Hypertrophic Cardiomyopathy (HCM) in Cats is the most commonly diagnosed form of heart disease in cats. It is characterized by abnormal thickening, or hypertrophy, of the muscular walls of the left ventricle, the main pumping chamber of the heart.
One of the most important aspects of feline HCM is that the disease can remain silent for a long period of time. Many affected cats appear completely healthy at home, maintain normal activity and appetite, and show no obvious signs of cardiovascular disease. In some cats, the first indication of a problem may be a heart murmur or abnormal heart rhythm detected during a routine veterinary examination. However, the absence of a murmur does not exclude heart disease, and some cats with clinically important HCM may have a completely unremarkable cardiac auscultation.
For this reason, appropriate cardiac assessment and, when indicated, echocardiography play an important role in identifying and evaluating feline cardiomyopathy.
What Happens to the Heart in HCM?
In HCM, the walls of the left ventricle become abnormally thick. This thickening can affect different areas of the ventricular muscle and may vary considerably between individual cats.
Although the heart may continue to contract strongly, the thickened ventricular muscle can become less compliant and more difficult to relax during diastole, the phase when the heart fills with blood. This impaired relaxation can increase filling pressures within the heart.
Over time, increased pressure may result in enlargement of the left atrium. Left atrial enlargement is particularly important because it can indicate more advanced disease and may be associated with an increased risk of complications such as congestive heart failure and arterial thromboembolism.
Some Hypertrophic Cardiomyopathy (HCM) in Cats also develop dynamic left ventricular outflow tract obstruction (LVOTO), often associated with systolic anterior motion of the mitral valve. This can produce turbulent blood flow and may be responsible for a heart murmur detected during physical examination.
The appearance and severity of HCM can therefore vary significantly from one cat to another. This is one reason why a heart murmur alone cannot determine the severity of the disease.
Which Cats Can Develop HCM?
HCM can occur in cats of different ages and breeds. Certain breeds, including Maine Coons and Ragdolls, have recognized inherited forms of the disease, while HCM is also frequently identified in other breeds and mixed-breed cats.
Genetic testing is available for specific mutations in some breeds. However, genetic testing does not replace echocardiography. A negative genetic test does not exclude all forms of HCM, and a positive result does not necessarily indicate the current severity of cardiac disease.
It is also important to recognize that increased ventricular wall thickness does not always represent primary HCM. Other conditions, particularly systemic hypertension and hyperthyroidism, may produce or contribute to cardiac hypertrophy. Therefore, blood pressure measurement and appropriate laboratory investigations may be recommended as part of a complete assessment, particularly in middle-aged and older cats.
Why Can HCM Be Difficult to Detect?
Cats are particularly good at hiding signs of illness. During the early stages of HCM, most cats behave normally and may have no respiratory or exercise-related signs.
Some cats have a heart murmur, gallop sound or arrhythmia, but others have none of these abnormalities.
This means that a normal physical examination cannot completely rule out cardiomyopathy.
When clinical suspicion exists, or when a cat has an abnormal cardiac examination, belongs to a predisposed breed, or requires further cardiovascular assessment before certain procedures, additional diagnostic testing may be appropriate.
Why Is Echocardiography Important?
Echocardiography is the principal imaging technique used to diagnose and characterize feline HCM.
Unlike auscultation, which primarily tells us what the heart sounds like, echocardiography allows us to directly evaluate its structure and function.
During an echocardiographic examination, several important features can be assessed, including:
- Left ventricular wall thickness and distribution of hypertrophy
- Left atrial size
- Systolic and diastolic cardiac function
- Mitral valve motion
- Presence and severity of dynamic outflow tract obstruction
- Intracardiac blood-flow patterns
- Evidence of spontaneous echo contrast or intracardiac thrombus
- Other structural abnormalities that may mimic or accompany HCM
Importantly, the purpose of echocardiography is not simply to determine whether HCM is present. It helps us understand how advanced the disease is and what potential complications may exist.
