HCM, or hypertrophic cardiomyopathy, is the most common heart disease in cats. It causes the muscular walls of the heart’s left ventricle to thicken over time, making the chamber smaller and stiffer. A large screening study of apparently healthy cats in rehoming centers found HCM in nearly 15% of them, and the condition becomes more common with age. Many affected cats show no outward signs, which is part of what makes the disease so concerning.
What Happens Inside the Heart
In a healthy cat heart, the left ventricle fills with blood between beats, then contracts to pump it out to the body. With HCM, the muscle cells in the ventricular wall become disorganized and excessive connective tissue builds up between them. This makes the wall thicker and less flexible. A normal left ventricular wall measures less than 5 millimeters. Walls between 5 and 6 millimeters are considered borderline, and anything above 6 millimeters is classified as hypertrophic.
The thickening isn’t always even. It’s often asymmetric, affecting some portions of the wall and the small muscles inside the chamber (papillary muscles) more than others. As the wall grows inward, the space inside the ventricle shrinks. The heart can’t fill with as much blood per beat, so it compensates by beating faster. Over months or years, this puts increasing strain on the heart and can lead to serious complications.
Breeds at Higher Risk
HCM affects cats of every background, and most diagnosed cats are actually mixed-breed domestic cats simply because they outnumber purebreds. That said, certain breeds carry a higher genetic risk: Maine Coons, Ragdolls, American Shorthairs, British Shorthairs, Persians, Bengals, Sphynx cats, Norwegian Forest cats, and Scottish Folds all appear more frequently in HCM studies.
Researchers have identified specific gene mutations linked to the disease in some of these breeds. Maine Coons were the first breed to have an HCM-associated mutation pinpointed, in a gene that helps build the protein scaffolding of heart muscle cells. A different mutation in the same gene was later linked to HCM in Ragdolls. In Sphynx cats, the culprit appears to be a mutation in a completely different gene. These mutations have also turned up in breeds not originally studied, including Munchkins, Exotic Shorthairs, and Minuets, suggesting the genetic picture is more complex than breed lines alone would predict.
Genetic testing is available for some of these mutations and can be useful for breeding decisions, but a negative test doesn’t guarantee a cat won’t develop HCM. Many cases involve mutations that haven’t been identified yet.
Why Many Cats Show No Symptoms
One of the most frustrating things about HCM is that many cats look and act completely normal, even with significant heart wall thickening. The heart compensates well for a long time, and cats are naturally good at hiding discomfort. A veterinarian might detect a heart murmur or abnormal rhythm during a routine exam, but even that isn’t always present.
When symptoms do appear, they typically signal that the disease has progressed to congestive heart failure, meaning fluid is building up in or around the lungs. At that point, you might notice rapid or labored breathing, open-mouthed breathing (which is always abnormal in cats), or a sudden drop in energy and activity. These signs can develop gradually or come on quickly.
Blood Clots: The Most Dangerous Complication
As the left side of the heart struggles to move blood efficiently, blood can pool and slow down, particularly in a small pouch called the left atrial appendage. When blood sits still long enough, clots can form. If a clot breaks loose, it travels through the bloodstream until it gets stuck somewhere, cutting off blood supply to whatever is downstream.
The most common place for this to happen is where the aorta splits to supply the hind legs, a location that gives this emergency its name: saddle thrombus. The signs are sudden and unmistakable. A cat will cry out in pain, lose the ability to use one or both back legs, and the affected paws may feel cold to the touch. The paw pads and nail beds may look pale or bluish compared to the unaffected limbs, and there will be no detectable pulse in the affected leg. This is a veterinary emergency that requires immediate care.
How HCM Is Diagnosed
An echocardiogram (heart ultrasound) is the gold standard for diagnosing HCM. It lets a veterinarian directly measure the thickness of the heart walls, assess how well the chambers fill and empty, and look for complications like an enlarged left atrium or blood flow obstruction. This is typically performed by a veterinary cardiologist or a vet with specialized ultrasound training.
For screening situations where an echocardiogram isn’t immediately available, a blood test measuring a molecule called NT-proBNP can help. This protein is released by heart muscle cells under stress. Levels below 49 pmol/L suggest hidden heart disease is unlikely, while levels above 100 pmol/L suggest structural heart changes are probable. Values in between are less definitive. The blood test is a useful screening tool, but it can’t replace an echocardiogram for a definitive diagnosis.
If a cat is already having breathing difficulties, the same blood test can help determine whether heart failure is the cause. Levels above roughly 270 pmol/L in a cat with respiratory signs point toward heart failure with about 93% sensitivity.
Treatment and Ongoing Management
There is no cure for HCM. Treatment focuses on slowing the disease’s progression, managing symptoms, and reducing the risk of life-threatening complications. What your cat needs depends on how far the disease has advanced.
Cats with mild, asymptomatic HCM may not need medication right away, though regular monitoring with echocardiograms is important to track changes. When the left atrium starts to enlarge, your vet will likely recommend an anti-clotting medication to reduce the risk of thromboembolism. This is one of the most important interventions because blood clots are often the first visible crisis in an otherwise stable cat.
If the thickened heart wall is creating an obstruction inside the ventricle, beta-blockers like atenolol can slow the heart rate and give the ventricle more time to fill, which helps blood flow more smoothly. Calcium channel blockers serve a similar role in some cases. For cats that develop congestive heart failure, diuretics are used to clear the fluid buildup in and around the lungs, making breathing easier. Some cats with advanced disease may also be placed on a medication that helps the heart contract more effectively, though this is used cautiously when obstruction is present.
What to Expect After Diagnosis
Prognosis varies enormously depending on the stage at diagnosis. Some cats with mild HCM live for years with minimal or no progression and enjoy a normal quality of life. The outlook changes significantly once complications develop. In one study of cats with symptomatic HCM, those who developed congestive heart failure had a median survival of about 51 days after the onset of symptoms. Cats who presented with a blood clot had a median survival of just 5.5 days, reflecting how devastating and sudden that complication can be.
These numbers represent cats who were already in crisis, not all cats with HCM. Early detection through routine veterinary exams and screening of at-risk breeds gives cats the best chance of being managed before they reach that point. Regular echocardiographic monitoring, typically every 6 to 12 months for known HCM cats, lets your veterinarian adjust the treatment plan as the disease evolves.