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Hypertrophic Cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is the most common heart disease seen in cats.

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What is hypertrophic cardiomyopathy?

Hypertrophic cardiomyopathy (HCM) is the most common heart disease seen in cats. Breeds predisposed to HCM include Maine Coons, Ragdolls, Sphynx, and British Shorthairs, though it is most often diagnosed in domestic short-haired or long-haired cats. HCM is often suspected to be due to a genetic predisposition, though similar changes to the heart can be caused by high blood pressure or high thyroid levels. HCM is most often diagnosed in middle age, and male cats are slightly more commonly affected.

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What are the consequences and symptoms of HCM?

HCM causes an abnormal thickening of the heart muscle. Over time, several outcomes can be related to HCM. If cats experience symptoms secondary to HCM, they are most likely to experience congestive heart failure (CHF). This occurs when the pressure within the heart becomes too great, and fluid backs up into the lungs or chest cavity. Cats with heart failure can have an increased breathing rate and effort, lethargy or decreased appetite.

Secondly, because of changes in blood flow and enlargement of the left atrium, cats with HCM are at increased risk for blood clot formation within the heart. Pieces of these clots can be swept by the bloodstream to other locations in the body (arterial thromboembolism or ATE). The most common site for ATE in cats is in the back legs (saddle thrombus); this leads to sudden onset weakness/paralysis and pain in these limbs.

The last outcome associated with HCM is irregular heart rhythms (arrhythmia) secondary to scar tissue formation. Unfortunately, this can lead to sudden death in a small percentage of these cats, or fainting can occur. Other types of arrhythmias such as atrial fibrillation (a fast and chaotic rhythm originating from the top of the heart) may also occur.

How is HCM diagnosed?

HCM screening can be challenging during routine health checks. It is important to note that many healthy cats have benign heart murmurs, and many cats with HCM have no murmurs. However, if your primary veterinarian notes abnormal heart sounds, symptoms of heart disease or elevation of a blood test called NT-proBNP, further evaluation by echocardiography (ultrasound of the heart) with a veterinary cardiologist is recommended. Testing for other causes of heart thickening (high blood pressure and high thyroid levels) is also recommended.  If heart failure is suspected, chest x-rays may be performed.

In predisposed breeds, annual screening is recommended, as the absence of a heart murmur does not eliminate the risk of heart disease. In Maine Coons, Ragdolls, and Sphynx cats, genetic testing for HCM is available. Please remember that genetic tests can help quantify risk or aid in breeding programs, but some cats who test negative for the affected gene go on to develop HCM and vice versa.

How is HCM treated?

Cats often remain in the asymptomatic phase of heart disease for many years. Cats with more advanced heart disease (ACVIM Stage B2) are treated with medication (clopidogrel) to reduce the risk of ATE.

If heart failure is present, treatment with diuretics +/- additional drugs to reduce scar tissue formation, support heart pumping function, control abnormal rhythms, or supplement electrolytes may be added. If fluid has accumulated within the chest cavity (pleural effusion), manual drainage under sedation may be recommended.

How should I care for my cat with HCM at home?

Before the onset of heart failure, most cats can live normally with no restrictions. Special diets are not required, though very salty foods (human foods, jerky treats) should be avoided.

Cats with HCM should be monitored for symptoms of the worsening heart disease listed above. If these signs are noted, please contact a veterinarian. Signs of ATE (sudden pain and loss of leg function) should prompt emergency evaluation. We will also ask you to monitor your cat’s sleeping breathing rate at home. A normal breathing rate (while sleeping) is less than 36 breaths/minute. There are apps available that can help you count and track this rate at home.

What is the prognosis of HCM?

Not all cats with HCM will go on to develop symptoms, and cats with HCM often remain asymptomatic for years. In one publication, approximately 20% of cats developed heart failure within 5 years of diagnosis of HCM, and 9% developed ATE during that time period.  After the onset of heart failure, with medications, most cats continue to have an excellent quality of life and can often live 6-18 months, though survival time varies widely based on the severity of the disease.

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