# Hypertrophic Cardiomyopathy HCM in Cats: Screening, Clot Prevention and Care ## Key Takeaways - Hypertrophic Cardiomyopathy (HCM) is the most common feline heart disease, characterized by concentric thickening of the left ventricle, leading to diastolic dysfunction and increased left atrial pressure. - Diagnosis relies on echocardiography, with a left ventricular wall thickness ≥6 mm being a key criterion, after ruling out secondary causes like hyperthyroidism and hypertension. - Arterial thromboembolism (ATE), or saddle thrombus, is a critical complication arising from blood clots in the enlarged left atrium, causing acute, severe pain and hind limb paralysis. - Antithrombotic therapy, primarily with clopidogrel, is a cornerstone of management for cats with significant left atrial enlargement (Stage B2) to mitigate ATE risk. - Prognosis is highly variable, ranging from years of good quality of life in asymptomatic cats to a guarded outlook for those with congestive heart failure or ATE. - Genetic predisposition is strong in breeds like Maine Coons and Ragdolls, but HCM can occur in any cat, necessitating screening in at-risk individuals and breeds. --- If your [cat](/knowledge/veterinary-medicine/clinical-methods/cat) has been diagnosed with hypertrophic cardiomyopathy (HCM), or if you are considering screening because of breed risk, the central questions are straightforward: What does this mean for my cat's lifespan? How do I prevent a fatal blood clot? And what does daily care look like? This article provides a definitive, source-grounded overview of HCM in cats, focusing on screening strategies, clot prevention, and long-term management. **Owner Triage Summary:** HCM is the most common heart disease in cats, causing thickening of the heart muscle [1][2]. Many cats live for years without symptoms, but the disease can lead to congestive heart failure (CHF) or a sudden, painful blood clot called arterial thromboembolism (ATE), often called a saddle thrombus [3][4]. If your cat suddenly cannot use its back legs, is vocalizing in pain, or is breathing rapidly with an open mouth, this is an emergency. If your cat is asymptomatic but has a heart murmur or is a high-risk breed (Maine Coon, Ragdoll, Sphynx, Persian), schedule an echocardiogram with a veterinary cardiologist to establish a baseline [1][5][6]. Clot prevention with medications like clopidogrel is a cornerstone of care for cats with severe left atrial enlargement [7]. **At a Glance: HCM in Cats** | Feature | Description | Clinical Relevance | | :--- | :--- | :--- | | **Definition** | Primary myocardial disease causing concentric hypertrophy (thickening) of the left ventricle [8]. | Leads to diastolic dysfunction and increased filling pressures. | | **Prevalence** | Ranges from 8% to 34% in various study populations; a recent Moroccan study found 9.88% in healthy cats [6]. A referral population in Iran showed a 9.1% clinical prevalence [9]. | Highly common; many cats are asymptomatic. | | **Key Breeds** | Maine Coon, Ragdoll, Sphynx, Persian, British Shorthair [1][5][6][10]. | Genetic predisposition is strong; screening is advised. | | **Major Risks** | Congestive heart failure (CHF), arterial thromboembolism (ATE), sudden death [3][4][11]. | ATE is a devastating complication with high morbidity. | | **Diagnosis** | Echocardiography (ultrasound) is the gold standard [8][12]. ECG and biomarkers (NT-proBNP, cTnI) are supportive [13][14]. | Early detection allows for proactive management. | | **Clot Prevention** | Antithrombotic therapy (e.g., clopidogrel) is used in high-risk cats [7][4]. | Aims to reduce the risk of fatal ATE. | | **Prognosis** | Highly variable; depends on stage. Cats with CHF or ATE have a guarded prognosis [7][9]. | Subclinical cats can have a normal quality of life for years. | ## Understanding the Disease: Anatomy and Pathophysiology HCM is a primary disease of the heart muscle. The defining feature is an increase in the thickness of the left ventricular free wall and/or the interventricular septum [8]. This thickening is not caused by another condition like high blood pressure or an overactive thyroid; it is a disease of the heart muscle itself [8][15]. The