Hypertrophic Cardiomyopathy In Cats

Written by Shula Berg BVSc CertAVP(GSAS) GPAdvCert(SASTS) MRCVS
Clinically reviewed by Elizabeth McLennan-Green BVM&S CertAVP(SAM) MRCVS

Table of Contents

- Overview
- Symptoms
- Diagnosis
- Treatment
- Outlook

Hypertrophic Cardiomyopathy (HCM) is the most common type of heart disease in cats, affecting up to 1 in 6 cats. The heart contains four chambers; two atria at the top, and two ventricles at the bottom. Deoxygenated blood from the body flows into the right atrium, before passing to the right ventricle which pumps it towards the lungs. Oxygenated blood from the lungs enters the left atrium, before passing to the left ventricle, which pumps it to the body. In HCM, the muscular walls of the ventricles become thickened. This reduces the capacity and the efficiency of the heart. 

It is not fully understood why some cats develop HCM, but a genetic predisposition is suspected in some cases as the condition is seen more commonly in Maine Coons, Ragdolls, British Shorthairs and Sphynx cats. Screening for HCM is often recommended before breeding these types of cat. 

A subset of cats with HCM will have what is known as Hypertrophic Obstructive Cardiomyopathy (HOCM). This occurs when the thickening of the heart wall is sufficient to partially obstruct the aorta, the major vessel leaving the left ventricle. In some cases, the pressure required for blood to escape the left ventricle is enough to pull the mitral valve, which sits between the left atrium and the left ventricle, into the outflow tract as well. This is known as Systolic Anterior Motion of the mitral valve or SAM. 

What are the symptoms of Hypertrophic Cardiomyopathy?

Many cats with HCM have no symptoms at all, even when examined by a vet. Some cats will have a heart murmur audible with a stethoscope. This abnormal sound is caused by turbulence in the heart, and is more common in cats with HOCM and/or SAM. Unfortunately, the absence of a heart murmur on examination does not mean that a cat does not have HCM. Some cats have a heart sound that is described as a gallop rhythm, as the heart sounds like a galloping horse with 3 sounds rather than 2. The gallop rhythm is caused by thickened heart muscle, so if this is heard it is advised to have the heart checked further. 

Heart disease is progressive and, eventually, some cats will reach a point where their heart is not pumping efficiently, known as Congestive Heart Failure (CHF). Blood backs up in the circulation and fluid leaks in to the lungs, known as Pulmonary Oedema. The most common symptoms of CHF are fast or laboured breathing, sometimes with panting or an open mouth. Pulmonary Oedema reduces the amount of oxygen entering the bloodstream, so cats with CHF often spend more time resting or sleeping, are less active than usual, and may have a reduced appetite. Cats are very good at hiding when they are unwell. Given the lack of symptoms with HCM, cats may appear to develop CHF “out of nowhere” and become very unwell very quickly. 

Cats with heart disease are at increased risk of blood clot formation. This can lead to a condition known as Thromboembolism, where a blood clot leaves the heart and blocks circulation to one or both back legs. Affected cats will suddenly lose use of their back legs; the feet may be cold to the touch and cats are often very vocal due to discomfort. This requires urgent veterinary treatment. 

Which tests are used to diagnose Hypertrophic Cardiomyopathy?

A diagnosis of HCM is only made using an ultrasound scan of the heart, known as echocardiography (commonly called an “echo”). This is advisable in all cases where a murmur or gallop rhythm is found. An echo allows us to see the chambers of the heart, assess how they are filling and contracting, and even directly observe abnormal movement of the valves. Measurements are recorded, allowing comparison with future scans to monitor progression of disease. 

Some other conditions can make the heart walls appear thicker, so it is common for a blood pressure measurement to be taken. Blood tests may be recommended to check the thyroid, and assess suitability for certain medications. Occasionally specific cardiac biomarkers, known as troponin I, and NT-proBNP, are measured to look for damage to the heart muscle and to indicate the degree of stretching to the heart muscle respectively. 

If CHF is suspected, chest x-rays may be advised. If the heart’s rhythm sounds abnormal, an ECG will be performed to check this. 

How is Hypertrophic Cardiomyopathy treated?

Unfortunately, no treatment currently available can reverse or slow the progression of HCM before symptoms develop. Depending on the findings during diagnosis, some treatment can be used to manage symptoms if present. Treatment goals include alleviating lung congestion (using diuretics such as Furosemide) and preventing formation of blood clots that can lead to thromboembolism (using anti-platelet drugs such as Clopidogrel). In cats with very high heart rates, beta-blockers (e.g. Atenolol) may be prescribed to try and slow the heart rate and allow calmer blood flow, with less turbulence, in cases with HOCM. 

Cats who are in CHF at the time of diagnosis will need more specific treatment. 

What is the outlook for cats with Hypertrophic Cardiomyopathy?

The prognosis for cats with HCM is very variable. Some cats will live their whole life without symptoms, many without ever obtaining a diagnosis. In other cats, the condition progresses and can significantly reduce lifespan. Unfortunately, there are no specific predictive factors for how an individual cat will be affected. It is recommended that cats with known HCM have an echo (heart scan) repeated yearly to monitor progression. 

The prognosis for cats who develop congestive heart failure is fairly guarded as the condition is progressive. Although some cats will remain stable on medication for the rest of their life, others may gradually stop responding to treatment. Most cats only survive for a few months after diagnosis of CHF. It is important that our pets maintain their quality of life, so if you feel like your cat is struggling please speak to your vet.                                   

One of the best measurements of congestive heart failure is an increase in respiratory (breathing) rate. Counting the respiratory rate is an easy way to monitor your pet at home. When your cat is calm and relaxed, count how many breaths they take in 15 seconds (in and out is one breath). Multiply this by four to get a respiration rate per minute. Normal cats take less than 30 breaths per minute; if you repeatedly count over 40, or see a steady increase over several days, contact your vet. The smartphone app “Cardalis” can help with monitoring and recording respiratory rates. 


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Page last reviewed: 23rd April 2024

Next review due: 23rd April 2026