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 an estimated 10-15% 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-aged cats between 5 and 7 years. 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. 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. It is important to note that 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.

Mildly elevated resting respiratory rate

An increase in the resting respiratory rate (counted during undisturbed sleep or relaxed rest) above the normal range of approximately 15-30 breaths per minute may be an early indicator of developing cardiac compromise. This subtle change reflects the beginning of elevated left-sided filling pressures before overt pulmonary oedema or pleural effusion develops. Monitoring sleeping respiratory rate at home has emerged as a valuable tool for early detection of disease progression.

Intermittent open-mouth breathing

Brief episodes of open-mouth breathing or panting that occur after minimal exertion or seemingly without provocation may represent early cardiovascular compromise. While open-mouth breathing in cats can have various causes, its occurrence in a cat with known or suspected heart disease warrants attention. These episodes may be transient and self-resolving in the early stages, making them easy to dismiss as situational.

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 hind limb paralysis from thromboembolism

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. 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.

Click to read about the biological mechanisms

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

Purpose: Echocardiography (cardiac ultrasound) is the definitive diagnostic tool for HCM, providing direct visualisation of left ventricular wall thickness, chamber dimensions, valvular function, and cardiac motion patterns. A diagnosis of HCM is typically made when the left ventricular wall or interventricular septum measures 6mm or greater in diastole in the absence of other causes of hypertrophy. Echocardiography also assesses left atrial size — a critical prognostic indicator — and can identify systolic anterior motion of the mitral valve, dynamic outflow tract obstruction, and the presence of spontaneous echo contrast ('smoke') or thrombi within the left atrium.
Considerations: Echocardiographic assessment requires specialised equipment and expertise, ideally performed by a veterinary cardiologist or a practitioner with advanced cardiac imaging training. Measurements can be affected by heart rate, hydration status, and the cat's level of stress during the examination. Some cats fall into an equivocal zone with borderline wall thickness measurements (5-6mm), where serial monitoring over time may be needed to determine whether the findings represent early disease or a normal variant.

Cardiac biomarkers (NT-proBNP and troponin)

Purpose: Blood tests measuring cardiac biomarkers can provide supportive diagnostic information and help assess disease severity. NT-proBNP (N-terminal pro-B-type natriuretic peptide) is released by the heart in response to myocardial stretch and elevated filling pressures, and elevated levels support the presence of significant cardiac disease. Cardiac troponin I (cTnI) indicates myocardial cell damage and may be elevated in cats with active myocardial injury, fibrosis, or ischaemia.
Considerations: Cardiac biomarkers are useful as screening tools and for monitoring but have limitations in sensitivity and specificity. Elevated NT-proBNP values support cardiac disease but can also be influenced by renal function, stress, and other non-cardiac conditions. Normal biomarker values do not definitively exclude HCM, particularly in early or mild disease. These tests are most valuable when interpreted alongside echocardiographic findings and clinical presentation.

Thoracic radiography

Purpose: Chest X-rays assess heart size and shape and can identify evidence of congestive heart failure including pulmonary oedema, pleural effusion, and pulmonary venous congestion. Radiography provides information about the overall cardiac silhouette and the presence of complications that may require specific treatment. In cats presenting with respiratory distress, radiography helps determine whether the signs are cardiac or respiratory in origin.
Considerations: Radiographic assessment of heart size in cats is less reliable than in dogs due to the normally elongated feline cardiac silhouette and breed-related variation. Significant myocardial hypertrophy can be present without obvious radiographic enlargement, making radiography relatively insensitive for detecting subclinical HCM. The primary value of radiography is in assessing for congestive heart failure complications rather than diagnosing the underlying myocardial disease.

Electrocardiography (ECG)

Purpose: ECG records the electrical activity of the heart and can identify arrhythmias, conduction disturbances, and patterns suggestive of ventricular enlargement. Left ventricular hypertrophy may produce characteristic voltage changes, though ECG is relatively insensitive for detecting HCM. The primary value of ECG is in identifying arrhythmias that may accompany HCM, including ventricular premature complexes, supraventricular tachycardia, or atrial fibrillation.
Considerations: A normal ECG does not exclude HCM, and many cats with echocardiographically confirmed hypertrophy have normal resting electrocardiograms. Conversely, stress and catecholamine release during veterinary visits can produce arrhythmias in normal cats. Ambulatory ECG monitoring (Holter monitoring) can provide more comprehensive rhythm assessment but requires specialised equipment and may be impractical in some cats.

Blood pressure and thyroid assessment

Purpose: Measuring blood pressure and thyroid hormone levels (T4) is essential to exclude secondary causes of left ventricular hypertrophy. Systemic hypertension can cause concentric left ventricular hypertrophy that is echocardiographically indistinguishable from primary HCM, and hyperthyroidism produces cardiac changes through the metabolic effects of excess thyroid hormone. Identifying and treating these secondary causes is critical, as the hypertrophy may be reversible once the underlying condition is managed.
Considerations: Both blood pressure and thyroid testing should ideally be performed before concluding that left ventricular hypertrophy represents primary HCM, particularly in cats over 8-10 years of age where both hypertension and hyperthyroidism become increasingly prevalent. Some cats may have concurrent HCM and secondary causes of hypertrophy, complicating interpretation. Repeat testing may be needed if initial results are equivocal.

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 optimal monitoring interval is debated and varies between cardiologists, typically ranging from 6 to 18 months for stable subclinical cases. 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 significant hypertrophy, 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.

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 preferred over aspirin based on the FATCAT study which showed superior efficacy. In some cases, dual antiplatelet therapy (clopidogrel plus aspirin) or anticoagulant therapy may be considered for very high-risk cats.

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 their role in feline heart failure is less well established than in dogs.

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 monitoring becomes essential, 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 initial period following ATE is critical, 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

Misconception:

"A cat without a heart murmur cannot have heart disease"

Reality:

A significant proportion of cats with echocardiographically confirmed HCM do not have audible heart murmurs on clinical examination. Studies have shown that up to 50-60% of cats with HCM may lack a detectable murmur, meaning that 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.

Misconception:

"HCM always leads to heart failure and shortened lifespan"

Reality:

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.

Misconception:

"Genetic testing can definitively determine whether a cat will develop HCM"

Reality:

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.

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. Research into feline HCM continues actively, and understanding of both the genetic basis and optimal management strategies is expected to develop further.

Last reviewed: 24 April 2026 · Dr Alastair Greenway MRCVS