from going on to develop HCM.