Hypertrophic cardiomyopathy (HCM) is the most common heart condition in cats, with familial (inherited) forms of the disease having been recognised in a number of breeds. The disease in cats is assumed to be similar to the disease in humans, where currently 4341 have been shown to cause HCM. Ragdolls, Persians and Maine coons are amongst breeds with the highest rates of disease due to HCM.
HCM causes abnormal thickening of the heart muscle, and this means that the heart cannot function properly. Thickening of the ventricle of the heart leads to muscle stiffness and less effective contraction. Blood tends to become “backed up” through the atrium and into the veins of the lungs. Eventually this leads to leakage of fluid into the air spaces, which is called pulmonary oedema. When this happens, the cat has congestive heart failure. As well as congestive heart failure, HCM can lead to arrhythmias (abnormal heart rhythms) and sometimes this can cause sudden death without any prior clinical signs being seen. Another uncommon complication of HCM is aortic thromboembolism, where a blood clot forms in the enlarged left atrium and at some point leaves the heart and lodges in the aorta - often where it narrows and branches to go to the hindlimbs. This leads to a lack of blood flow to and subsequent paralysis of the hind legs, and is very painful. Clots may less commonly lodge elsewhere, such as in the lungs or brain.
HCM can occur at any age, although is more often seen in adult to middle aged cats. The exception is in the ragdoll, where it is common to see heart failure by 2 - 3years of age. Diagnosis of HCM is based on a cardiac ultrasound (echocardiogram), and should generally be performed by a specialist.
There is no cure for HCM, although if thickening of the heart muscle is secondary to another disease, such as hyperthyroidism, treatment of the primary condition may resolve the cardiac condition. Treatment of HCM aims to manage signs of congestive heart failure, and reduce the abnormality of muscle relaxation as much as possible. Recent studies have shown that calcium channel blockers have shown good results at improving heart function and blood flow around the body. Drugs to control arrhythmias are given if needed. Therapy is often given to try to prevent blood clots from forming in the heart (e.g. aspirin), although studies have shown that this treatment is not all that effective, and will not get rid of clots that have already formed. This treatment must be monitored carefully, as it can also lead to an increased risk of bleeding.
Genetic testing is available for several breeds, including MYBPC3 mutations that occur in the ragdoll and Maine coon. These are both dominant mutations with variable expression. There is also a test for the HCM2 mutation that occurs in several breeds (British shorthair, Maine coon, Norwegian forest cat, Persian, ragdoll) however this mutation has low penetrance and cats with this mutation may never go on to develop clinical disease. Ultrasound screening remains important in breeds with this form of cardiomyopathy.