Hypertrophic Cardiomyopathy in Cats: Causes, Symptoms and Treatment

Hypertrophic cardiomyopathy can be a genetic condition, especially in breeds such as Maine Coon and Ragdoll cats. Gorodenkoff / Shutterstock

Hypertrophic cardiomyopathy (HCM) is the most common heart condition seen in cats. It causes abnormal thickening of the heart wall, making it harder for the heart to pump blood around your cat’s body. Most cats with HCM show no symptoms until it progresses to the point of heart failure. This means detecting HCM in the early phases can be difficult.

Some breeds are more prone to HCM, so we know genetics plays a role. It can also be caused by high blood pressure (hypertension) or a form related to hyperthyroidism. But it can happen in cats with none of these risk factors. This means taking your cat for regular check-ups is important – whatever their breed, age or health.

Although HCM is a serious and ultimately life-threatening disease, many cats live comfortably with it for a long time, especially with treatment.

What is Hypertrophic Cardiomyopathy in Cats?

Your cat’s heart is essentially a pump made of muscle. It consists of four chambers (right and left ventricle and right and left atrium) that fill with blood, and muscular walls that contract to pump that blood out of the heart and around the body.

With HCM, the heart muscle becomes abnormally thickened (hypertrophy). This makes the heart walls stiffer, making it harder to contract. The thickening of the walls also means the chambers within the heart become smaller, so they cannot fill with enough blood. Overall, as the disease progresses it becomes harder and harder for your cat’s heart to pump blood effectively.

Your cat’s body will compensate for a long time, meaning they may not show signs of being unwell. But eventually, the heart will struggle to cope, and symptoms will develop. These will usually be signs of the complications of HCM as opposed to HCM itself.

Causes of Hypertrophic Cardiomyopathy in Cats

Hypertrophic cardiomyopathy can be an inherited disease and is especially common in breeds such as Maine Coon cats and Ragdoll cats. Peredniankina / Shutterstock.com

We know HCM is an inherited disease. It is the most common inherited heart disease in both people and cats. The CatScan study looked at the prevalence of cardiomyopathy in 78 apparently-healthy cats, finding HCM in around 15%. Other studies have found similar results, meaning around 1 in 7 cats are thought to have it.

Certain breeds are known to be more commonly affected:

• Maine Coon
• Ragdoll
• Sphynx
• British Shorthair
• British Longhair

Research is underway to identify exactly what genes may play a part – which will be important to improving genetic screening.

The CatScan study showed several other risk factors for developing HCM, including a heart murmur, being male, middle-aged or older, and being overweight. However, HCM has been shown to occur in cats of all ages, sexes, and conditions. Other conditions such as high blood pressure and hyperthyroidism can also cause HCM. So, regular check-ups of your cat’s heart are important even if they aren’t high-risk.

Symptoms of Hypertrophic Cardiomyopathy in Cats

Most cats with HCM have no outward symptoms in the early stages. As the heart struggles to pump enough blood it will compensate by increasing the speed at which it beats. The earliest signs of HCM in cats are increased heart rate and the development of a heart murmur. Regular check-ups with your veterinarian can pick up on these subtle signs. However, not all cats with HCM will have any detectable changes.

As a cat owner, you will most likely not see any signs of illness in your cat until they develop a complication of HCM, such as congestive heart failure or a blood clot. Unfortunately, sudden death is also possible from undiagnosed HCM.

Congestive Heart Failure

As HCM progresses, the heart struggles to pump blood effectively around the body. This can lead to fluid backing up in the lungs (pulmonary edema or pleural effusion) resulting in congestive heart failure. This is the most common outcome of HCM in cats. Symptoms of heart failure in cats can come on suddenly or over time and include breathing difficulties, appetite loss and lethargy.

Although new early treatment options are emerging, treating heart failure remains the mainstay of treatment for cats with HCM. Although both HCM and heart failure are serious and life-threatening conditions, medication can successfully stabilize cats for months to years.

Blood Clots

As well as heart failure, cats with HCM are at risk of developing blood clots within the heart. Pieces of these clots can travel through the bloodstream and get lodged causing a rapidly life-threatening condition called Feline Arterial Thromboembolism (FATE). This most often affects the back legs, causing sudden onset of symptoms:

• Pain
• Paralysis/weakness
• Cold
• Pallor/blue tinge
• Stiffness

FATE is a devastating condition that commonly results in euthanasia – although there are recent advancements in treatment.

Sadly, cats with HCM are also at a greater risk of sudden death with no other symptoms of illness beforehand. This may be due to irregular heart rhythms (arrhythmia) or undetected clots.

Diagnosis of Hypertrophic Cardiomyopathy in Cats

Hypertrophic cardiomyopathy is diagnosed with an ultrasound of your cat’s heart, called an echocardiogram, or ‘echo’. Sungthong / Shutterstock

Your veterinarian may suggest screening for HCM if your cat is an at-risk breed or if they have noticed another abnormality such as a heart murmur or fast/irregular heartbeat. It is also recommended for cats suffering from high blood pressure or hyperthyroidism.