thickening makes the left ventricle stiff and less compliant. This stiffness, known as diastolic dysfunction, means the heart cannot relax properly to fill with blood between beats. As a result, pressure inside the left atrium increases. Over time, this chronic pressure overload causes the left atrium to enlarge and remodel [8][4]. This is a critical step in disease progression. A severely enlarged left atrium is a primary risk factor for blood clot formation [3][4]. The pathophysiological complexity of HCM is still being unravelled. Recent proteomic studies have identified dysregulation in plasma proteins associated with innate and humoral immune responses, blood coagulation (fibrinogen, fibulin-1), lipid metabolism, and inflammation [2]. This suggests that HCM is not just a structural problem but involves systemic inflammatory and prothrombotic states [2][4]. Research is also exploring the role of the tissue renin-angiotensin system (RAS) within the myocardium, which may contribute to disease progression [16]. ## Causes and Differentials: Why Does My Cat Have HCM? HCM is a genetic disease. Breed-associated variants in the MYBPC3 gene have been reported in Maine Coon and Ragdoll cats [5]. A variant in the ALMS1 gene has been associated with HCM in Sphynx cats [1]. However, genetics are not the whole story. For example, a study in Sphynx cats in Thailand found the ALMS1 variant present in 44.68% of the population, but it did not show a significant association with the echocardiographic features of HCM [1]. This indicates that the genetic basis is complex and that other genes or environmental factors are involved. Before a diagnosis of primary HCM can be made, a veterinarian must rule out other causes of a thickened heart wall. These are known as "phenocopies" or secondary causes of hypertrophy [8][15]. The most common differentials are: - **Hyperthyroidism:** An overactive thyroid gland can cause a hypertrophic cardiomyopathy phenotype (HCMP). Interestingly, hyperthyroid cats can also develop other phenotypes, such as a restrictive cardiomyopathy phenotype (RCMP) [15]. - **Systemic Hypertension:** Chronic high blood pressure forces the heart to work harder, leading to thickening of the heart muscle [17][15]. The HCM phenotype is defined by concentric hypertrophy, but it is a heterogeneous disease. The thickening can be distributed differently, for example, confined to the left ventricular apex, a rare form called apical HCM [7]. It is important to understand that HCM is a clinical diagnosis based on echocardiography, not a single genetic test result. ## Risk Factors and Clinical Presentation HCM is most commonly diagnosed in middle-aged cats, with a median age around 4 to 6 years in some studies [6][9]. There is a clear male overrepresentation, with males making up 75% of affected cats in one study [6]. While certain breeds are predisposed, HCM can occur in any cat, including domestic shorthairs [3][9]. The clinical presentation is highly variable. Many cats with HCM are asymptomatic, and the disease is detected when a heart murmur is found during a routine physical exam [7][6]. In other cases, the first sign of disease is a catastrophic event. ### Subclinical HCM (Stages B1 and B2) These cats show no outward signs of heart disease. They may have a heart murmur or an arrhythmia. Echocardiography reveals left ventricular hypertrophy. The distinction between B1 and B2 is based on the severity of left atrial enlargement. Cats in stage B2 have more severe hypertrophy and are at higher risk of progression to congestive heart failure [6]. ### Symptomatic HCM (Stage C) Cats in this stage have developed congestive heart failure. The most common sign is respiratory distress due to pulmonary edema (fluid in the lungs) or pleural effusion (fluid in the chest cavity) [8][10]. Other signs include: - Lethargy and weakness - Reduced appetite (anorexia) - Rapid or labored breathing (tachypnea, dyspnea) - Open-mouth breathing - Collapse ### Arterial Thromboembolism (ATE) ATE, or "saddle thrombus," is a devastating complication. A blood clot forms in the heart, usually in the enlarged left atrium, and is ejected into the bloodstream. It most commonly