Hypertrophic cardiomyopathy ("HCM") is a disease of the myocardium (the muscle of the heart) in which a portion of the myocardium is hypertrophied (thickened) without any obvious cause. It is perhaps most well known as a leading cause of sudden cardiac death in young athletes. A cardiomyopathy is a primary disease that affects the muscle of the heart. With hypertrophic cardiomyopathy (HCM), the sarcomeres (contractile elements) in the heart increase in size, which results in the thickening of the heart muscle.

Feline HCM:

Feline hypertrophic cardiomyopathy (HCM) is the most common heart disease in cats; the disease process and genetics are believed to be similar to the disease in humans.^[

It is characterised by an abnormally thickened (hypertrophic) myocardium, the muscle that forms the heart wall.

Because of its increased thickness, the myocardium becomes stiffer and does not distend enough to allow complete, so a lower blood volume will be pumped into circulationafter each heart beat eventually, the upper heart chambers (atria) dilate in response to the increased pressure in the lower chambers (ventricles).

Initially, the affected cat does not present any noticeable symptoms (compensatory phase). However, with time, the pressure may  also increase in the left atrium and, subsequently, in the vessels that carry blood from the lungs to the left atrium. These vessels become engorged with blood and, eventually, fluid will leak from the vessels into the lungs (pulmonary oedema) or chest cavity (pleural effusion).

If fluid starts building up in the lungs or chest cavity, the cat starts breathing with difficulty and at a faster rate (usually more than 40-50 breaths per minute). This symptomatic phase of the disease is called “congestive heart failure” or simply CHF.

While medication is not commonly given to cats with HCM that have no clinical signs,  it has been shown that an ACE inhibitor is beneficial when heart failure is present  (at which time a diuretic is most beneficial). Diltiazem generally produces no demonstrable benefit and  Atenolol is commonly administered when systolic anterior motion of the mitral valve is present.

Unfortunately, death can occur without any other signs present making the disease a difficult and sometimes a deadly one.

While there is no cure for HCM, early detection and regular echocardiograms are key to trying to ward off life-threatening problems.

Possible complications of severe HCM

Thromboembolic disease (TED) is relatively common sequelae of Feline HCM. The aetiology remains a little uncertain, but it is thought that ischemic damage to the hypertrophied left ventricular myocardium facilitates thrombus formation and subsequent embolism. Classically the embolus lodges at the iliac bifurcation of the aorta, occluding either one or both of the common iliac arteries. Clinically this presents as a cat with complete loss of function in one or both hindlimbs. The hindlimbs are cold, and the cat is in considerable pain. This pain derives from the exaggerated inflammatory response to the embolus at the point of impact, and the inflammatory mediators released generally have a vasoconstrictor effect further exacerbating the problem.

Treatment of TED is variable - typically very low doses of aspirin may be prescribed (aspirin however is extremely toxic to cats and should only be prescribed and administered by a veterinary surgeon). Plavix is also another widely used drug that may or may not prevent clot formation in HCM cats. The FATCAT study at Purdue University is addressing the efficacy of aspirin vs. Plavix for the prevention of a second clot in cats that have already experienced a clot. Thrombolytic agents (e.g., tissue plasminogen activators) have been used successfully, but their cost is usually prohibitively high in veterinary medicine. Despite the relative efficacy of treatment, the prognosis for cats with TED is poor as they are likely to have significant HCM already, and a recurrent bout of TED is very likely. For this reason unfortunaltym euthanasia is often considered in TED cats.