CONDITION
Hypertrophic Cardiomyopathy in Cats
A condition where the heart muscle thickens abnormally, impairing blood flow and potentially causing sudden circulatory complications.
Why this matters now
Hypertrophic cardiomyopathy (HCM) is the most commonly diagnosed cardiac disease in cats, affecting around 15% (about 1 in 7) of the general feline population based on echocardiographic screening studies. The condition can present at any age, from young adulthood through to old age, though clinical presentation often occurs in middle age. Certain breeds carry a significantly higher prevalence, with Maine Coons, Ragdolls, British Shorthairs, Sphynx, Persian, and Bengal cats particularly represented. Specific genetic mutations have been identified in Maine Coons (MYBPC3 A31P) and Ragdolls (MYBPC3 R820W), and genetic testing is available for these breeds. Male cats appear to be affected more frequently and may show earlier onset of clinical disease compared to females.
The progression of HCM is highly variable and unpredictable. Some cats live their entire lives with mild, stable hypertrophy that never produces clinical signs, while others may present acutely with life-threatening complications as their first indication of heart disease. The condition may remain subclinical for years before progressing to a stage where the thickened heart muscle compromises diastolic filling, leading to elevated filling pressures and eventually congestive heart failure, typically manifesting as pleural effusion or pulmonary oedema. Aortic thromboembolism (ATE, commonly called a 'saddle thrombus') represents another devastating potential complication, where blood clots form within the enlarged left atrium and lodge in peripheral arteries, most commonly at the aortic trifurcation. The risk of sudden death, though difficult to quantify, exists even in cats without prior clinical signs. Once heart failure begins, a cat can move from slightly faster breathing at rest to laboured, open-mouth breathing within hours, and cats tend to hide breathing difficulty until little reserve remains. Decompensation can follow stress, general anaesthesia, intravenous fluids or corticosteroid treatment. Whether fluid has collected in or around the lungs cannot be judged by watching at home, and it takes examination and imaging to establish. Thromboembolism begins over minutes, with severe pain and sudden loss of use of one or both hind legs or, less often, one front leg. Among cats that are treated, around 35-45% survive to leave hospital, rising to as many as 70-80% when only one limb is affected, and the outlook is poorer when more than one limb is involved, motor function is absent or body temperature is low at presentation. This spectrum of possible outcomes, from lifelong subclinical disease to acute, life-threatening events, makes predicting any individual cat's trajectory challenging.
Signals & patterns
Early signals
Heart murmur detected incidentally
Many cats with HCM are initially identified when a heart murmur is detected during routine veterinary examination for vaccination, dental assessment, or pre-anaesthetic screening. The murmur may be soft and variable, sometimes present at one visit and absent at another, particularly in the early stages. Not all cats with HCM have audible murmurs, and conversely, not all murmurs in cats indicate HCM. Physiological (innocent) murmurs and murmurs from other causes also occur.
Subtle exercise intolerance
Mildly affected cats may show reduced willingness to engage in vigorous play or may tire more quickly during activity than expected. Given the naturally variable activity levels of cats and their tendency to self-regulate exercise, this change can be extremely subtle and easy to overlook. Owners may notice that a cat who previously engaged in energetic play sessions now prefers to observe rather than participate, or that bursts of activity are shorter and followed by longer rest periods.
Gallop rhythm on auscultation
A gallop rhythm, an additional heart sound (S3 or S4) creating a three-beat rhythm rather than the normal two-beat pattern, may be detected during cardiac auscultation. This finding reflects abnormal diastolic filling dynamics and can be a more specific indicator of myocardial disease than a murmur alone. The gallop sound may be intermittent and rate-dependent, sometimes only audible when the heart rate is within a certain range.
Sleeping breathing rate repeatedly above 30 per minute
Healthy cats, and cats whose heart failure is well controlled, usually take fewer than 30 breaths per minute during undisturbed sleep. A sleeping rate that is repeatedly above 30, or that climbs over a day or two, can be the first sign of fluid collecting in or around the lungs and can come before breathing difficulty is obvious. Once fluid has started to collect, breathing can become laboured within hours.
Open-mouth breathing at rest or after little exertion
Although it can be the first change an owner notices, open-mouth breathing or panting at rest, or after only brief play, is not a minor sign in cats. Unlike dogs, cats rarely breathe with the mouth open, and in a cat with heart disease it can mark fluid in or around the lungs. Episodes can settle within minutes, which makes them easy to dismiss, but settling does not show that fluid is absent, and that cannot be judged at home. Feline asthma, marked stress and overheating can look similar and are separated by examination and imaging.