lodges at the bifurcation of the aorta, blocking blood flow to the hind legs [4]. The onset is acute and extremely painful. Signs include: - Sudden paralysis or paresis of one or both hind legs - Vocalization and signs of severe pain - Cold hind limbs - Pale or blue nail beds - Absent femoral pulses A recent case report described a lingual artery thromboembolism causing tongue infarction in a cat with HCM, highlighting that clots can affect other areas of the body [3]. The pathophysiology of ATE is linked to Virchow's triad: endothelial injury, blood stasis, and hypercoagulability. These factors create a prothrombotic microenvironment even before spontaneous echo contrast (SEC) is visible on an ultrasound [4]. ## Veterinary Examination and Diagnostics: From Screening to Diagnosis The diagnostic pathway for HCM involves several steps, from a basic physical exam to advanced imaging. ### Physical Examination and History A thorough physical exam is essential. The presence of a systolic heart murmur, a gallop sound, or an arrhythmia can raise suspicion of heart disease [3][15]. However, the absence of a murmur does not rule out HCM. A detailed history can reveal subtle changes in activity or appetite. ### Blood Tests and Biomarkers Routine blood tests are used to rule out other diseases, particularly hyperthyroidism and kidney disease. There is no pathognomostic biomarker for HCM, but cardiac biomarkers are gaining importance [8][13]. - **NT-proBNP (N-terminal pro-B-type natriuretic peptide):** This is a marker of myocardial wall stress. It is frequently elevated in cats with HCM and is associated with clinical status and echocardiographic findings [13]. - **Cardiac Troponin I (cTnI):** This is a marker of myocardial damage. It is also commonly elevated in cats with HCM [13]. These biomarkers are useful for screening and risk stratification, but they cannot replace echocardiography for a definitive diagnosis [13]. ### Electrocardiography (ECG) An ECG measures the electrical activity of the heart. It can detect arrhythmias and changes in heart muscle mass. A 6-lead ECG can be used to identify subclinical HCM, with a higher R wave amplitude and the presence of ST segment elevation being independently associated with the disease [14]. While useful, ECG is less sensitive and specific than echocardiography. ### Echocardiography: The Gold Standard Echocardiography (ultrasound of the heart) is the gold standard for diagnosing HCM [8][12]. It allows the veterinarian to: - Measure the thickness of the left ventricular free wall and interventricular septum. A diagnosis of HCM is typically made when the wall thickness is ≥6 mm [9][18]. - Assess left atrial size and volume [11]. - Evaluate for the presence of spontaneous echo contrast (SEC) or thrombus formation [3]. - Assess diastolic function. - Measure left ventricular outflow tract obstruction. The imaging view and method used for measurements can affect the results. A study comparing different echocardiographic methods found clinically relevant bias between 2D and M-mode measurements, and between long-axis and short-axis views, particularly in cats with HCM [12]. This highlights the importance of having a skilled and consistent operator for serial evaluations. Advanced echocardiographic techniques, such as two-dimensional speckle-tracking echocardiography (STE), can detect subtle myocardial dysfunction before it is visible on standard measurements. Global longitudinal strain (GLS) and global radial strain (GRS) are significantly lower in HCM cats compared to healthy controls, indicating early systolic dysfunction [19]. Left atrial stiffness, a novel parameter derived from STE, is also higher in cats with HCM [18]. The left atrioventricular coupling index (LACI) is another novel volumetric ratio that combines LA remodeling and LV chamber size, and it increases with disease severity [11]. ## Evidence-Based Management and Care The management of HCM depends on the stage of the disease. There is no cure, so the goals of therapy are to: - Control clinical signs (if present). - Slow disease progression. - Prevent complications, especially ATE. - Improve quality of life. ### Management of Subclinical