A Quiet Threat: Hypertrophic Cardiomyopathy in Cats
When people think about heart disease in pets, they often picture dogs—but cats can suffer from heart conditions too, and one of the most common and serious is Hypertrophic Cardiomyopathy (HCM).
Veterinary student Jeremiah Pouncy explains it simply: “HCM is a thickening of the heart muscle. The scary part is that many cats look perfectly healthy until the disease is advanced. By the time they show signs, it can already be serious.”
What Is HCM?
Hypertrophic Cardiomyopathy is a disease where the walls of the heart—especially the left ventricle—become abnormally thickened. This thickening makes the heart less flexible and reduces the amount of blood it can hold and pump out with each beat.
Because the heart can’t relax and fill properly, pressure builds up inside, which can lead to heart failure or blood clotsthat block blood flow to the legs or other organs.
HCM is the most common heart disease in cats, and it can affect cats of any breed or age, although it’s often seen in middle-aged males.
Which Cats Are at Risk?
While any cat can develop HCM, certain breeds are genetically predisposed, including:
Maine Coon
Ragdoll
British Shorthair
Bengal
Sphynx
Norwegian Forest Cat
Purebred cats are more likely to carry inherited gene mutations that cause the heart muscle to thicken. However, domestic shorthairs and longhairs can also develop HCM with no clear genetic cause.
Other factors—such as high blood pressure or overactive thyroid (hyperthyroidism)—can worsen the condition.
Early Signs and Symptoms
One of the most challenging things about HCM is that it can remain silent for years. Cats often don’t show any symptoms until the disease is advanced.
When signs do appear, they may include:
Rapid or labored breathing
Open-mouth breathing (a medical emergency in cats)
Lethargy or hiding more than usual
Loss of appetite
Collapse or fainting
Sudden paralysis of the back legs, caused by a blood clot (saddle thrombus)
Jeremiah recalls his first case during veterinary rotations:
“The cat came in suddenly unable to move her back legs. It turned out to be a blood clot from HCM. It was heartbreaking—and a powerful reminder of how dangerous this disease can be.”
How HCM Is Diagnosed
A heart murmur or abnormal rhythm heard during a checkup often gives veterinarians the first clue. From there, diagnostic tests help confirm HCM and assess its severity:
Echocardiogram (heart ultrasound): The gold standard for diagnosis. It measures wall thickness and chamber size.
Chest X-rays: Show fluid buildup or heart enlargement.
Electrocardiogram (ECG): Detects abnormal heart rhythms.
Blood tests: Check thyroid levels and cardiac biomarkers like NT-proBNP.
Blood pressure measurement: To rule out secondary causes.
Regular screening—especially for at-risk breeds—is the best way to catch HCM before it becomes life-threatening.
Treatment and Management
There is no cure for HCM, but treatment can help control symptoms and reduce complications.
Veterinarians may use:
Diuretics to remove excess fluid (furosemide).
Beta-blockers or calcium channel blockers to slow the heart rate and help it fill better.
ACE inhibitors to reduce strain on the heart.
Antithrombotic drugs (like clopidogrel) to prevent blood clots.
Oxygen therapy or hospitalization during acute crises.
“The goal,” Jeremiah says, “is to reduce stress on the heart, manage fluid buildup, and prevent blood clots. With careful monitoring, many cats can live comfortably for years.”
New Hope: Felycin®-CA1
In 2025, the FDA granted conditional approval for Felycin®-CA1, the first drug designed specifically for cats with subclinical hypertrophic cardiomyopathy—that is, cats with early disease but no symptoms yet.
Felycin-CA1 contains sirolimus, a compound that may help reduce heart muscle thickening and slow disease progression. It’s given once weekly under veterinary supervision.
While not a cure, Jeremiah says this marks an exciting step:
“Until now, our treatments only managed the effects of HCM. Felycin-CA1 could actually help change the course of the disease if used early.”
The drug should not be used in cats with liver disease or diabetes, and ongoing studies are collecting more long-term safety and effectiveness data.
Living With an HCM Diagnosis
Cats with HCM need regular veterinary checkups—usually every 6–12 months, depending on the stage of disease. Echocardiograms help track progression and guide medication adjustments.
Owners should monitor for breathing changes, weakness, or sudden loss of leg movement and seek emergency care immediately if they occur.
Most importantly, Jeremiah emphasizes:
“An HCM diagnosis isn’t the end. With early detection and new treatment options like Felycin-CA1, cats are living longer, more comfortable lives than ever before.”
Talk to your vet for more information.
