Hypertrophic Cardiomyopathy (HCM) is the most common form of heart disease in cats. Studies have shown that up to 1 in 7 cats may have some form of this disease, but most will never become visibly ill from it. When symptoms develop, however, they can be extremely serious and worsen rapidly. If you notice an unexplained change in your cat’s routine or breathing, speak to a vet.

In cats with HCM, the left side of the heart becomes thickened and stiff. This makes it difficult to move the blood through the heart properly, which then makes the problem worse over time. Complications may develop, such as mitral valve disease or arrhythmias. Currently, there is no specific treatment for HCM, but many cats have a mild form of the disease for many years without any symptoms. When the disease starts to progress or cause symptoms, there are medications that can improve quality of life or prevent serious complications, at least for some time. 

What causes Hypertrophic Cardiomyopathy in cats?

• Any cat can develop this disease, especially as they get older.

• Specific genetic mutations have been shown to cause this disease in some breeds. In such cases, the disease is usually severe and develops at a young age.

• The initial changes start when the heart muscle cells become disorganised or are replaced by connective tissue.

• As the shape of the heart changes, it becomes less effective at moving the blood through.

• The increased effort needed to move the blood worsens the changes to the heart muscle, creating a vicious cycle.

• These changes may lead to mitral valve problems and obstruction of the blood flow, leading to a severe form of disease called Obstructive HCM.

In mild cases, cats may be able to pace themselves without showing obvious symptoms. Sometimes they’ll become noticeably less active or struggle to catch their breath after exercise or stress. 

Unfortunately, two serious complications may develop from HCM: 

• Aortic thromboembolism: where blood clots form in the heart and then move into a major artery, blocking it and causing sudden and severe symptoms like paralysis and pain.

• Congestive Heart Failure: where poor heart function leads to the build-up of fluid in the lungs. This develops slowly, but symptoms can appear very suddenly, such as breathlessness and open-mouth breathing. 

Other diseases that affect the heart may also lead to a secondary form of HCM. These include Primary hypertension, Hyperthyroidism and Acromegaly. 

Symptoms of Hypertrophic Cardiomyopathy in Cats

Symptoms may be quite different from cat to cat. They depend on the severity and stage of the disease. They also change if complications develop. Common symptoms include:

• Faster breathing than normal, even when resting (The normal breathing rate for cats is 15 to 30 breaths per minute at rest)

• “Panting” (open-mouth breathing) or difficult breathing

• Changes in routine (avoiding previously enjoyed activities because they have become too tiring)

• Reduced energy levels or interactivity

• Reduced appetite

• Faster heart rate

• Fainting

In cases that develop a blood clot, we may see:

• Sudden and severe pain in one or both back legs

• Weakness or paralysis of the back legs

• Cold and pale back paw(s)

Cats at higher risk of Hypertrophic Cardiomyopathy

• Specific genetic mutations have been detected in Maine Coon and Ragdoll cats, and have been associated with severe forms of HCM and early onset of symptoms.

• Other breeds that seem to develop HCM more frequently include the Persian, Sphynx, Birman, American and British Shorthair, Scottish Fold, Norwegian Forest Cat and Bengal.

• It is suspected that most cases of HCM have a genetic component, but no other abnormal genes have been identified so far.

• In all cats, HCM becomes more common with age.

• Male cats develop severe complications from this disease more frequently, especially Aortic Thromboembolism. 

Diagnosis of Hypertrophic Cardiomyopathy in Cats

• Often the first symptom of HCM is detected during a routine physical examination. This can be a heart murmur or an abnormal heart rhythm. The vet may recommend further tests to check the cause and severity of the problem.

• Echocardiography (a heart scan) is the most important and useful test for heart problems in cats. It allows assessment of the heart muscle, blood flow and heart valves.

  • Repeated scans over time monitor the progression of the disease. There is a classification system based on scan results and symptoms (Grade A-B1-B2-C-D). This helps guide treatment decisions.

• Electrocardiography (ECG) is a test to assess the heart rhythm. This test may provide valuable additional information, especially if an arrhythmia is present.

• Chest x-rays may be recommended, especially if the vet is worried about fluid build-up in the lungs.

• Blood tests are recommended in cases of suspected HCM for two reasons:

  • Thyroid and general blood tests are recommended to rule out hyperthyroidism and other metabolic conditions as the underlying cause for the heart disease.

  • NT-proBNP and Troponin blood tests can be used to detect and monitor heart disease. These parameters go up when the heart muscle is struggling and can be used to confirm that the primary issue is heart disease. NT-proBNP can also be used to monitor cases before symptoms develop. 

• Genetic testing is available for mutations causing HCM in Maine Coon and Ragdoll cats. However, these only tell us if a cat is at higher risk of developing HCM. They don’t tell us anything about the current heart function or if heart disease is the cause of any symptoms a cat may have. 

Veterinary Treatment of Hypertrophic Cardiomyopathy in Cats

Currently there is no cure for HCM, but treatments may improve quality of life and prevent or delay serious complications. New treatments (Rapamycin) are currently being tested and may become available soon. These aim to improve the function of the heart muscle itself, and possibly reverse or delay the disease.

