Hypertrophic Cardiomyopathy (HCM) in Cats: The Silent Heart Disease That Can Strike Without Warning

Hypertrophic cardiomyopathy (HCM) is the most commonly diagnosed cardiac disease in cats, and it can remain silent for years — a cat with moderate left ventricular hypertrophy may have a normal physical exam and no clinical signs — until a catastrophic event occurs. The left ventricular wall thickens from a primary muscle disorder, making the ventricle stiff and unable to fill properly. Pressure backs up into the left atrium and lungs, and blood clots can form in the enlarged atrium, break loose, and cause sudden, painful hind-limb paralysis (Cornell Feline Health Center, n.d.). Mutations in the MYBPC3 gene have been identified in Maine Coons and Ragdolls, but HCM occurs in all breeds (UC Davis, 2021).

What is actually happening inside a cat's heart with HCM?

Hypertrophic cardiomyopathy (HCM) is the most commonly diagnosed cardiac disease in cats, and at its core, it is a disease of muscle gone wrong. The left ventricular wall thickens — not from high blood pressure or a narrowed aortic valve, but from a primary disorder of the heart muscle itself. That thickening makes the ventricle stiff. A stiff ventricle cannot relax properly between beats, which means it fills poorly, and the pressure backs up into the left atrium and eventually the lungs. This is diastolic dysfunction, and it is the central problem in HCM (Cornell Feline Health Center, n.d.).

The disease has a strong genetic component. Mutations in the myosin-binding protein C (MYBPC3) gene have been identified in Maine Coon and Ragdoll cats, and genetic testing is available for both breeds. However, HCM occurs in all cat breeds, including domestic shorthairs, and can be diagnosed in cats as young as 4 months and as old as 16 years (UC Davis School of Veterinary Medicine, 2021). Males are overrepresented. The disease can remain silent for years — a cat with moderate left ventricular hypertrophy may have a normal physical exam, normal thoracic radiographs, and no clinical signs — until a catastrophic event occurs.

What are the signs that a cat's heart is failing from HCM?

Many cats with HCM are asymptomatic at the time of diagnosis, and the disease is discovered incidentally when a veterinarian hears a heart murmur, gallop rhythm, or arrhythmia during a routine exam. When signs do appear, they fall into three categories. The first is congestive heart failure: fluid accumulates in the lungs (pulmonary edema) or around the lungs (pleural effusion), and the cat presents with rapid, labored, or open-mouth breathing, often hiding, reluctant to move, and adopting a crouched posture with elbows out and head extended (Cornell Feline Health Center, n.d.).

The second presentation is arterial thromboembolism (ATE), also called a saddle thrombus. A blood clot forms in the enlarged left atrium, breaks loose, and lodges at the aortic bifurcation where the aorta splits to supply the hind legs. The result is sudden, painful paralysis of one or both hind limbs. The cat screams in pain, drags its back legs, and the affected limbs are cold, firm, and pulseless. This is one of the most devastating emergencies in feline medicine, with a survival-to-discharge rate of only about 40-50% even with aggressive treatment (UC Davis School of Veterinary Medicine, 2021). The third presentation is sudden death, which can occur without any prior warning signs.

How do veterinarians diagnose HCM and rule out other heart diseases?

Echocardiography — cardiac ultrasound — is the definitive diagnostic tool. It allows direct visualization and measurement of left ventricular wall thickness, left atrial size, and systolic and diastolic function. A left ventricular wall thickness greater than 6 mm in diastole is generally considered diagnostic for HCM in cats, provided other causes of hypertrophy (systemic hypertension, hyperthyroidism, aortic stenosis) have been excluded (Cornell University College of Veterinary Medicine, n.d.).

Thoracic radiographs can show an enlarged cardiac silhouette and evidence of pulmonary edema or pleural effusion, but a cat can have significant HCM with normal radiographs. The NT-proBNP blood test measures a biomarker released by stretched heart muscle and can be used as a screening tool: a normal NT-proBNP makes clinically significant HCM unlikely, while an elevated result warrants echocardiography. Blood pressure measurement and thyroid testing are essential to rule out hypertensive cardiomyopathy and thyrotoxic heart disease, both of which can mimic HCM (UC Davis School of Veterinary Medicine, 2021).

What treatments can slow HCM down or prevent catastrophe?

There is no cure for HCM, and treatment is aimed at controlling symptoms, preventing complications, and improving quality of life. For cats in congestive heart failure, furosemide is the cornerstone — it removes excess fluid from the lungs and pleural space. The ACE inhibitor benazepril or enalapril may be added to reduce afterload and counteract neurohormonal activation. For cats with significant left atrial enlargement, the antiplatelet drug clopidogrel is recommended to reduce the risk of thrombus formation; the FAT CAT trial demonstrated that clopidogrel significantly reduced the recurrence rate of arterial thromboembolism compared to aspirin (Cornell Feline Health Center, n.d.).

