Hypertrophic cardiomyopathy (HCM) is a fairly common disease in cats, but the cause is yet unknown.  The walls of the ventricles are abnormally thickened and the chamber is abnormally narrowed. As a result, the ventricle (the section of the heart that pumps blood to the body) cannot effectively fill with blood.  In response, the atria swell to accommodate this back-up of blood.  This increase in atrial size results in sluggish blood flow within the atria and increases the chance of coagulation (clot formation). 

If a clot forms in the atria, it can enter the systemic circulation and lodge within a blood vessel.  Most commonly, this clot, or thromboembolism, will lodge at the terminal bifurcation of the aorta where it branches to supply the blood vessels of the hind legs.  This is called a "saddle thromboemboli".  Thromboembolism of the terminal aorta manifests as acute hind limb weakness or paralysis, pain, and cold/pale foot pads. 

Occasionally, a clot may lodge elsewhere in the body and can result in acute renal failure or sudden death.To help prevent the initial blood clot formation, cats with HCM are frequently put on anticoagulant drugs (Fragmin, aspirin, or heparin are most commonly used).  Another potential outcome of this disease is congestive heart failure which can result in difficulty breathing. 

It is impossible to predict how individual cats will progress with this disease, and life expectancy is greatly varied.  There is a very good chance that your cat will be on lifelong medication.  To help us judge how your cat is responding to therapy, regular recheck visits (physical exams and echocardiographs) every 6-8 months are important.