Feline Hypertrophic Cardiomyopathy: The Complete Guide for Cat Parents

Medically reviewed by Kevin Toman, DVM — 40+ years of clinical veterinary experience, founder of the Pet Longevity Institute. Last reviewed August 2026.

There are FOUR (4) Diseases that threaten almost every older cat, and we have a summary of them for you here.  HCM gets two pages -- one on the disease, one on what I have learned in its treatment:

• HCM University
• 300 Cases of HCM Treated -- What I have learned
• Cat CKD University
• Cat Hypertension University
• Cat Hyperthyroidism University

OK, NOW FEATURING HCM--

HCM causes the wall of the heart's main pumping chamber — the left ventricle — to thicken abnormally, which can interfere with how well the heart fills and pumps blood. Some cats live a normal lifespan with HCM and never show a symptom. Others progress to congestive heart failure, blood clots, or dangerous heart rhythms. The difference almost always comes down to early detection, correct staging, and a monitoring plan that catches change before it becomes a crisis.

This page is meant to be the single resource you need to understand HCM in cats — what causes it, how it's diagnosed and staged, what the treatment options actually do, and what to realistically expect. Where our own protocols differ from — or add to — standard of care, that's noted explicitly, along with the reasoning behind it.

What Is Feline Hypertrophic Cardiomyopathy?

HCM is a disease of the heart muscle itself (a "cardiomyopathy"), not of the valves or arteries. In HCM, the walls of the left ventricle thicken — sometimes evenly, sometimes in specific regions — which stiffens the chamber and makes it harder for the heart to relax and fill between beats. Over time this can raise pressure inside the heart, enlarge the left atrium, encourage clot formation, and, in advanced cases, lead to congestive heart failure or a saddle thrombus (a blood clot that lodges where the aorta splits toward the hind legs).

In most cats, the underlying cause is a genetic mutation affecting one of the proteins in the heart muscle's contractile machinery, though not every cat with HCM has an identified mutation. HCM can also occur secondary to other conditions — most commonly hyperthyroidism or high blood pressure — which is why a full workup rules those out before a primary HCM diagnosis is confirmed.

Symptoms of HCM in Cats

HCM is notoriously silent in its early stages. Many cats show no outward signs at all until the disease is fairly advanced, which is exactly why screening — not waiting for symptoms — matters.

Early or subtle signs

• A heart murmur or gallop rhythm found incidentally at a wellness exam
• Mild lethargy or reduced activity/play, often dismissed as normal aging
• Occasional rapid or labored breathing after exertion or stress
• Reduced appetite

Emergency signs — seek care immediately

• Open-mouth breathing or persistent panting
• Breathing that looks labored, rapid, or effortful even at rest
• Sudden hind-limb weakness, dragging, coldness, or paralysis (possible saddle thrombus/ATE)
• Collapse, extreme lethargy, or crying out in distress
• Blue or pale gums

Because so many cats with HCM show no symptoms until a crisis, we recommend cardiac screening as part of routine senior wellness care — not only when a murmur is heard. See our Functional Health Worksheets for what a comprehensive senior-cat workup should include.

Which Cat Breeds Are Most at Risk?

HCM can occur in any cat, but certain breeds carry a meaningfully higher risk, and in some breeds specific genetic mutations have been identified:

• Maine Coon — a mutation in the MYBPC3 gene (cardiac myosin-binding protein C) is well documented and genetic testing is available
• Ragdoll — a different MYBPC3 mutation than the Maine Coon variant, also testable
• Sphynx
• British Shorthair
• American Shorthair
• Domestic shorthair/mixed-breed cats — HCM is not limited to purebreds; mixed-breed cats make up a large share of cases seen in practice

If you have a Maine Coon or Ragdoll, ask your veterinarian about genetic testing — a positive result doesn't guarantee disease, but it does mean earlier and more frequent cardiac screening is warranted.

How Is HCM Diagnosed?

No single test diagnoses HCM on its own. Diagnosis typically combines:

• Echocardiogram (cardiac ultrasound) — the definitive test. It directly measures wall thickness and chamber function and is the only way to confirm and stage HCM.
• Auscultation (listening with a stethoscope) — useful for catching a murmur or gallop, but has real limits: a meaningful percentage of cats with confirmed HCM have no audible murmur, and murmurs can also occur in cats with entirely normal hearts. A clean auscultation does not rule HCM out.
• NT-proBNP blood test — a biomarker that rises when the heart muscle is under strain; useful as a screening or triage tool, especially to help distinguish cardiac from respiratory causes of breathing difficulty, but it's a supporting test, not a replacement for echocardiography.
• Blood pressure and thyroid testing — to rule out secondary causes of wall thickening before confirming primary HCM.
• Chest X-rays — to evaluate for fluid in or around the lungs if congestive heart failure is suspected.