HCM (Stages B1 and B2) For asymptomatic cats, the primary focus is on monitoring and clot prevention. - **Monitoring:** Regular rechecks with echocardiography are essential to track disease progression. The frequency depends on the severity of the disease. - **Clot Prevention:** For cats with significant left atrial enlargement (Stage B2), antithrombotic therapy is recommended. Clopidogrel is a common antiplatelet medication used for this purpose [7]. The decision to start therapy is based on the risk of thromboembolism, which is strongly linked to left atrial size [4][11]. - **Pimobendan:** This medication is a positive inotrope and vasodilator. It is often used in cats with stage B2 HCM, particularly those with congestive heart failure, to help manage the disease [7]. ### Management of Congestive Heart Failure (Stage C) If a cat develops CHF, it is a medical emergency requiring hospitalization. - **Diuretics:** Furosemide is the mainstay of therapy to remove fluid from the lungs [10]. - **Oxygen Therapy:** Cats in respiratory distress require supplemental oxygen. - **Other Medications:** Pimobendan is commonly used to improve heart function [7]. Other drugs, such as ACE inhibitors, may be considered, though their benefit in feline HCM is less well-established. ### Clot Prevention: A Deeper Dive Preventing blood clots is one of the most critical aspects of HCM care. The risk of ATE is not uniform; it is highest in cats with severe left atrial enlargement and spontaneous echo contrast [3][4]. The presence of SEC is a sign of blood stasis and a late finding in the prothrombotic state [4]. Antiplatelet therapy, such as clopidogrel, is the standard of care for most cats at risk. It works by preventing platelets from clumping together to form a clot. The decision to use a more potent anticoagulant, such as rivaroxaban, is typically made by a veterinary cardiologist based on the individual cat's risk profile. The goal is to prevent the initial clot formation that can lead to a fatal saddle thrombus. ### Emerging Therapies and Research Research is ongoing to find better treatments for HCM. One area of interest is the mTOR pathway, which is involved in cardiac remodeling. In silico studies have identified potential feline-specific mTOR inhibitors, such as dihydro-alpha-ergocryptine, that may be more effective than existing drugs [20]. However, these are still in the discovery phase and are not yet available for clinical use. ## Unsafe Home Remedies and What to Avoid When a cat is diagnosed with HCM, owners may be tempted to try supplements or home remedies. It is crucial to understand that these are not substitutes for veterinary care and can sometimes be harmful. - **Do not give human medications:** Never give your cat human heart or blood pressure medications without veterinary supervision. Doses are different and some are toxic. - **Avoid over-the-counter supplements:** Many supplements have not been proven to help and can interact with prescribed medications. The tissue RAS system is complex, and unregulated supplements could have unintended effects [16]. - **Do not restrict exercise unless advised:** For asymptomatic cats, normal activity is generally fine. However, avoid stressful situations that can cause a surge in adrenaline. - **Do not ignore respiratory signs:** If your cat is breathing faster than 30 breaths per minute at rest, contact your veterinarian immediately. Do not wait to see if it improves. ## Prevention and Prognosis ### Prevention Since HCM is a genetic disease, it cannot be "prevented" in an individual cat. However, responsible breeding practices are key to reducing the prevalence of the disease in high-risk breeds. Genetic testing for known MYBPC3 variants in Maine Coons and Ragdolls can identify carriers [5]. However, a negative genetic test does not guarantee that a cat will not develop HCM, as there are likely many other mutations that have not yet been identified [1][5]. Screening breeding cats with echocardiography is a more comprehensive approach. ### Prognosis The prognosis for a cat with HCM is highly variable. - **Asymptomatic Cats:** Cats with mild to moderate HCM can live for many years with a good quality