[Written by a Veterinarian] Feline Hypertrophic Cardiomyopathy: Symptoms, Diagnosis, and Treatment Guide

Feline hypertrophic cardiomyopathy (HCM) is a disease where the heart muscle thickens, making it difficult for the heart to pump blood throughout the body. This disease is known to occur genetically in some cats.

This article will provide a detailed explanation of feline hypertrophic cardiomyopathy.

What is Feline Hypertrophic Cardiomyopathy?

Feline hypertrophic cardiomyopathy is the most common heart disease in cats. It is a cardiac condition that requires extremely urgent attention.

Basic Information on Hypertrophic Cardiomyopathy

The heart is divided into right and left chambers; blood is pumped from the right chamber to the lungs, and from the left chamber to the rest of the body. To circulate blood, the heart muscle (myocardium) constantly contracts and expands.

In feline hypertrophic cardiomyopathy, the left heart muscle thickens, disrupting its normal contraction and expansion. This makes it difficult for blood to enter the heart, and simultaneously, the heart struggles to pump blood throughout the body. Additionally, since blood from the lungs flows into the left chamber, hypertrophic cardiomyopathy can cause blood from the lungs to accumulate, leading to pulmonary congestion. Furthermore, blood stagnation in the heart can lead to the formation of blood clots, which can then block blood vessels (thromboembolism).

Hypertrophic cardiomyopathy often shows no significant symptoms in its early stages, so by the time symptoms appear, the disease has often progressed and the cat's condition is severely deteriorated. Many cats succumb to the disease because treatment is initiated too late, and medication must be continued for life.

Genetic Factors and Risk of Onset

Feline hypertrophic cardiomyopathy is known to occur genetically in some cat breeds. While Maine Coons are particularly well-known, it is also known to occur genetically in Ragdolls, American Shorthairs, Persians, Norwegian Forest Cats, and Scottish Folds. In addition to these breeds, it tends to be more common in short-haired cats and male cats.

Hypertrophic cardiomyopathy caused by genetic abnormalities arises from a mutation in part of a gene, and gene mutations have been identified in Maine Coons and Ragdolls. This gene mutation is inherited in an autosomal dominant pattern. This means that if one of the two paired genes has the mutation, the disease will develop. If one of the parents has the gene mutation, 50% of the offspring will develop the disease. Therefore, it is crucial to avoid breeding parent cats with the hypertrophic cardiomyopathy gene mutation or cats that have developed hypertrophic cardiomyopathy.

Other causes besides genetic abnormalities include hyperthyroidism, hypertension, and taurine deficiency.

Symptoms and Diagnosis of Hypertrophic Cardiomyopathy

Feline hypertrophic cardiomyopathy often shows no symptoms in its early stages. While heart murmurs can sometimes be heard in cases of heart disease, approximately 15% of cats with hypertrophic cardiomyopathy do not have heart murmurs. Consequently, it is often not detected during routine health check-ups, and in most cases, it is only diagnosed when the condition has significantly worsened.

Symptoms of Hypertrophic Cardiomyopathy

Hypertrophic cardiomyopathy often presents no symptoms in its early stages. If there is a genetic predisposition to hypertrophic cardiomyopathy, signs such as easy fatigue, quick cessation of play, or poor appetite may be observed from a young age. The onset age can be as early as 4 months. The average age of diagnosis is around 5 years.

The main symptoms of hypertrophic cardiomyopathy are reluctance to move, open-mouth breathing, loss of appetite, and vomiting.

If a thromboembolism occurs, it most often affects the arteries of the hind legs, leading to hind leg paralysis, cold and purple paw pads, and severe pain. If a blood clot blocks a renal artery, the cat may stop producing urine.

Hypertrophic cardiomyopathy is an emergency and a very dangerous condition. It has a very high mortality rate.

Diagnosis of Hypertrophic Cardiomyopathy

Cats with hypertrophic cardiomyopathy are often in very poor condition, so various tests are performed while administering oxygen. First, a medical history, auscultation, palpation, chest X-ray, blood pressure measurement, electrocardiogram, blood tests, and urinalysis are conducted.