Later signals
Respiratory distress from congestive heart failure
When HCM progresses to congestive heart failure, cats typically develop respiratory difficulty from pulmonary oedema (fluid in the lungs) or pleural effusion (fluid around the lungs). Signs include rapid, laboured breathing, open-mouth breathing, reluctance to lie down, and visible abdominal effort during respiration. The onset can be gradual or acute, and some cats present in severe respiratory distress as their first clinical sign of heart disease, with no prior history of cardiac concerns.
Acute limb paralysis from thromboembolism, usually the hind legs
Aortic thromboembolism (ATE) presents dramatically with sudden onset of hind limb paralysis or paresis, typically affecting both hind legs when the clot lodges at the aortic trifurcation. Less often a single leg, sometimes a front leg, is affected, which can look like a sudden lameness or injury. The affected limbs become cold, painful, and pale (the nail beds may appear blue or white rather than pink), and the femoral pulses are absent or severely diminished. This is an acutely painful event, and affected cats typically vocalise in distress and may become aggressive when handled due to the severity of the pain.
Lethargy and appetite loss
Progressive cardiac compromise can produce generalised lethargy, reduced appetite, and withdrawal from normal activities as cardiac output becomes insufficient to meet the body's metabolic demands. Cats may spend increasing amounts of time resting in quiet locations and show diminished interest in food or social interaction. Weight loss may accompany these changes, reflecting both reduced food intake and the metabolic demands of cardiac disease.
Syncope or collapse episodes
Fainting episodes (syncope) may occur due to arrhythmias or dynamic outflow tract obstruction, producing sudden, brief periods of collapse or loss of consciousness. These events are typically transient, with the cat recovering within seconds to minutes, but they indicate significant cardiac compromise. Syncope may occur during activity, excitement, or apparently at rest, and can be confused with seizure activity by owners who witness the events.
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How this is usually investigated
Investigation of suspected HCM typically centres on echocardiography as the primary diagnostic tool, supplemented by additional tests to assess the systemic impact and identify concurrent conditions. Importantly, the investigation also aims to exclude secondary causes of left ventricular hypertrophy, particularly systemic hypertension and hyperthyroidism, which can mimic the appearance of HCM.
Echocardiography
Cardiac biomarkers (NT-proBNP and troponin)
Thoracic radiography
Electrocardiography (ECG)
Blood pressure and thyroid assessment
Options & trade-offs
Management of HCM in cats depends heavily on the stage of disease and the presence or absence of clinical complications. For cats with subclinical HCM, the evidence base for prophylactic treatment remains limited and somewhat debated, while cats presenting with congestive heart failure or thromboembolism require more intensive intervention.
Monitoring subclinical disease
For cats with mild, subclinical HCM (modest hypertrophy without left atrial enlargement or clinical signs), periodic echocardiographic monitoring may be the primary approach. This allows tracking of disease progression and enables timely intervention if the condition advances. Monitoring sleeping respiratory rate at home has become an increasingly valued tool, as an upward trend can provide early warning of developing heart failure before clinical signs become overt.
Trade-offs: The uncertainty of not knowing whether or when the disease will progress can be emotionally challenging for owners. The monitoring interval varies between cardiologists. The 2020 ACVIM consensus statement suggests cats with subclinical disease and no more than mild left atrial enlargement are re-checked about once a year. Once the left atrium is moderately or severely enlarged, the consensus puts more weight on home monitoring of sleeping breathing rate than on a fixed re-scan interval, partly because repeat examinations are themselves stressful, and practice varies. The cost of regular echocardiography can accumulate over time. Some owners find the 'watch and wait' approach anxiety-provoking, particularly after learning about the potential complications of HCM.
Beta-blocker therapy (atenolol)
Beta-blockers, particularly atenolol, are commonly used in cats with HCM, especially those with dynamic outflow tract obstruction or tachyarrhythmias. These medications reduce heart rate, allowing more time for diastolic filling, decrease myocardial oxygen demand, and may help manage dynamic outflow tract obstruction by reducing contractile vigour. They are among the most frequently prescribed cardiac medications for feline HCM. In the UK, atenolol is a human medicine with no veterinary licence, so it is prescribed under the veterinary medicines cascade.