of life. The progression of the disease is slow in many cases. - **Stage B2 Cats:** These cats are at a higher risk of developing CHF or ATE. The median survival time is shorter than for stage B1 cats. - **Cats with CHF:** The prognosis is guarded. The median survival time after a diagnosis of CHF is often measured in months, but some cats respond well to treatment and live for a year or more. - **Cats with ATE:** The prognosis is poor. ATE is a very painful and often fatal condition. Even with aggressive treatment, many cats do not survive, and recurrence is common [9]. Euthanasia is often elected due to the severity of the condition and the poor long-term outlook [3][7]. ## Limitations and When to Contact a Veterinarian This article provides a comprehensive overview, but it cannot replace individualized veterinary care. The information presented here is educational and is not a substitute for veterinary diagnosis or treatment. **Important Limitations:** - **Breed-level genetic information cannot predict individual outcomes.** As seen in the Sphynx and Maine Coon studies, genetic variants do not always correlate with the severity of the disease [1][5]. - **Echocardiographic measurements are operator-dependent.** The results can vary between different veterinarians and different machines, so it is best to have serial evaluations performed by the same specialist [12]. - **Every cat is different.** The response to medication and the rate of disease progression are unpredictable. **Contact your veterinarian immediately if you notice any of the following:** - **Rapid or labored breathing** (more than 30 breaths per minute at rest). - **Open-mouth breathing.** - **Sudden weakness or collapse.** - **Inability to use one or both hind legs.** - **Vocalizing or signs of severe pain.** - **Loss of appetite or lethargy.** Early intervention can be life-saving, especially in the case of congestive heart failure. ## Frequently Asked Questions ### How is hypertrophic cardiomyopathy (HCM) diagnosed in cats? HCM is definitively diagnosed using echocardiography (ultrasound of the heart), which measures the thickness of the left ventricular wall [8][12]. A wall thickness of 6 mm or greater, after ruling out other causes like hyperthyroidism and hypertension, confirms the diagnosis [9][18]. ### What are the first signs of HCM in a cat? Many cats with HCM show no signs at all. When signs do appear, they are often related to congestive heart failure (rapid breathing, lethargy) or a blood clot (sudden paralysis of the hind legs) [3][8]. ### Can a cat live a normal life with HCM? Yes, many cats with subclinical HCM (no symptoms) can live a good quality of life for years. The key is regular monitoring and appropriate medication to manage the disease and prevent complications [7][6]. ### What is a saddle thrombus in cats? A saddle thrombus, or arterial thromboembolism (ATE), is a blood clot that travels from the heart and lodges at the base of the aorta, blocking blood flow to the hind legs. It causes sudden, severe pain and paralysis [4][11]. ### How can I prevent blood clots in my cat with HCM? Clot prevention is managed with antithrombotic medications, most commonly clopidogrel. Your veterinarian will recommend this if your cat has significant left atrial enlargement, which is a major risk factor for clot formation [7][4]. ### Is HCM painful for cats? The disease itself is not painful in its early stages. However, complications are painful. Congestive heart failure causes respiratory distress, and a saddle thrombus causes severe pain due to lack of blood flow to the limbs [3]. ### Are certain cat breeds more likely to get HCM? Yes, certain breeds have a genetic predisposition, including Maine Coon, Ragdoll, Sphynx, and Persian cats [1][5][6]. However, HCM can occur in any breed, including domestic shorthairs [3]. ### What should I do if my cat is diagnosed with HCM? First, work with your veterinarian or a veterinary cardiologist to establish a monitoring and treatment plan. This will include regular echocardiograms and possibly medication. At home, monitor your cat's resting respiratory rate and contact your vet immediately if you see any signs of distress [8][7]. ## Sources [1] [Prevalence and Clinical Relevance of Alström Syndrome Protein 1 Gene Variant and Feline Hypertrophic Cardiomyopathy in Sphynx Cats in Thailand.](https://pubmed.ncbi.nlm.nih.gov/42353426/) [2] [Understanding Pathophysiological Complexity of Feline Hypertrophic Cardiomyopathy Using SWATH-MS Plasma Proteomics.](https://pubmed.ncbi.nlm.nih.gov/41828989/) [3] [Tongue infarction in suspected lingual artery thromboembolism in a cat with hypertrophic cardiomyopathy.](https://pubmed.ncbi.nlm.nih.gov/41978704/) [4] [Early activation of Virchow's triad in feline hypertrophic cardiomyopathy: beyond spontaneous echo contrast and insights into translational thromboembolism.](https://pubmed.ncbi.nlm.nih.gov/42465720/) [5] [Development and validation of PCR-RFLP assays for detection of MYBPC3 variants associated with hypertrophic cardiomyopathy in Maine Coon and Ragdoll cats.](https://pubmed.ncbi.nlm.nih.gov/42501791/) [6] [First Description of Hypertrophic Cardiomyopathy Phenotype in Apparently Healthy Cats in Morocco: An Echocardiographic Prevalence Study.](https://pubmed.ncbi.nlm.nih.gov/42353413/) [7] [Echocardiographic diagnosis of hypertrophic cardiomyopathy confined to the left ventricular apex in a cat.](https://pubmed.ncbi.nlm.nih.gov/41394092/) [8] [Linking clinical and imaging diagnostic assessments of the feline hypertrophic cardiomyopathy phenotype.](https://pubmed.ncbi.nlm.nih.gov/41487485/) [9] [Hypertrophic cardiomyopathy in cats: clinical prevalence, risk factors, and outcomes of arterial thromboembolism in a referral population in Tehran, Iran (2020-2024).](https://pubmed.ncbi.nlm.nih.gov/41888795/) [10] [Presumed acute myocardial damage in a British Shorthair cat with previously diagnosed hypertrophic cardiomyopathy.](https://pubmed.ncbi.nlm.nih.gov/42174688/) [11] [Left Atrioventricular Coupling Index in Feline Hypertrophic Cardiomyopathy: Association with Disease Severity and Arterial Thromboembolism.](https://pubmed.ncbi.nlm.nih.gov/42188963/) [12] [Effect of echocardiographic imaging view and methods on left ventricular wall-thickness measurements in normal cats and cats with hypertrophic cardiomyopathy.](https://pubmed.ncbi.nlm.nih.gov/41964609/) [13] [Cardiovascular biomarkers in feline hypertrophic cardiomyopathy phenotype: evidence from the last decade.](https://pubmed.ncbi.nlm.nih.gov/42467289/) [14] [Accuracy of 6-lead electrocardiography in identifying subclinical hypertrophic cardiomyopathy in cats.](https://pubmed.ncbi.nlm.nih.gov/41742508/) [15] [Hypertrophic cardiomyopathy is not the sole echocardiographic phenotype associated with hyperthyroidism in cats: a retrospective study in 147 cats (2005-2025).](https://pubmed.ncbi.nlm.nih.gov/41742580/) [16] [Characterization of the myocardial and renal renin-angiotensin system in normal cats and cats with hypertrophic cardiomyopathy.](https://pubmed.ncbi.nlm.nih.gov/42105302/) [17] [Echocardiographic evaluation of aortic root size in cats with hypertrophic cardiomyopathy.](https://pubmed.ncbi.nlm.nih.gov/42036317/) [18] [Left atrial stiffness as a novel echocardiographic parameter of atrial dysfunction in cats with hypertrophic cardiomyopathy: A two-dimensional speckle-tracking study.](https://pubmed.ncbi.nlm.nih.gov/41716159/) [19] [Comparative Study of Speckle Tracking Echocardiography in Normal and Hypertrophic Cardiomyopathy Cats.](https://pubmed.ncbi.nlm.nih.gov/41893695/) [20] [In Silico Discovery of mTOR Inhibitors as Potential Therapeutics for Feline Hypertrophic Cardiomyopathy.](https://pubmed.ncbi.nlm.nih.gov/42371166/) ## Related Clinical & Scientific Guides * [Pancreatitis in Dogs: High-Fat Danger, Fluid Therapy and Bland Diets](/knowledge/veterinary-medicine/internal-medicine/pancreatitis-in-dogs-high-fat-danger-fluid-therapy-and-bland-diets) * [Dilated Cardiomyopathy DCM in Dogs: Diet-Related Taurine Gaps and Recovery](/knowledge/veterinary-medicine/internal-medicine/dilated-cardiomyopathy-dcm-in-dogs-diet-related-taurine-gaps-and-recovery) * [Addisons Disease in Dogs: Hypoadrenocorticism Crisis Electrolyte Balance](/knowledge/veterinary-medicine/internal-medicine/addisons-disease-in-dogs-hypoadrenocorticism-crisis-electrolyte-balance)