Beta-blockers such as atenolol are sometimes prescribed to slow the heart rate and improve diastolic filling, though evidence for a survival benefit in asymptomatic cats is limited. Asymptomatic cats with mild to moderate HCM may not require medication, but they should be monitored with echocardiography every 6 to 12 months. Cats with dynamic left ventricular outflow tract obstruction may benefit from beta-blockade to reduce the obstruction gradient (UC Davis School of Veterinary Medicine, 2021).

What does the long-term outlook look like for a cat with HCM?

Prognosis is highly variable. Cats with mild, non-progressive HCM may live for years without ever developing clinical signs. Cats that present in congestive heart failure have a median survival time of approximately 6 to 18 months with treatment, though some live considerably longer. Cats that survive an arterial thromboembolism have a guarded prognosis: median survival after a first ATE event is approximately 3 to 6 months, and recurrence is common. The most important prognostic indicators are left atrial size (larger atria = higher risk of CHF and ATE) and the rate of disease progression on serial echocardiograms (Cornell Feline Health Center, n.d.).

Owners of cats with HCM should learn to monitor the resting respiratory rate at home. A sleeping cat should breathe fewer than 30 times per minute; a sustained increase above 30-40 breaths per minute is an early warning sign of developing congestive heart failure and warrants immediate veterinary attention. Stress reduction — keeping the cat indoors, minimizing changes to the household, and using low-stress handling techniques at veterinary visits — is an underappreciated but important part of long-term management.

Hypertrophic cardiomyopathy is the most commonly diagnosed cardiac disease in cats, characterized by concentric thickening of the left ventricular myocardium without chamber dilation. The disease has a strong genetic component, with mutations in the myosin-binding protein C gene identified in Maine Coon and Ragdoll cats, though HCM can occur in any breed, including domestic shorthairs. The pathophysiology involves impaired diastolic function, meaning the heart cannot relax and fill properly between contractions, which leads to increased left atrial pressure and eventual left atrial enlargement. Many cats with HCM remain asymptomatic for years, and the disease may be discovered incidentally during routine examination when a heart murmur, gallop rhythm, or arrhythmia is detected. However, the first clinical sign can be catastrophic, including sudden death, arterial thromboembolism causing acute hindlimb paralysis, or congestive heart failure with pulmonary edema presenting as acute respiratory distress (Merck Veterinary Manual, 2025; Cornell University College of Veterinary Medicine, 2021). Diagnosis relies on echocardiography, which allows measurement of left ventricular wall thickness, assessment of left atrial size, and evaluation of systolic and diastolic function. Thoracic radiographs help identify pulmonary edema or pleural effusion when heart failure is present. NT-proBNP, a cardiac biomarker, can be useful as a screening test in at-risk breeds or when echocardiography is not immediately available. Treatment focuses on managing clinical signs rather than reversing the underlying hypertrophy. Cats in congestive heart failure require diuretics, typically furosemide, along with ACE inhibitors to reduce afterload. Clopidogrel is recommended to reduce the risk of arterial thromboembolism. Beta-blockers or calcium channel blockers may be used to improve diastolic filling and reduce heart rate in selected cases (Merck Veterinary Manual, 2025).

The long-term outlook for cats with hypertrophic cardiomyopathy varies considerably depending on the severity of disease at diagnosis, the presence of clinical signs, and the response to treatment. Cats with mild, asymptomatic HCM may live for years without progression, while cats presenting with congestive heart failure or arterial thromboembolism have a more guarded prognosis. Regular echocardiographic monitoring allows assessment of disease progression and adjustment of therapy. Owners of cats with HCM should be educated about monitoring resting respiratory rate at home, as an increase above 30 breaths per minute during sleep can be an early indicator of developing congestive heart failure. They should also understand the signs of arterial thromboembolism, including sudden hindlimb paralysis, pain, and cold extremities, as this complication requires immediate emergency care. With appropriate management, many cats with HCM can maintain a good quality of life, and the goal of treatment is to keep them comfortable and stable for as long as possible (Merck Veterinary Manual, 2025; Cornell University College of Veterinary Medicine, 2021).

References

• Cornell Feline Health Center. (n.d.). Hypertrophic cardiomyopathy. Cornell University College of Veterinary Medicine. https://www.vet.cornell.edu/departments-centers-and-institutes/cornell-feline-health-center/health-information/feline-health-topics/hypertrophic-cardiomyopathy
• Cornell University College of Veterinary Medicine. (n.d.). Hypertrophic cardiomyopathy (HCM). https://www.vet.cornell.edu/hospitals/services/cardiology/hypertrophic-cardiomyopathy-hcm
• UC Davis School of Veterinary Medicine. (2021, April 29). Hypertrophic cardiomyopathy. Animal Health Topics. https://healthtopics.vetmed.ucdavis.edu/health-topics/hypertrophic-cardiomyopathy