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

Feline hypertrophic cardiomyopathy (HCM) is a disease in which the heart muscle thickens, making it difficult to pump blood throughout the body. It is known that this disease occurs genetically in some cats.

In this article, we 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 highly urgent condition.

Basic Information on Hypertrophic Cardiomyopathy

The heart is divided into right and left chambers; the right side sends blood to the lungs, and the left side sends it to the rest of the body. To circulate blood, the heart muscle—known as the myocardium—constantly repeats cycles of contraction and relaxation.

In feline hypertrophic cardiomyopathy, the left ventricular heart muscle thickens, preventing normal contraction and relaxation. Consequently, it becomes difficult for blood to enter the heart, and at the same time, blood cannot be pumped out to the rest of the body. Furthermore, because the left chamber receives blood returning from the lungs, HCM causes blood from the lungs to back up, leading to a state of fluid overload (pulmonary congestion). Additionally, because blood pools inside the heart, blood clots (thrombi) can form and obstruct blood vessels (thromboembolism).

Because major symptoms are not visible in the early stages of hypertrophic cardiomyopathy, the disease is often advanced and in a critical state by the time symptoms appear. It is not uncommon for cats to pass away before treatment can be effective, and medication must be continued for life.

Genetic Factors and Risk of Onset

Feline hypertrophic cardiomyopathy is known to occur genetically in certain breeds. The Maine Coon is particularly well-known, but it is also known to occur genetically in Ragdolls, American Shorthairs, Persians, Norwegian Forest Cats, and Scottish Folds. In addition to these breeds, there is a tendency for the disease to be more common in shorthaired cats and male cats.

Hypertrophic cardiomyopathy caused by genetic abnormalities arises from mutations in certain genes; these mutations have been identified in Maine Coons and Ragdolls. This gene mutation is inherited in an "autosomal dominant" pattern. This means that if there is a mutation in either of the two paired genes, the disease will occur. If one parent carries the gene mutation, 50% of the kittens born will develop the disease. Therefore, it is important to avoid using parent cats with the HCM gene mutation or cats that have developed HCM for breeding.

Beyond genetic abnormalities, the disease can also be caused by hyperthyroidism, hypertension, or taurine deficiency.

Symptoms and Diagnostic Methods for Hypertrophic Cardiomyopathy

Feline hypertrophic cardiomyopathy shows no symptoms in its early stages. While heart murmurs can sometimes be detected in cases of heart disease, approximately 15% of cats with hypertrophic cardiomyopathy do not have murmurs. Therefore, it is often missed during routine health checks, and in most cases, it is diagnosed only after the condition has deteriorated.

Symptoms of Hypertrophic Cardiomyopathy

Symptoms are often absent in the early stages of hypertrophic cardiomyopathy. If a cat has a genetic predisposition, signs such as fatigue, stopping play quickly, or loss of appetite may appear from a young age. Onset can occur as early as four months of age, with the average age at diagnosis being around five years.

The main symptoms of hypertrophic cardiomyopathy include lethargy, open-mouth breathing, loss of appetite, and vomiting.

If arterial thromboembolism occurs, it most often happens in the arteries of the hind limbs, leading to immobility of the back legs, cold and purplish paw pads, and severe pain. If a blood clot obstructs the renal artery, it can lead to an inability to urinate.

Hypertrophic cardiomyopathy is a high-urgency, life-threatening condition. The probability of death is extremely high.

Diagnostic Methods for Hypertrophic Cardiomyopathy

Because cats with hypertrophic cardiomyopathy are often in critical condition, various tests are performed while administering oxygen. Initial steps include a medical history interview, auscultation, palpation, chest X-rays, blood pressure measurement, electrocardiogram, blood tests, and urine tests.

An echocardiogram (heart ultrasound) is essential for diagnosing hypertrophic cardiomyopathy. A diagnosis is made if findings such as thickening of the heart muscle are present.

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 to D), and treatment based on these guidelines is recommended.

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

Stage A

Stage A refers to cats with a predisposition to cardiomyopathy but no symptoms. No treatment is necessary in this stage.

Stage B1

Stage B1 refers to cases where there is a low risk of chronic heart failure or thromboembolism. No specific treatment is recommended for this stage, but performing an echocardiogram at least once a year is advised.

Stage B2

Stage B2 refers to a state with 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 the state where heart failure or thromboembolism has occurred.

If pulmonary edema or pleural effusion is present, immediate administration of diuretics or thoracentesis (draining the fluid) is required. To avoid excessive stress, oxygen and sedatives are administered, and tests are performed while monitoring the cat's condition. Until the patient is stabilized and out of heart failure, treatment with diuretics and cardiotonic drugs is provided, requiring hospitalization. Antiplatelet drugs are also recommended to prevent blood clots.

Stage D

Stage D represents refractory heart failure that is resistant to treatment. Adjustments such as changing diuretics or cardiotonic agents are considered.

Arterial Thromboembolism

Arterial thromboembolism has the worst prognosis among all complications of cardiomyopathy. When it occurs, it leads not only to paralysis of the limbs but also to severe pain and a shortened life expectancy. Therefore, euthanasia is often recommended when arterial thromboembolism is confirmed.

However, if pain management is adequate, there is potential for improvement (no hypothermia, no chronic heart failure, unilateral onset), and the owner fully understands the risks and prognosis, treatment for the blood clot may be pursued.

Treatment for arterial thromboembolism involves pain management, anticoagulants, and antiplatelet drugs. Depending on the severity of the obstruction, the affected limb may undergo necrosis. Care tailored to the condition of the limb is necessary.

Early Detection and Care for Hypertrophic Cardiomyopathy

Get Regular Health Checkups

Hypertrophic cardiomyopathy is rarely detected in its early stages. Therefore, we recommend periodic echocardiograms and cardiac biomarker tests. Especially for high-risk breeds or cats with a family history of cardiomyopathy, heart examinations should begin before one year of age.

Care for Cats with Hypertrophic Cardiomyopathy

Cats diagnosed with hypertrophic cardiomyopathy will require lifelong medication and regular checkups. Since there is a risk of respiratory distress recurring after hospital discharge, consider renting an oxygen chamber for home use.

Cats with heart disease are prone to "cardiac cachexia"—a condition of weight loss and muscle wasting—which is known to impact prognosis. Switch to a food that provides appropriate protein and sufficient calories to ensure nutritional intake. There are prescription diets specifically formulated for heart disease that are sodium-restricted.

Cats that have experienced arterial thromboembolism may suffer from residual paralysis or necrosis of the limbs. Care such as protecting these areas from injury is necessary.

It is also important to bear in mind that, unfortunately, the life expectancy for cats with hypertrophic cardiomyopathy is not long, and there is a risk of sudden death.

Conclusion

Hypertrophic cardiomyopathy is the most common and one of the poorest-prognosis heart diseases in cats. It often goes unnoticed until symptoms manifest, and once symptoms appear, there is a very high probability of death; even if the condition is stabilized, there is still a risk of sudden death.

Early detection and early treatment are critical to the prognosis of hypertrophic cardiomyopathy. Regular health checkups, establishing a relationship with a regular veterinarian, and identifying clinics that offer night or holiday services are all essential measures.