HCM is more common, but RCM overall is considered the second most common feline cardiomyopathy. The precise frequency of the endomyocardial subtype is unknown.
When to see a vet urgently
Sudden cardiac signs may result from heart failure, arrhythmia or a blood clot. Seek emergency care if:
- your cat breathes with its mouth open or clearly struggles to breathe at rest;
- it collapses, loses consciousness or becomes very weak;
- a limb suddenly becomes cold, painful or paralysed;
- mucous membranes are bluish or very pale and condition deteriorates.
Contact an emergency vet and avoid unnecessary handling. Do not wait for every sign. You can find a vet near you.
Overview
The endocardium is a thin lining covering heart chambers and valves. During prolonged inflammation or degeneration, connective tissue grows within it. The walls lose elasticity, valve function is disrupted and blood flow deteriorates. In cats, endocardial fibrosis can complicate chronic endocardial inflammation or result from systemic disease.
However, the origin of endomyocardial RCM is usually unclear. Identifying fibrosis alone does not establish previous infection.
In restrictive cardiomyopathy, the main problem is poor ventricular compliance and filling. Contraction strength may remain preserved. When the stiff left ventricle cannot fill normally, atrial pressure rises and the atrium may enlarge. Pressure also affects pulmonary circulation, potentially causing pulmonary oedema or fluid around the lungs. Good-looking contraction on ultrasound therefore does not alone exclude serious heart disease.
RCM does not always appear as uniform thickening of the entire heart wall. In the endomyocardial form, connective tissue may form thick bands within the chamber, changing the left ventricle's shape or reducing its space. In another form, fibrosis predominantly affects the heart muscle itself. Distinguishing them may need ultrasound by a vet experienced in cardiology.
Endocardial fibroelastosis is a separate, rare condition described particularly in young animals. It should not be used as a synonym for every feline endocardial fibrotic change. The more precise diagnosis in an examination report helps clarify the finding.
Causes
The main causes proposed for feline endocardial fibrosis are:
- previous inflammation of the endocardium and myocardium, or endomyocarditis; bacterial endocarditis is uncommon in cats;
- congenital or acquired heart defects;
- age-related degeneration.
These are possible associations rather than explanations for every cat. The cause of endomyocardial RCM often remains unknown and should not automatically be assumed to be previous bacterial infection or old age. A link between inflammation and later scarring has been proposed, but not every histological study has found evidence of inflammation or the viruses investigated. Such results also do not prove that inflammation can never contribute.
Risk factors
- Elderly cats.
- Cats with chronic infections.
- Untreated or poorly controlled heart disease.
Precise risk factors and the importance of inheritance are poorly understood. Disease has been described in pedigree and non-pedigree cats and in young individuals. Chronic infection or another cardiac disease alone does not establish this form of fibrosis. Investigating symptoms and cardiac findings matters more than belonging to a risk group.
Typical symptoms
Endocardial fibrosis often develops gradually, so early signs may be mild:
- lethargy and reduced activity;
- poor exercise tolerance;
- fast breathing and breathlessness;
- coughing when fluid accumulates in the lungs;
- pale or bluish mucous membranes;
- murmurs heard when listening to the heart;
- oedema and ascites in advanced disease;
- fainting and weakness.
A lack of symptoms does not always indicate minor changes. Reduced activity may be so subtle that you notice a problem only once breathing becomes difficult. There may be no obvious murmur either. An abnormal rhythm or additional heart sounds may nevertheless prompt investigation.
Coughing is not a typical feline heart failure sign. Airway disease also needs investigation, rather than automatically attributing a cough to diagnosed fibrosis. Fluid accumulation often causes faster breathing or increased effort.
Complications and consequences
Endocardial fibrosis can cause serious problems:
- chronic heart failure;
- arrhythmias;
- valve dysfunction;
- arterial thromboembolism;
- sudden death.
Atrial enlargement and slow flow can promote clot formation. A clot may detach and enter an artery, disrupting hindleg circulation, for example. Only one limb may be affected. An arrhythmia can cause weakness or fainting without every symptom resulting from lung fluid.
Not every cat develops every complication. Risk is assessed from atrial size, filling and function, any clot and clinical signs. Mention of fibrosis alone does not establish disease stage.
Diagnosis at the vet
Diagnosis uses:
- listening for murmurs and arrhythmias;
- ECG to assess rhythm and conduction;
- heart ultrasound, the main method for seeing endocardial thickening and valve dysfunction;
- chest X-rays to assess heart enlargement and lung fluid;
- blood tests, including cardiac markers.
Read more about heart muscle diseases in the article Cardiomyopathies in cats.
The role and limitations of ultrasound
Ultrasound assesses ventricular size, atrial enlargement, valves and blood flow as well as the endocardium. Doppler measurements examine filling and valve leakage. Obvious connective tissue bands within a chamber may support the endomyocardial diagnosis. However, not every change is equally clear, and different cardiomyopathies can partly resemble one another.
