Hypertrophic Cardiomyopathy (HCM)

Hypertrophic cardiomyopathy is the single most common heart disease in domestic cats, and one of the most important for owners to understand because it so often gives no warning. In HCM the muscular walls of the heart's main pumping chamber, the left ventricle, become abnormally thick. A thickened, stiff heart cannot fill and relax properly, so despite looking powerful it actually moves blood less efficiently — and the strain gradually backs pressure up into the lungs and the rest of the circulation.

What makes HCM particularly dangerous is how quietly it can progress. Many cats show no outward signs at all for years; the first indication that anything is wrong may be a heart murmur or an abnormal 'gallop' rhythm picked up during a routine examination — or, tragically, sudden death or a sudden crisis in a cat that had always seemed perfectly well. As the disease advances, fluid can accumulate around or within the lungs (congestive heart failure), causing fast or laboured breathing, and the abnormal blood flow can allow a clot to form. If that clot lodges where the main artery divides to the back legs, it causes the sudden, extremely painful hind-limb paralysis known as a saddle thrombus — one of the most distressing emergencies in feline medicine.

There is a strong hereditary component. The disease is especially associated with Maine Coons and Ragdolls, in which specific causative gene mutations have been identified and can be DNA tested, and it is seen more frequently in British Shorthairs, Persians, and Sphynxes among others. That said, HCM is by no means limited to pedigree cats. Because hyperthyroidism and high blood pressure can produce similar heart-muscle thickening, your vet will usually check for these treatable conditions too.

Diagnosis rests on an echocardiogram — an ultrasound scan of the beating heart — which measures the thickness of the heart walls and assesses how well the heart is working. While HCM cannot be cured, a great many cats live comfortably for years with monitoring and medication that helps the heart relax, controls fluid build-up, and lowers the risk of clots. The single most useful thing an owner can do is learn to count their cat's resting breathing rate at home: a sustained rise above roughly 30 breaths per minute while the cat is asleep is an early red flag that warrants a same-day veterinary call.