Cats with stage A and B1 disease usually do not require treatment. Cats at stage B1 are usually monitored 1-2 times per year to detect further changes as soon as they occur. 

For cats at stage B2 or worse, commonly used treatments include:

• Medication to prevent blood clots, like Clopidogrel. This is to try to prevent the most serious complication of HCM, arterial thromboembolism, which is unfortunately quite common otherwise.

• Anti-arrhythmia medications, when necessary. These help pace the heart and may also improve blood flow. Atenolol is the most frequently used.

• Diuretics if lung congestion has developed or is a concern. These help the body eliminate the excess fluid that builds up from poor blood flow, especially in the lungs. Furosemide is the most commonly used, but Torasemide and Spironolactone may be used in severe cases.

• Pimobendan may be prescribed in some cases of HCM (with the exception of cases where Obstructive HCM is present). This is a medication that is very effective at improving heart strength and blood flow in dogs, but its use in HCM in cats is still under investigation. Studies so far have not shown any safety concerns and have suggested there may be some benefits.

• In cases where the disease is very advanced, some dietary changes may be recommended. These include Taurine supplementation and salt restriction. Electrolytes may need to be monitored with blood tests, and supplementation of potassium may be needed, especially in cats requiring high doses of diuretics. 
  

Cases that develop arterial thromboembolism will require intensive emergency care, including hospitalisation, strong painkillers and injections of anticoagulants. Intensive care is often required for 3-7 days. Sadly, only about 50% of cats survive their first episode of thromboembolism, and recovery from repeated episodes is even more difficult. 

Cats with congestive heart failure (CHF) may develop an emergency complication, pulmonary oedema. Such cases will need immediate hospitalisation for treatment with oxygen and high doses of diuretics. 

Cases with CHF may also benefit from additional treatments like benazepril (Angiotensin-converting enzyme inhibitors).

Home Treatment of Hypertrophic Cardiomyopathy in Cats

• Many cats with mild or early disease only need to be monitored. 

• Along with regular vet visits and monitoring tests, it may also be useful to monitor the resting breathing rate.

  • Count how many times your cat’s chest moves in a minute, while they’re relaxing somewhere flat. Normal rate is 15-30 breaths per minute. If you count more than 30 breaths multiple times, speak to a vet for further advice.

• Cats are usually very good at pacing themselves, especially if they’re not feeling well. But if your cat has been diagnosed with heart disease, try to avoid situations that appear to cause them to become breathless or start panting. 

• It’s also especially important to make sure your cat has a safe and calm place to retreat to if they’re feeling stressed or threatened. 

• Avoid stressful situations when possible. This can be things like changing their environment or forcing them into a situation they find unpleasant (vet visits can’t be completely avoided, unfortunately). 

• Cats that are eating a good quality, age-appropriate and complete diet will probably not benefit from a diet change in the early stages of heart disease. 

  • This may change when the disease becomes more severe, especially if they are taking multiple medications like diuretics. Your vet can provide further advice for your cat’s specific situation. 

Prevention of Hypertrophic Cardiomyopathy in Cats

• In most cases, there isn’t much that can be done to prevent the development of HCM. 

• Maintaining a healthy diet, body condition (weight) and being active are always good strategies to improve health.

• For those few cases where there is a known mutation behind HCM (Ragdoll and Maine Coon), testing cats for these mutations before breeding them is highly recommended. It’s also advisable that cats with these mutations have regular vet checks to detect signs of disease as early as possible.

What to expect for my cat with Hypertrophic Cardiomyopathy

• If the disease is detected before any symptoms develop, it’s difficult to predict what may happen. Many cats continue to have a mild form of disease for many years, and may never develop symptoms. However, given how dangerous the disease can be, regular monitoring is advised.

• When symptoms develop, these are often very serious, and the disease usually progresses quickly.

  • Cats whose disease is detected during routine check-ups (before serious symptoms) tend to do fairly well, and treatment may manage the condition for a year or more.

  • If Thromboembolism is the first symptom of the disease, the outcome is unfortunately poor. More than 50% of cats do not survive the first episode, and many have further episodes within months.

  • Cats with pulmonary oedema (or other signs of congestive heart failure) are more likely to recover from the initial crisis. Unfortunately, the disease commonly becomes unmanageable after about 6-9 months.

When to worry about Hypertrophic Cardiomyopathy in Cats

Seek help from an emergency vet if:

• Your cat is struggling to breathe, is open-mouth breathing or gasping for air

• Has blue or grey gums

• Is very tired and you are struggling to wake them up

• Is meowing in pain

• Can’t move their back legs
  

Call a vet as soon as possible if your cat:

• Has a resting respiratory rate consistently above 30 breaths per minute

• Is losing weight or has less appetite

• Is spending a lot of time sleeping
  

Joii can help if:

• You need help looking after your cat with heart disease in a stress-free manner

• You need guidance on how to take your cat’s resting respiratory rate

• You need advice on feeding your cat with heart disease

• You need tips on how to give medication to your cat