Feline hypertrophic cardiomyopathy: the most common heart disease in cats
Hypertrophic cardiomyopathy is the most common heart disease in cats, and many have it without anyone knowing. It often causes no symptoms for years, until it suddenly appears with breathing difficulty or paralysis of the back legs. Knowing about it means you can catch it in time.
What it is
In hypertrophic cardiomyopathy (HCM), the wall of the heart's left ventricle, the chamber that pumps blood to the body, thickens. A thicker muscle sounds stronger, but the opposite happens: it becomes stiff, relaxes poorly and fills less well with blood. Over time, the left atrium, the chamber above it, dilates under the pressure, and blood pools inside it. That is where the two serious complications come from: heart failure, with fluid in the lungs or the chest, and the formation of clots that can leave the heart. In most cats with HCM, however, the disease progresses little or not at all and never causes symptoms.
Who it affects
It is common in the general cat population: in studies of apparently healthy cats it has been found in around one in seven, especially middle-aged and older cats, and more in males. Some breeds have an inherited predisposition, and in two of them specific mutations are known: the Maine Coon and the Ragdoll. It is also described more often in the British Shorthair, Sphynx, Persian, Bengal, Norwegian Forest Cat and others. But by sheer numbers, ordinary moggies account for the most cases. In addition, hyperthyroidism and high blood pressure can thicken the heart in a similar way, so they must be ruled out.
Why it usually goes unnoticed
Many cats with HCM have no symptoms for years, or for life. Some have a heart murmur or a gallop rhythm that the vet hears when listening with a stethoscope, but not every cat with a murmur has HCM and not every cat with HCM has a murmur. Cats also adapt by reducing their activity, so lower exercise tolerance goes unnoticed. The disease is usually discovered at a check-up, before an anaesthetic, or when a complication appears out of the blue. That is why heart check-ups are recommended for predisposed breeds and for cats with a murmur.
The warning signs
Heart failure shows mainly as fast or laboured breathing, with the belly moving a lot, open-mouth breathing, tiredness, hiding or not eating. Coughing is rare in cats with heart disease, unlike in dogs. Sometimes the first symptom appears after stress, an anaesthetic or treatment with corticosteroids or fluids. Arterial thromboembolism, the most dramatic complication, presents as sudden paralysis of one or both back legs, which are cold, with pale or bluish pads, and with excruciating pain: the cat cries out. There can also be fainting and, in some cats, sudden death.
Count their breaths
This is the most useful tool you have at home if your cat has HCM or belongs to an at-risk breed. When they are asleep or resting calmly, count how many times the chest rises in 30 seconds and multiply by two. A healthy cat at rest normally breathes fewer than 30 times a minute. If they repeatedly breathe more than 30 times a minute while asleep, see the vet soon; if it is over 40, or they breathe with effort or with the mouth open, it is an emergency. There are phone apps that help you keep a record. This simple count detects fluid on the lungs days before the cat becomes seriously ill.
How it is diagnosed
The key test is echocardiography, an ultrasound of the heart that measures wall thickness, the size of the atrium, how well it relaxes and whether there are clots or "smoke" inside, which indicates a risk of thrombi. Ideally it is done by a vet experienced in cardiology. A blood test called NT-proBNP, which rises when the heart is under strain, can be used for screening in cats with a murmur or in at-risk breeds, although it does not replace echocardiography. Blood pressure is also measured and, in older cats, thyroid hormone. If they are struggling to breathe, an X-ray or a quick chest ultrasound shows whether there is fluid.
Stages and treatment
The ACVIM consensus statement on feline cardiomyopathies classifies the disease into stages: A, cats with a predisposition but no disease; B1, disease without symptoms and at low risk; B2, no symptoms but at higher risk, mainly because of a dilated atrium; C, cats that have had heart failure or a thromboembolism; and D, heart failure that no longer responds well to treatment. Low-risk cats are not usually treated, only monitored. In those at higher risk, the vet will usually prescribe medication to prevent clots. In heart failure, diuretics and other heart drugs. There is not yet a treatment that reverses the thickening, although new options are being researched.
Breeds, breeders and genetic tests
In the Maine Coon and Ragdoll there are genetic tests for the known mutations, and responsible breeders test their breeding cats. But a negative result does not guarantee the cat will not develop HCM, because there are other, unidentified genetic causes, and for other breeds there is no reliable test. That is why the best practice in predisposed breeds is for breeding cats to have regular echocardiograms by a cardiologist. If you buy a cat of one of these breeds, ask about both: the genetic test where one exists, and the parents' echocardiograms.
What it costs
As a guide in Spain in 2026: an echocardiogram with a veterinary cardiologist, 120 to 250 €; an NT-proBNP test, 50 to 100 €; an HCM genetic test, 30 to 70 €. Clot-prevention medication, 10 to 30 € a month; treatment for heart failure, 30 to 80 € a month plus check-ups. An emergency for fluid on the lungs with hospitalisation and oxygen, 500 to 1,500 €; a thromboembolism, a similar or higher figure, with a guarded prognosis. Pet insurance taken out before diagnosis can cover a good part of these costs, because pre-existing conditions are usually excluded.
Hypertrophic cardiomyopathy is the most common heart disease in cats and often causes no symptoms until a serious complication appears. Count their breaths while asleep: over 30 a minute, see the vet; over 40 or open-mouth breathing, it is an emergency. And sudden, painful paralysis of the back legs is an emergency.
Educational content from Michipedia, translated from the Spanish original. It does not replace the advice of a vet or a feline behaviour specialist. Every cat is an individual.