Stages of Feline HCM (ACVIM Staging System)

The most widely used staging framework comes from the 2020 American College of Veterinary Internal Medicine (ACVIM) consensus statement on feline cardiomyopathies. Staging matters because treatment recommendations differ meaningfully at each stage.

Treatment Options by Stage

Standard-of-care treatment is staged and targeted at the specific risk each cat faces — there is no single "HCM medication." Commonly used treatments include:

• Beta-blockers (e.g., atenolol) — sometimes used for outflow obstruction or significant arrhythmias, more often in symptomatic cats than in stage A/B1
• Antithrombotic therapy (e.g., clopidogrel) — used from stage B2 onward to reduce the risk of a thromboembolic event
• Diuretics (e.g., furosemide) — used in stage C/D to manage fluid buildup from congestive heart failure
• ACE inhibitors / pimobendan — used selectively in CHF management depending on the individual cat's echocardiographic findings

Felycin-CA1 and Rapamycin: The Longevity Approach to HCM

Standard-of-care drugs above largely manage the consequences of HCM — arrhythmia risk, clotting risk, fluid overload. They don't target the underlying hypertrophic (muscle-thickening) process itself. This is where our practice takes a different approach.

Rapamycin is an mTOR inhibitor with a growing body of research — including studies specifically in cats with HCM — suggesting it can improve markers of diastolic heart function and, in some cases, reduce wall thickness. Felycin-CA1, the cat-specific rapamycin formulation developed within our practice, was created to make this approach practical and precisely dosed for feline patients. You can see how it works in our video on Felycin for cats and our video on rapamycin for cats.

This isn't a replacement for standard-of-care staging and monitoring — it's an addition, used alongside conventional treatment where appropriate for the cat's stage. Cats on Felycin or rapamycin still need the same echocardiographic monitoring described below. Explore both options on our Felycin-CA1 product page and rapamycin for cats product page, or see targeted nutraceutical support in our Heart & HCM Support collection.

Monitoring Schedule

Monitoring frequency depends on stage, and should be set by your veterinarian based on your cat's specific echocardiographic findings. As a general framework:

• Stage A (at-risk breed, no findings yet): screening echo roughly every 1–2 years, or per breed-specific genetic testing guidance
• Stage B1 (subclinical, low risk): recheck echo roughly every 12 months
• Stage B2 (subclinical, higher risk): recheck echo roughly every 6 months
• Stage C/D (symptomatic): much closer monitoring, often including bloodwork, X-rays, and exams every 1–3 months or sooner depending on stability

These are general starting points, not a substitute for an individualized plan from the veterinarian managing your cat's case.

Prognosis: What to Expect

Prognosis in feline HCM varies enormously by stage, which is the main reason staging (not just "diagnosis") is the right framework for thinking about this disease.

• Many cats identified at stage A or B1 live a normal or near-normal lifespan, especially with regular monitoring to catch progression early.
• Cats at stage B2 carry meaningfully higher risk of a heart failure event or thromboembolism, but many remain stable for extended periods with appropriate monitoring and preventive treatment.
• Cats that progress to congestive heart failure (stage C) have a more variable and generally shorter prognosis, ranging from months to a few years depending on how well they respond to treatment, though individual outcomes vary widely.
• A thromboembolic event (saddle thrombus) is a medical emergency with a guarded prognosis regardless of prior stage.

The single biggest lever you have over your own cat's prognosis is catching progression early — which is the entire rationale behind stage-appropriate monitoring rather than a "wait for symptoms" approach.

Frequently Asked Questions

Additional Resources & Sources

From Dr. Kevin

• HCM Digital Course (a $75 value, included free)
• HCM video showing actual heart changes
• HCM treatment flow chart
• Video: Felycin for cats
• Video: Rapamycin for cats
• Get your cat's Longevity Score
• Join the Cat Longevity Collective
• Feline CKD University — HCM and kidney disease are frequently comorbid in senior cats

Primary and reference sources

• Luis Fuentes V, et al. ACVIM Consensus Statement Guidelines for the Classification, Diagnosis, and Management of Cardiomyopathies in Cats. J Vet Intern Med. 2020;34:1062–1077. (Open access; source of the staging system referenced above.)
• Cornell Feline Health Center — general feline cardiac disease resources
• Winn Feline Foundation — feline health research, including breed genetic screening studies

This page is for educational purposes and does not replace an individualized diagnosis, staging, or treatment plan from a veterinarian who has examined your cat.