This information is essential when deciding whether a cat requires medication, additional investigations, or simply periodic monitoring.
Additional Diagnostic Tests
Echocardiography provides valuable structural and functional information, but other tests may also contribute to a comprehensive cardiac evaluation.
Blood pressure measurement is particularly important because systemic hypertension can cause cardiac hypertrophy and may require specific treatment.
NT-proBNP is a cardiac biomarker that can provide useful information regarding myocardial stress. It can be particularly helpful as a screening or supportive test, although it does not replace a complete echocardiographic examination when definitive characterization of the heart is required.
Cardiac troponin may provide additional information regarding myocardial injury in selected cases.
An electrocardiogram (ECG) may be recommended when an abnormal rhythm is suspected. For intermittent arrhythmias, longer-term ECG monitoring such as a Holter examination may provide more information than a short in-clinic ECG.
Thoracic radiographs are particularly valuable when congestive heart failure is suspected, as they can help evaluate the lungs and identify pulmonary edema or pleural effusion.
Blood testing, including assessment of thyroid function, may also be appropriate, especially in older cats.
What Are the Possible Complications?
The clinical course of Hypertrophic Cardiomyopathy (HCM) in Cats varies considerably. Some cats with mild disease remain stable for many years and may never develop clinical signs. Others may experience progressive cardiac changes.
One of the most important complications is congestive heart failure (CHF). When pressure within the left atrium and pulmonary circulation becomes sufficiently elevated, fluid can accumulate within the lungs or chest cavity. Affected cats may develop an increased resting respiratory rate, rapid or difficult breathing, reduced activity or lethargy.
Another serious complication is arterial thromboembolism (ATE). Abnormal blood flow within an enlarged left atrium can contribute to blood clot formation. A clot may subsequently enter the systemic circulation and obstruct an artery, most commonly affecting blood flow to the hind limbs. This can result in sudden pain, weakness or paralysis and requires immediate veterinary attention.
Cardiac arrhythmias and, less commonly, sudden cardiac death may also occur in affected cats.
Does Every Cat With HCM Need Medication?
No.
One of the most important principles in managing feline HCM is that treatment should be based on the individual cat’s disease phenotype, severity and risk of complications rather than simply the presence of ventricular hypertrophy.
A cat with mild HCM and no significant left atrial enlargement may require monitoring rather than immediate medication.
In contrast, cats with significant left atrial enlargement, congestive heart failure, thromboembolic risk, clinically relevant arrhythmias or other important abnormalities may require specific therapy.
This is why accurate diagnosis and staging are essential before deciding on treatment.
When Should a Cat Have a Cardiac Evaluation?
A comprehensive cardiac assessment should be considered when a cat has a heart murmur, gallop rhythm, abnormal heart rhythm, unexplained rapid breathing, breathing difficulty, weakness, collapse or suspected cardiac enlargement.
Assessment may also be appropriate in cats from breeds with known predisposition to HCM, particularly when they are being considered for breeding.
Any cat that develops persistent rapid or difficult breathing should receive prompt veterinary attention. Similarly, sudden weakness, pain or paralysis of one or both hind limbs may represent arterial thromboembolism and should be considered an emergency.
The Importance of Early Detection
The goal of cardiac screening is not simply to find disease earlier. The real value lies in identifying which cats are at greater risk of disease progression or complications and determining the appropriate follow-up for each individual patient.
A cat diagnosed with mild HCM may only require periodic reassessment, while another cat may benefit from closer monitoring or medical therapy.
Because feline HCM can remain clinically silent, relying exclusively on visible symptoms may delay diagnosis until complications occur.
Through a combination of physical examination, echocardiography, blood pressure measurement, ECG, cardiac biomarkers, thoracic imaging and appropriate laboratory testing, veterinarians can build a more complete picture of cardiovascular health.
Early recognition, accurate characterization and individualized follow-up allow us to make informed decisions and provide the most appropriate care for each cat throughout the course of the disease.