Trade-offs: The evidence that beta-blockers alter the natural history of HCM or prevent complications in subclinical cats remains limited, and their use in this population is debated among cardiologists. Side effects can include bradycardia, lethargy, and reduced appetite, though many cats tolerate atenolol well at appropriate doses. Beta-blockers are contraindicated in cats with overt congestive heart failure, where their negative inotropic effects can worsen cardiac output. Twice-daily dosing is typical, which requires consistent owner compliance.
Antithrombotic therapy
For cats at increased risk of thromboembolism, typically those with moderate to severe left atrial enlargement or spontaneous echo contrast visible on echocardiography, antithrombotic medication may be considered. Clopidogrel (an antiplatelet agent) is the most commonly used medication for this purpose, often chosen over aspirin because the FATCAT study found fewer recurrent thromboembolic events with clopidogrel. In some cases, dual antiplatelet therapy (clopidogrel plus aspirin) or anticoagulant therapy may be considered for very high-risk cats. In the UK, clopidogrel and aspirin are human medicines with no veterinary licence for cats, so they are prescribed under the veterinary medicines cascade.
Trade-offs: Clopidogrel requires daily oral administration and can cause gastrointestinal side effects including inappetence and vomiting in some cats. The medication reduces but does not eliminate thromboembolism risk, and some cats experience ATE despite appropriate antithrombotic therapy. Determining the threshold at which the benefit of antithrombotic therapy outweighs the risks and burdens of medication involves clinical judgement, as clear evidence-based guidelines for initiation are still evolving.
Congestive heart failure management
When HCM progresses to congestive heart failure, management typically involves diuretics (primarily furosemide) to reduce fluid accumulation, along with continued or modified cardiac medications. Furosemide reduces pulmonary oedema and pleural effusion by promoting renal fluid excretion, and the dose is titrated to the minimum effective level to maintain comfort while preserving renal function. ACE inhibitors (benazepril or enalapril) may be added to reduce neurohormonal activation and afterload, though in a placebo-controlled trial in cats with heart failure, benazepril did not delay treatment failure, so their role is uncertain. Furosemide is licensed for cats in the UK. Benazepril is licensed for cats only to reduce protein loss in kidney disease, and enalapril has no current UK veterinary authorisation, so the use of either in feline heart failure is under the veterinary medicines cascade.
Trade-offs: Diuretic therapy requires careful dose titration: too little provides inadequate fluid control, while too much can cause dehydration, electrolyte imbalances, and renal compromise. Cats in congestive heart failure typically require multiple daily medications, and the complexity of the treatment regimen can be challenging. Renal function is monitored closely, as diuretics can exacerbate pre-existing or developing kidney disease. The prognosis for cats with HCM-related heart failure is variable but generally guarded, with median survival times often cited in the range of several months to a year or more with appropriate management.
Thromboembolism management and recovery
Cats presenting with aortic thromboembolism require intensive management including pain control (typically opioid analgesics), supportive care, antithrombotic therapy to prevent clot extension and recurrence, and management of any concurrent heart failure. The first days after ATE carry particular risk, as affected cats may develop reperfusion injury, hyperkalaemia, or metabolic complications as blood flow is restored to ischaemic tissues. Physiotherapy and supportive care during recovery can help optimise limb function in surviving cats.
Trade-offs: The prognosis for ATE is guarded, with survival rates varying significantly depending on the number of limbs affected, the presence of concurrent heart failure, body temperature, and the severity of metabolic derangement. The recurrence rate for ATE is significant even with appropriate antithrombotic therapy. The initial presentation is extremely painful and distressing for both the cat and the owner, and quality of life considerations during and after treatment are important factors in decision-making. Some cats achieve meaningful recovery of limb function, while others retain permanent deficits.
Common misconceptions
"A cat without a heart murmur cannot have heart disease"
A significant proportion of cats with echocardiographically confirmed HCM do not have audible heart murmurs on clinical examination. In published studies, between about 20% and 70% of cats with HCM had no murmur, depending on the population studied, so the absence of a murmur provides limited reassurance. This is partly because the pattern of hypertrophy does not always create the turbulent blood flow patterns that produce audible murmurs. Some cats present with their first sign of heart disease being congestive heart failure or thromboembolism, without any prior detection of cardiac abnormality on routine examination.