Echocardiogram (Heart Ultrasound)

HCM is diagnosed using an ultrasound of your cat’s heart – called an echocardiogram, or ‘echo’. Echocardiography is usually performed by a specialist veterinary cardiologist who will measure the thickness of the heart walls. They will also check for signs of chamber enlargement and the development of blood clots inside the heart. 

Some general practice veterinarians or those who specialize in ultrasound specific practice may also be able to perform this ultrasound, especially in places where a veterinary cardiologist is not available. Their ultrasound images may still be sent to a cardiologist for review.

proBNP test

A blood test called an NT-proBNP (N-terminal prohormone brain natriuretic peptide) can be used as a screening test for HCM. NT-proBNP is a hormone that is produced by the muscle cells in the ventricles of the heart, which are the larger chambers at the bottom. The left ventricle especially is the one that develops severe muscle thickening with HCM.

The ventricular muscle produces NT-proBNP in amounts proportional to the severity of muscle thickening. Checking these levels is one way of screening for heart muscle thickening.

There are two forms of this test. The SNAP proBNP gives a positive or negative (abnormal/normal) result and can be used as a fast and simple “yes or no” screening test at any veterinary clinic. A positive result, greater than 150pmol/L, will cause the spot on the test to appear darker than the control spot (see photo below).

The SNAP proBNP test is one way to screen for heart disease in cats. The test above shows a result positive for heart disease.

There is also a form that provides a numerical, or quantitative, result. This form is called the Cardiopet feline proBNP. This form can be used to initially screen for heart muscle thickening but can also be used to monitor progression over time.

The proBNP tests cannot diagnose HCM specifically. But by screening for heart muscle thickening which occurs most often in cats with HCM, abnormal/positive results can indicate the need to reevalaute a cat patient’s health and consider more specific diagnostic testing with an echo.

X-rays

A chest x-ray can be used to screen for heart enlargement. Vertebral heart score (or scale, VHS) is a way to measure if a heart is “too big”. It can also be used to monitor size enlargement over time.

This is a less sensitive way to check for HCM in cats as changes may occur to the heart structure before an overall size enlargement is apparent. It also cannot diagnose HCM specifically. But it is one screening tool available to nearly all veterinary practices to screen for heart disease and warrant further investigation.

X-rays are also a primary way to see if your cat may be suffering from heart failure, as the fluid build up in the lungs heart failure causes can be see on x-ray. Ultrasound can also be utilized but is less available and requires more training.

Other Testing

As part of a full cardiac assessment, your cat should have their blood pressure checked, as well as blood tests and an ECG (checking heart rate and rhythm).

Genetic Screening for Cats with Hypertrophic Cardiomyopathy

We know that many cases of HCM may have a genetic basis. The specific genes involved have been identified in certain breeds (including the Maine Coon and the Ragdoll), meaning genetic screening is available.

If your cat is found to have this gene mutation, they have an increased chance of developing HCM. However, not all cats with the gene mutation will go on to develop HCM. Therefore, it is recommended that cats with a positive genetic screen also have regular ultrasound scans of their heart to identify changes early. As HCM can occur at any stage of life, a single normal ultrasound does not rule out the risk of developing it at a later stage.

It is recommended that cats with positive genetic screening should not be bred from, as there is a high chance of them passing the mutated gene on to their kittens. If you are considering breeding from a Ragdoll or Maine Coon cat, genetic screening for HCM is recommended.

Treating Cats with Hypertrophic Cardiomyopathy

In cats with an underlying condition, such as high blood pressure or hyperthyroidism, treating this successfully can improve or even reverse changes that have resulted in heart disease.

In cats predisposed to HCM (i.e. genetically), there is no way to prevent the disease. However, there is a newer and very promising medication to treat cats in earlier stages of HCM. There is now a way to slow the disease down, and even reverse some of the changes it causes to the heart.

In 2023, Dr. Joshua Stern, a veterinary cardiologist and associate Dean at NC State, completed a study in HCM cats where they were treated with a medication called rapamycin, also called sirolimus.

Called the RAPACAT study, the trial involved about 40 cats with subclinical HCM. This means they have heart muscle thickening or other structural heart changes related to the disease but have not developed congestive heart failure. 

After 6 months, there was a significant reduction in left ventricular wall muscle thickness in cats taking sirolimus/rapamycin compared to the placebo. Side effects appear to be minimal, especially as it was the low dose of the medication given once a week that was the most effective.

The results of the trial study were so compelling, that sirolimus was given conditional approval by the FDA under the brand felycin-CA-1. 

Continuing through the end of 2026, the same researchers are conducting the HALT study. This broader study will involve hundreds of client-owned cats and provide a greater understanding of sirolimus’ efficacy and safety for treating cats with HCM before they develop more serious disease.

