HCM in Cats: Hypertrophic Cardiomyopathy Explained
- Hypertrophic cardiomyopathy, or HCM, is the most common heart disease in cats. It causes the heart muscle, usually the left ventricle, to thicken and fill less efficiently.
- Many cats have no obvious signs early on. Others develop fast or labored breathing, lethargy, fainting, or sudden painful hind-leg weakness from a blood clot.
- An echocardiogram is the test that confirms HCM. Chest X-rays, blood pressure, thyroid testing, NT-proBNP, and ECG may help your vet stage the disease and look for related problems.
- Treatment depends on whether your cat has no symptoms, congestive heart failure, an outflow obstruction, or clot risk. Options may include monitoring, diuretics, clopidogrel, oxygen support, and other heart medications chosen by your vet.
- See your vet immediately if your cat is breathing with effort, breathing with an open mouth, collapses, or suddenly cannot use one or both back legs.
What Is HCM?
Hypertrophic cardiomyopathy, usually called HCM, is a disease where the muscular wall of the heart becomes abnormally thick. In cats, the left ventricle is most often affected. When that wall thickens, the chamber cannot relax and fill normally between beats, so the heart may move less blood with each contraction.
HCM is the most common heart disease in cats. Some cats stay in a subclinical phase for months or years, meaning the heart changes are present but there are no obvious symptoms at home. Others develop complications such as congestive heart failure, abnormal heart rhythms, blood clots, or sudden death.
One reason HCM can be hard for pet parents to spot is that cats often hide illness well. A cat may seem normal until breathing changes, exercise intolerance, or a painful clot event happens. That is why a murmur, gallop rhythm, or abnormal screening test often leads your vet to recommend a heart ultrasound even if your cat still seems comfortable.
Symptoms of HCM
- No visible symptoms at all in early or subclinical disease
- Heart murmur, gallop rhythm, or irregular heartbeat found during an exam
- Resting breathing rate consistently over about 35 breaths per minute
- Rapid, labored, or open-mouth breathing, which can signal fluid in or around the lungs
- Lethargy, hiding more, or tiring faster during normal activity
- Weakness, collapse, or fainting episodes
- Decreased appetite in cats feeling unwell from heart failure
- Sudden crying out, hind-leg pain, cold back feet, dragging the rear legs, or paralysis from an arterial thromboembolism
- Sudden death in severe cases
Many cats with HCM look normal until the disease is advanced, so subtle changes matter. A faster resting breathing rate, less interest in play, or unusual fatigue can be early clues. If your cat is sleeping, count breaths for 30 seconds and double it. A repeated resting rate above about 35 breaths per minute deserves a prompt call to your vet.
See your vet immediately if your cat has labored breathing, open-mouth breathing, collapse, or sudden hind-leg weakness or pain. Those signs can happen with congestive heart failure or a blood clot and should be treated as emergencies.
What Causes HCM?
In many cats, the exact cause of HCM is not fully known. What is clear is that genetics play an important role in some lines and breeds. Maine Coons and Ragdolls are the best-known examples, but familial disease has also been reported in breeds such as Persians, Sphynx, Norwegian Forest Cats, Bengals, Turkish Vans, and some American and British Shorthairs.
HCM is not always present at birth, and it can develop over time. Some cats are diagnosed as young adults, while others are not identified until middle age or later. Males and females can both be affected, though some sources note males may develop more severe disease earlier.
Your vet also has to consider conditions that can mimic or contribute to heart muscle thickening. Hyperthyroidism and high blood pressure are two important examples. Less commonly, inflammation of the heart muscle or excess growth hormone can be involved. That is why a full workup often includes blood pressure measurement and lab testing rather than assuming every thickened heart is primary HCM.
How Is HCM Diagnosed?
The test that confirms HCM is an echocardiogram, which is an ultrasound of the heart. This lets your vet or a veterinary cardiologist measure the thickness of the heart walls, look at how well the chambers fill, and check for changes such as left atrial enlargement or obstruction to blood flow.