Echocardiography is essential for hypertrophic cardiomyopathy. If myocardial thickening is observed, it is diagnosed as hypertrophic cardiomyopathy.

Treatment of Feline Hypertrophic Cardiomyopathy

In April 2020, the American College of Veterinary Internal Medicine (ACVIM) published global guidelines for the diagnosis and treatment of cardiomyopathy. Cardiomyopathy is classified into four stages (A-D), and treatment based on these guidelines is recommended.

Hypertrophic cardiomyopathy can be caused by hyperthyroidism or hypertension. If these diseases are the cause, treatment based on those underlying conditions is necessary.

Stage A

Stage A refers to a state where there is a predisposition to cardiomyopathy but no symptoms. In this case, no treatment is necessary.

Stage B1

Stage B1 refers to a low risk of chronic heart failure or thromboembolism. No specific treatment is recommended at this stage, but annual or more frequent echocardiographic examinations are advised.

Stage B2

Stage B2 indicates a high risk of chronic heart failure or thromboembolism. If the risk of thromboembolism is high, antithrombotic therapy is recommended.

Stage C

Stage C is a state where heart failure or thromboembolism has occurred.

If pulmonary edema or pleural effusion is present, immediate administration of diuretics or thoracentesis is necessary. Oxygen therapy and sedatives should be administered to avoid excessive stress, and examinations should be conducted while monitoring the cat's condition. Until the cat can be stabilized from heart failure, treatment with diuretics and cardiotonics will be administered, requiring hospitalization. Antiplatelet drugs are also recommended to prevent blood clots.

Stage D

Stage D refers to a state of heart failure that is resistant to treatment. Changes in diuretics and cardiotonics should be considered.

Arterial Thromboembolism

Arterial thromboembolism is one of the complications of cardiomyopathy with the worst prognosis. When thrombosis occurs, it not only causes leg paralysis but also severe pain and a shortened lifespan. Therefore, euthanasia is recommended when arterial thromboembolism is identified.

However, if pain relief is adequate, there is a prospect of improvement (no hypothermia, not chronic heart failure, only unilateral onset), and the owner fully understands the risks and prognosis, then treatment for the thrombus will be initiated.

Treatment for arterial thromboembolism involves analgesia, anticoagulants, and antiplatelet drugs. The affected leg, depending on the extent of the clot, may develop necrosis. Care tailored to the condition of the leg will be necessary.

Early Detection and Care for Hypertrophic Cardiomyopathy

Undergo regular health check-ups.

Hypertrophic cardiomyopathy is often not detected in its early stages. Therefore, it is recommended to undergo regular echocardiograms and cardiac biomarker tests. Especially if your cat belongs to a predisposed breed or has relatives with cardiomyopathy, cardiac examinations should begin before 1 year of age.

Caring for Cats with Hypertrophic Cardiomyopathy

Cats with hypertrophic cardiomyopathy will require lifelong medication and regular veterinary visits. Since there is a possibility of respiratory distress recurring after discharge, consider renting an at-home oxygen chamber.

Cats with heart disease are known to develop cardiac cachexia due to decreased appetite and muscle mass, which affects their prognosis. Switch to a diet with appropriate protein and sufficient calories to ensure they receive adequate nutrition. There are prescription diets specifically for heart disease that are restricted in sodium.

Cats that have experienced arterial thromboembolism may suffer from residual leg paralysis or necrosis. Care is needed to protect the affected limb from injury.

It is also important to be aware that cats with hypertrophic cardiomyopathy unfortunately do not have a long life expectancy and are at risk of sudden death.

Summary

Hypertrophic cardiomyopathy is the most common heart disease in cats and has a very poor prognosis. It is often not noticed until symptoms appear. When symptoms do appear, the mortality rate is very high, and even if the condition stabilizes, sudden death can occur.

For hypertrophic cardiomyopathy, early detection and prompt treatment significantly affect the prognosis. Regular health check-ups, finding a primary veterinarian, and securing an animal hospital that offers night and holiday care are also important.