By Dr Duncan Houston
A cat with hypertrophic cardiomyopathy can look completely well. That makes an unexpected heart murmur or ultrasound finding unsettling, especially when the same diagnosis can describe very different levels of risk.
The useful questions are how the heart is functioning, whether complications are developing and what you should watch for at home.
Quick Answer
Hypertrophic cardiomyopathy, or HCM, causes abnormal thickening of the heart muscle and can interfere with filling and blood flow. Some cats remain symptom-free for years; others develop heart failure or arterial blood clots. Heart ultrasound is central to diagnosis and risk assessment, and treatment depends on the findings. Difficulty breathing, collapse or sudden painful limb weakness needs emergency veterinary care.
What happens to the heart in HCM?
The thickened left ventricle may become stiff and fill less effectively between beats. Pressure can rise behind it, enlarging the left atrium and eventually allowing fluid to accumulate in or around the lungs. Changes in blood flow can also encourage clot formation.
Some cats have obstruction to blood leaving the ventricle, sometimes involving abnormal movement of the mitral valve. This is a possible feature, not an inevitable stage that every cat reaches.
Cornell's feline HCM guidance explains the wide variation in signs and outcome. A diagnosis does not automatically mean the cat is currently in heart failure.
Which cats can develop HCM?
HCM affects pedigree and non-pedigree cats. Inherited variants are recognised in some breeds, including Maine Coons and Ragdolls, but many affected cats have no identified genetic cause.
A negative breed-specific DNA test cannot rule out every form of HCM. Likewise, a normal heart examination today does not guarantee lifelong freedom from disease. Screening advice should reflect the individual cat and, for breeding animals, the family history.
How urgent are the signs?
| What you notice | What it may mean | Action |
|---|---|---|
| Murmur found at a check-up, otherwise comfortable cat | Heart disease is possible, but a murmur alone is not a diagnosis | Arrange the recommended investigation |
| Sleeping breathing rate repeatedly above the agreed limit or rising from baseline | Possible developing fluid accumulation or another illness | Contact the veterinary team promptly the same day |
| Open-mouth or laboured breathing, collapse, sudden painful weak or paralysed limb | Heart failure, a clot or another serious emergency | Seek emergency veterinary care immediately |
How do vets diagnose and assess HCM?
Echocardiography, an ultrasound examination of the heart, assesses wall thickness, chamber size, filling, pumping and any outflow obstruction. Left atrial changes and evidence of sluggish blood flow help the clinician assess complication risk.
Blood pressure and thyroid testing help identify other causes of heart-muscle thickening. Blood biomarkers such as NT-proBNP can support investigation, but do not replace a full assessment or reliably exclude every mild case. An ECG investigates abnormal rhythms. Chest imaging helps assess lung or chest fluid, although a breathless cat may need stabilisation before stressful tests.
What else could explain these findings?
- A heart murmur: may occur without HCM or with other heart disorders; its loudness does not reliably grade disease severity.
- Thickened heart walls: can be associated with high blood pressure, hyperthyroidism or other conditions that need distinguishing from primary HCM.
- Rapid breathing: can also reflect asthma, lung disease, pain, fever or other chest disease.
- Sudden limb weakness: may be neurological or traumatic, but a painful cold limb raises concern for an arterial clot.
These alternatives are reasons to investigate, not reasons to delay when a cat is struggling.
When is this an emergency?
Go straight to an emergency vet for breathing difficulty, collapse or sudden painful limb weakness or paralysis. Keep handling to a minimum, use a secure carrier and call ahead. Let a breathless cat maintain the position in which they can breathe most comfortably.
Do not force tablets, food or water into a struggling cat. Do not massage a cold paralysed leg or give aspirin at home. A cat with a suspected clot needs urgent pain relief and assessment.
How is HCM treated?
There is no single medication list suitable for every stage. A cat with low-risk, symptom-free disease may need monitoring, while higher clot risk may justify an antiplatelet medicine. Heart failure requires treatment to reduce fluid accumulation, with oxygen or drainage of chest fluid when necessary.
Other medicines may be selected for important obstruction, rhythm disturbances or particular functional problems. Kidney function, hydration and electrolytes influence treatment safety. Do not start a diuretic, alter a dose or add a supplement without the treating team's plan.
What about sirolimus?
The US FDA conditionally approved a delayed-release sirolimus product to manage ventricular hypertrophy in cats with subclinical HCM. This is a specific prescription indication, not a cure or an emergency treatment for heart failure.
Conditional approval should not be presented as proof that every treated cat will live longer. The vet must confirm suitability, review the current label and arrange monitoring. The FDA advises screening for liver disease and says the product should not be used in cats with pre-existing liver disease or diabetes. Availability and authorisation differ between countries.
How to monitor your cat at home
- Learn the sleeping breathing rate. When your cat is asleep and not purring or actively dreaming, count each rise-and-fall of the chest as one breath for a full minute.
- Record the trend. Ask for a personalised call threshold. The ACVIM cardiomyopathy consensus uses a goal below 30 breaths per minute in monitoring cats with controlled heart failure. Repeated readings above 30 or a clear rise from baseline warrant a prompt call; effort matters regardless of the number.
- Track practical changes. Note appetite, weight, medication administration, activity and comfort.
- Keep follow-up individual. Recheck timing depends on stage, treatment changes and progress, rather than a fixed schedule for every cat.
- Make daily life easy. Provide calm resting areas and accessible resources. Let the cat choose gentle activity instead of forcing exercise.
Common mistakes and preventive care
- Assuming no murmur means no heart disease.
- Treating all HCM cats with the same drugs or restrictive diet.
- Waiting for a high breathing count despite obvious breathing effort.
- Replacing prescribed medication with supplements.
- Giving extra fluids without discussing the heart condition with the vet.
HCM cannot reliably be prevented through lifestyle changes. Sensible screening, appropriate breeding decisions and monitoring can improve recognition and care. Offer complete food your cat eats reliably; discuss any dietary restrictions rather than imposing a poor-appetite risk.
Frequently Asked Questions
Can a cat with HCM live a normal life?
Some symptom-free cats remain well for years. Prognosis depends on the individual findings and complications, so a universal survival estimate is misleading.
Does every cat with HCM need medication?
No. The reason for each treatment should be linked to the cat's risk and current condition.
Can HCM cause coughing?
It can, but coughing is less typical than breathing changes in feline heart failure. Persistent coughing still needs investigation for respiratory and other disease.
Does a negative DNA test rule out HCM?
No. It excludes the tested variants, not all possible causes of HCM.
Focus on the cat in front of you
A clear diagnosis, an individual treatment plan and a simple home record make HCM easier to manage. Know your emergency route, then use everyday appetite, breathing and comfort to guide timely conversations with your vet.
For support understanding your cat's care plan, explore ASK A VET™. Breathing difficulty or sudden painful weakness should go directly to an emergency veterinary service.
The ASK A VET™ Tracker logs live location and activity trends, so a change in routine never goes unnoticed.
Every ASK A VET article is written and reviewed by qualified veterinarians.