Diagnosis usually starts with a physical exam. Your vet may hear a murmur, a gallop rhythm, or an irregular heartbeat, but not every cat with HCM has an obvious abnormal sound. Chest X-rays can help look for fluid in or around the lungs if congestive heart failure is suspected. An ECG may be used if an arrhythmia is a concern.
Blood pressure testing and blood work are also important because hyperthyroidism and systemic hypertension can cause secondary thickening of the heart. Some cats also have NT-proBNP testing to help assess whether heart disease is likely or severe, especially when symptoms are vague or respiratory signs could have more than one cause.
Once HCM is diagnosed, follow-up matters. Your vet may recommend repeat imaging, breathing-rate tracking at home, and periodic lab work to monitor progression and response to treatment.
Treatment Options for HCM
Spectrum of Care means you have options. Here are treatment tiers at different price points.
Budget-Conscious Care
- Primary care exam and repeat auscultation
- Resting respiratory rate tracking at home
- Basic blood work, thyroid testing, and blood pressure check
- NT-proBNP screening when available
- Targeted medications only if your vet feels they are needed based on stage and symptoms
- Referral timing discussion if an echocardiogram is not immediately possible
Recommended Standard Treatment
- Echocardiogram to confirm HCM and stage severity
- Chest X-rays if breathing changes are present
- ECG if arrhythmia is suspected
- Blood pressure, CBC/chemistry, and thyroid testing
- Stage-based medication plan such as diuretics for congestive heart failure, clopidogrel for clot-risk cases, and selected heart medications when indicated
- Scheduled rechecks with repeat imaging or lab monitoring
Advanced / Critical Care
- Emergency stabilization for respiratory distress
- Oxygen therapy and injectable diuretics for congestive heart failure
- Thoracocentesis if fluid around the lungs is impairing breathing
- Specialty cardiology consultation and repeat echocardiography
- Hospital monitoring for arrhythmias, blood pressure, kidney values, and response to therapy
- Discussion of newer options such as conditionally approved sirolimus delayed-release for selected cats with subclinical HCM, when appropriate and available through your vet
Cost estimates as of 2026-03. Actual costs vary by location, clinic, and individual case.
Questions to Ask Your Vet About HCM
Bring these questions to your vet appointment to get the most out of your visit.
- Does my cat need an echocardiogram now, or are there screening tests we should start with first?
- Is this likely primary HCM, or do we need to rule out high blood pressure, hyperthyroidism, or another cause of heart thickening?
- Is my cat in a subclinical stage, or are there signs of congestive heart failure or left atrial enlargement?
- What symptoms should make me seek urgent care right away, especially for breathing changes or possible blood clots?
- Should I monitor my cat's resting breathing rate at home, and what number is too high for my cat?
- Does my cat need medication now, and what is each medication meant to do?
- What follow-up schedule do you recommend for rechecks, imaging, and lab work?
- What cost range should I expect for the next step in diagnosis and for ongoing management over the next year?
How to Prevent HCM
There is no guaranteed way to prevent HCM in an individual cat, especially when genetics are involved. Still, early detection can make a real difference. Cats in higher-risk breeds or family lines may benefit from screening discussions with your vet, particularly before breeding, anesthesia, or if a murmur is found.
Good preventive care also means looking for diseases that can worsen or mimic heart thickening. Regular exams, blood pressure checks when indicated, and senior blood work can help identify problems such as hyperthyroidism or hypertension earlier. Feeding a complete commercial cat food is also important for overall heart health, even though taurine deficiency is linked more strongly with dilated cardiomyopathy than HCM.
At home, focus on observation rather than trying to manage this condition alone. Track your cat's resting breathing rate if your vet recommends it, keep recheck appointments, and report changes in stamina, appetite, or breathing promptly. For breeding cats, responsible screening and avoiding breeding affected lines are the most meaningful prevention steps currently available.