The Silent Disease Most Cat Owners Have Never Heard Of
Hypertrophic cardiomyopathy (HCM) affects an estimated one in seven adult cats. It is the most common heart disease diagnosed in the species worldwide, and it kills without obvious warning in a significant number of cases. Yet many owners remain unaware of it until a cat collapses, develops sudden hind limb paralysis, or is found dead. Understanding HCM — what it does, which cats face the highest risk, and how it is monitored — is one of the most important things a cat owner can know.
What HCM Does to the Heart
Unlike DCM in dogs, where the heart muscle weakens, HCM involves pathological thickening of the heart muscle — primarily the walls of the left ventricle. The thicker the wall, the stiffer the chamber becomes. A stiff chamber cannot relax and fill properly between beats. Pressure builds in the left atrium as it struggles to push blood into an unyielding ventricle; eventually that pressure backs up into the lungs, causing fluid accumulation (congestive heart failure). The enlarged left atrium is also prone to forming blood clots, which can travel to distant arteries and cause sudden, catastrophic blockages — most commonly at the aortic trifurcation supplying the hind legs.
Which Cats Are at Greatest Risk
HCM can affect any cat of any age, but certain populations carry a substantially higher risk.
Breed Predispositions
- Maine Coon: A specific mutation in the MYBPC3 gene (A31P) has been identified and a DNA test is available. Prevalence in unscreened populations can exceed 30 percent.
- Ragdoll: A different MYBPC3 mutation (R820W) has been identified; again, genetic testing is available.
- British Shorthair, Persian, Sphynx, and Chartreux: All show elevated prevalence, though the genetic basis is less well characterised in most.
- Domestic shorthairs: Because they make up the bulk of the cat population, they also account for a large proportion of HCM cases in absolute numbers.
Age and Sex
Male cats are overrepresented in HCM diagnoses and tend to develop more severe disease at a younger age. Females may develop HCM later in life and can present differently. Middle-aged to older cats are at greatest risk overall, though young cats — including kittens — are occasionally diagnosed.
The Problem of Silent Disease
The most troubling feature of HCM is that many cats show no signs whatsoever until the disease is advanced. A cat may eat normally, play, and appear entirely well while significant cardiac changes develop silently. Some cats are discovered to have a heart murmur during a routine veterinary examination; others have no murmur at all despite significant structural disease. This means a normal clinical examination does not rule out HCM in a high-risk cat.
Symptoms to Recognise
When HCM does produce signs, these are the most common:
- Increased resting respiratory rate or laboured breathing — the most important and reliable sign of developing congestive heart failure. A resting rate consistently above 30 breaths per minute in a sleeping cat warrants urgent veterinary assessment.
- Open-mouth breathing, which in cats is always abnormal and often urgent.
- Sudden hind limb paralysis, cold painful back legs, and crying out — these are signs of aortic thromboembolism (ATE), a devastating complication.
- Lethargy and reduced appetite in the days preceding a decompensation event.
- Sudden death, without any prior signs in some cats.
Diagnosis
Echocardiography performed by a veterinary cardiologist is the definitive diagnostic tool. It measures the thickness of the heart walls directly and assesses chamber dimensions, valve function, and the presence of clots in the left atrium. A wall thickness exceeding 6mm in diastole is the most commonly cited threshold for diagnosis, though interpretation requires clinical context.
A biomarker blood test measuring NT-proBNP (a peptide released by stretched cardiac cells) can provide useful supporting information and is sometimes used as a screening tool between echocardiograms. Genetic testing is recommended for Maine Coons and Ragdolls, though a negative result does not guarantee freedom from HCM, as other causative mutations exist.
Chest X-rays help identify pulmonary oedema when congestive heart failure is present or suspected.
Management and What to Expect
There is currently no treatment proven to slow HCM progression in cats. Medical management focuses on controlling congestive heart failure when it occurs and reducing the risk of clot formation.
- Diuretics are the cornerstone of treating pulmonary oedema, removing excess fluid from the lungs.
- Antiplatelet therapy — typically aspirin at low dose or clopidogrel — reduces clot risk; studies suggest clopidogrel is more effective at preventing recurrent ATE.
- Medications to control heart rate or reduce outflow tract obstruction may be used in specific presentations.
Prognosis ranges from years of comfortable life in cats with mild, stable disease to months in those with severe HCM or a thromboembolic event. Regular echocardiographic monitoring — every six to twelve months for known HCM cats — allows treatment to be adjusted as the disease evolves.
For high-risk breeds, annual echocardiographic screening beginning at approximately two years of age is widely recommended by veterinary cardiologists. Discuss a screening plan with your vet, and monitor your cat's resting respiratory rate at home every week — it is the most practical early warning system available to owners.