"HCM always leads to heart failure and shortened lifespan"
The clinical trajectory of HCM varies enormously between individual cats. Many cats with mild HCM detected on screening live normal lifespans without developing clinical complications. The risk of progression to heart failure or thromboembolism is influenced by factors including the degree of hypertrophy, left atrial size, the presence of specific genetic mutations, and individual variation that remains poorly understood. While HCM is indeed a serious condition with potentially life-threatening complications, a diagnosis does not automatically equate to a poor prognosis, and many affected cats live comfortably for years with appropriate monitoring and, when indicated, management.
"Genetic testing can tell whether a cat will develop HCM"
Available genetic tests identify specific known mutations (such as the MYBPC3 mutations in Maine Coons and Ragdolls), but a negative result does not exclude HCM. Many cases of HCM, particularly in mixed-breed cats and breeds without identified mutations, have a genetic basis that current testing cannot detect. Conversely, cats that test positive for a known mutation carry increased risk but may not inevitably develop clinically significant disease, as the expression of the mutation is influenced by other genetic and environmental factors. Genetic testing is a valuable tool, particularly for breeding decisions, but it complements rather than replaces echocardiographic screening.
Related conditions
Hyperthyroidism in Cats
Hyperthyroidism is a common cause of secondary thickening of the left ventricular wall in older cats, which can look like HCM on echocardiography and may lessen once thyroid levels are controlled. Thyroid testing is part of investigating a thickened heart in older cats, and some cats have both conditions.
Chronic Kidney Disease in Cats
Chronic kidney disease and hypertrophic cardiomyopathy often co-occur in older cats, and the two conditions can influence one another through shared effects on blood pressure and fluid balance. Monitoring both organ systems together is a common clinical consideration.
Pericardial Effusion
In cats, congestive heart failure from cardiomyopathy is the most common cause of pericardial effusion. The fluid is usually a small volume that rarely compresses the heart, and other causes in cats include feline infectious peritonitis and lymphoma.
Feline Asthma
Cats with feline asthma may present with breathing difficulty that can, in some cases, resemble the respiratory signs seen when heart muscle thickening leads to circulatory congestion. Distinguishing between primary airway disease and cardiac causes of respiratory distress is a recognised diagnostic consideration.
Aortic Thromboembolism in Cats
HCM is the most common underlying cause of aortic thromboembolism in cats: clots form in an enlarged left atrium and most often lodge where the aorta divides, causing sudden severe pain and loss of use of the hind legs. Less often a clot blocks the artery to a front leg. Thromboembolism can be the first sign that a cat has heart disease.
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Systemic Hypertension in Cats
High blood pressure causes thickening of the left ventricle that can look identical to HCM on echocardiography. Blood pressure measurement is part of distinguishing the two, and the thickening may lessen once pressure is controlled.
Pleural Effusion
Fluid around the lungs is one of the two main forms congestive heart failure takes in cats with HCM, causing fast, laboured breathing. Its presence and cause are established by imaging and, often, by sampling the fluid.
Acromegaly in Cats
Excess growth hormone in cats with acromegaly can thicken the heart muscle and mimic HCM. Acromegaly is often found in cats with diabetes that is difficult to control.
Restrictive Cardiomyopathy in Cats
Restrictive cardiomyopathy is another feline heart muscle disease in which a stiff ventricle leads to an enlarged left atrium, with the same risks of congestive heart failure and thromboembolism. The two are told apart by echocardiography.
Understanding HCM involves recognising the tension between its high prevalence and the uncertainty inherent in predicting its clinical trajectory for any individual cat. Many cats with echocardiographically confirmed HCM live normal lifespans without complications, while others face life-threatening events that can occur without warning. This unpredictability can be emotionally challenging for owners, particularly when a heart murmur or screening test reveals subclinical disease. Ongoing monitoring with periodic echocardiography helps track the condition's evolution and allows management decisions to evolve alongside the disease. Between examinations, the sleeping breathing rate is a measure an owner can count at home. A count repeatedly above 30 breaths per minute, any open-mouth breathing, or sudden pain with weakness or coldness in a leg, most often a hind leg, marks a change that can progress over minutes to hours rather than days. Because anaesthesia, intravenous fluids and corticosteroids can tip a cat with HCM into heart failure, the diagnosis is relevant information for any future procedure or medication. Research into feline HCM continues actively, and understanding of both the genetic basis and management strategies is expected to develop further.
Last reviewed: 10 October 2026 · Dr Alastair Greenway MRCVS