Sirolimus is available for veterinarians to prescribe for cats with HCM. However, given its current only conditional approval by the FDA, specific criteria must be met before it can be prescribed:

1. Cats must be “otherwise healthy”, subclinical (for HCM) and without systemic hypertension, other causes of compensatory myocardial hypertrophy (heart muscle thickening), current or historical symptoms of congestive heart failure, arterial thromboembolism, and severe left ventricular outflow obstruction (which can only be seen with an ultrasound of the heart)
2. Echocardiographic measurement of left ventricular wall thickness to diagnose subclinical hypertrophic cardiomyopathy (defined as greater than or equal to 6 millimeters at end diastole by 2D or M-mode assessment (with heart ultrasound))

Some general practice veterinarians may be able to meet these criteria, but involving a veterinary cardiologist will be needed in many cases.

A heart murmur alone, even in a genetically predisposed cat is not enough evidence to start sirolimus/rapamycin. Cats can commonly have murmurs when under stress, especially during veterinary exams, even without true heart disease. Conversely, heart disease in cats may be present without any outward signs. 

Screening cats at risk for HCM at an earlier age can be crucial for finding the disease, especially now that there is a treatment for even the earliest stages.

Although sirolimus/rapamycin is a very promising treatment now available for HCM, other treatment approaches may be needed as well. Treatment of more advanced stages of HCM is aimed at treating the consequences the disease causes (if present), by:

• Reducing the risk of blood clots
• Treating high blood pressure
• Treating signs of heart failure

For cats with no symptoms and only mild changes seen on echo, The American College of Veterinary Internal Medicine (ACVIM) guidelines for cardiomyopathies in cats currently only recommends annual monitoring with heart scans. They consider these cats “low risk of congestive heart failure or arterial thromboembolism, and in general, treatment is not recommended”.

When the HALT study is completed and further approvals are granted for rapamycin/sirolimus as a treatment for HCM, this consensus statement is likely to change. Even cats at low risk for heart failure or blood clots will be eligible for treatment if they have structural heart changes present that are caused by HCM.

For cats who have more significant changes seen on echo, such as enlarged heart chambers, starting clopidogrel is recommended. This drug helps to prevent the development of clots. These cats are at risk of both blood clots and congestive heart failure, so they should be monitored closely.

As HCM progresses, it causes congestive heart failure. Diuretics such as furosemide are the main treatment for heart failure in cats. They reduce the fluid build-up in the lungs allowing your cat to breathe more easily. Other drugs your veterinarian may consider include ACE-inhibitors and pimobendan. Rapamycin/sirolimus has not been trialed in cats who have already developed congestive heart failure. It’s not clear if it will have the same benefits in the late stages of HCM.

Caring For Your Cat with Hypertrophic Cardiomyopathy

Cats who have been diagnosed with, or at risk of, hypertrophic cardiomyopathy should have regular check-ups with their veterinarian, including heart scans. Gorodenkoff / Shutterstock

If you know your cat is at risk of HCM or has early signs of HCM, you should take them for regular check-ups with your veterinarian, including an annual ultrasound of their heart. Even an annual quantitative BNP test can be useful to monitor progression.

Alongside ensuring they receive any medication it may help to feed a diet designed for cats with heart disease. These contain omega-3 fatty acids, optimal levels of taurine, reduced sodium, and high-quality, highly digestible protein. It is sensible to avoid salty treats, as these can cause fluid retention which may worsen heart failure.

Preventing Hypertrophic Cardiomyopathy in Cats

There is no true treatment that can prevent the development of HCM in predisposed cats. However, with the advent of rapamycin/sirolimus and ongoing studies, the disease can be treated in its earlier stages.

Recommendations for treatment and prevention of HCM will likely change in the coming years with new treatment options. Now that treatment for early stages of HCM is available, it’s more important than ever to have cats screened long before they show signs of illness. Here are some guidelines to consider:

• If your cat is an at-risk breed (i.e. Maine Coon, Ragdoll, Sphynx) consider an echocardiogram with a veterinary cardiologist around a year of age even if no signs of illness are present. This can help screen for genetic/inherited heart abnormalities.
• If your cat is an at-risk breed, consider a feline pro-BNP screening test around a year of age. If it’s normal, repeat annually as a way for early detection. 
• When a pro-BNP test is abnormal, consider the quantitative pro-BNP test. The number elevation can shed some light on how much muscle thickening is present and can be used to monitor progression.
• Some general practice veterinarians may be able to do more basic ultrasounds of the heart to get measurements of muscle thickness and chamber sizes. While not a full echocardiogram, this may at least provide enough evidence to warrant early treatment for HCM.
• If your cat is not considered an at-risk breed, you should still consider screening for HCM starting around 7-10 years of age. Many cats who are not known to be genetically at risk can still develop HCM, more often in their older years.
• If your vet hears a heart murmur during an exam, this may or may not be related to true heart disease. But it’s always best to check with an early screening test with either a heart ultrasound scan or a feline NT pro-BNP

It is also important to remember that secondary HCM can be prevented – and even reversed – through quick treatment of high blood pressure and hyperthyroidism. Annual wellness screens with bloodwork and checking blood pressure are an important part of safeguarding heart health, especially for at-risk cats.