Ultrasound differs from microscopic tissue examination. Imaging provides a clinical assessment of structure and function; histology can confirm the precise tissue lesion. Routine treatment decisions often rely on ultrasound, symptoms and other investigations without taking a cardiac tissue sample.
ECG assesses rhythm but does not alone confirm fibrosis. X-rays particularly help assess fluid accumulation and other respiratory causes. Cardiac markers can support further investigation needs, but one blood value does not identify endomyocardial fibrosis specifically. Blood pressure and thyroid function are assessed when needed to identify other conditions increasing cardiac workload.
Treatment and follow-up
Managing symptoms and complications
No established medicine removes existing scar tissue. Treatment targets its functional consequences: fluid accumulation, arrhythmias and clot risk. Concurrent diseases are treated according to their own diagnoses. Fibrosis itself does not justify antibiotics without confirmed or well-founded suspected infection.
A breathless cat may need hospitalisation, oxygen and treatment to reduce excess fluid. Chest cavity fluid may require drainage. Once more stable, medication and check-ups are tailored to cardiac function, atrial size, kidney condition and other findings. One medication plan does not suit every cat.
Monitoring may include kidney function, blood electrolytes and repeat ultrasound or ECG. Tell the vet if medication cannot be given or appetite declines. Changing treatment yourself from a breathing measurement or temporary improvement can destabilise management.
Home observations and daily arrangements
Monitor breathing during quiet rest or sleep without purring. Record rate and compare it with your cat's previous baseline. The vet gives individual instructions for changes. Obvious effort or open-mouth breathing requires urgent contact whether or not you have counted the rate.
Also record eating, weight changes and activity. Heart failure can reduce appetite and muscle mass. Diet is planned for disease stage and other illnesses to ensure adequate nutrition. Treats and foods offered with medicines count too. Poor eating is not resolved simply by adding restrictions.
Assessing prognosis
Prognosis particularly depends on heart failure, clots and response. Published endomyocardial case series often focus on severely ill cats or those examined after death, so survival times cannot be directly applied to every new patient. Disease courses have varied in studies. Follow-up assesses comfortable breathing and eating and participation in ordinary daily life.
Prevention
- Treat infections that may damage the heart promptly.
- Monitor blood pressure in at-risk cats.
- Arrange regular veterinary examinations, particularly for older cats.
- Treat hormonal disorders, including hyperthyroidism and diabetes, promptly.
- Maintain a healthy weight and sensible nutrition.
These steps support general health and manage diagnosed underlying illnesses. They have not been proven to reliably prevent endomyocardial RCM of unknown cause. Good care also does not remove established fibrosis, even if symptoms improve.
Important for cat owners
Endocardial fibrosis is chronic, progressive and cannot be completely cured. However, timely diagnosis and treatment help slow progression and preserve quality of life. Remember that regular cardiac examinations are particularly important for elderly cats and those with chronic illnesses.
In practice, treatment benefit often means controlling heart failure and other complications. Medication cannot be promised to stop connective tissue formation itself. Review the plan if repeated emergency visits are needed, eating fails or breathing cannot remain comfortable.
This article provides general information and does not replace veterinary care. If you notice signs of illness in your cat, contact a vet.
Frequently asked questions
Is fibrosis the same as bacterial endocarditis?
No. Fibrosis means increased connective tissue, whereas infective endocarditis involves microbes damaging the lining or valves. Previous inflammation is one proposed contributor but cannot be established as the cause of every feline fibrotic heart lesion.
Can the heart contract normally despite serious disease?
Yes. Restrictive cardiomyopathy often primarily affects relaxation and filling. Atrial and pulmonary circulatory pressures may rise even when ventricular contraction appears preserved.
Does young age rule it out?
No. The endomyocardial form has also been described in young cats. Young animals additionally have other separate endocardial diseases. Diagnosis depends on findings rather than age alone.
This article is for general information only and is not a guide to treatment. If you notice the first signs of illness, contact your vet.
Paw4care editorial team
Sources
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- Pathological Features and Pathogenesis of the Endomyocardial Form of Restrictive Cardiomyopathy in Cats, Journal of Comparative Pathology / PubMed Central, accessed 2026-11-05.
- Endocardial Fibroelastosis in Dogs and Cats, MSD Veterinary Manual, accessed 2026-11-05.
- Heart Disease – Cats, Tufts University Cummings School of Veterinary Medicine, accessed 2026-11-05.
- Cardiomyopathy, Cornell Feline Health Centre, accessed 2026-11-05.
- Heart Disease Diagnosis, Tufts University Cummings School of Veterinary Medicine, accessed 2026-11-05.
- Arterial Thromboembolism in Dogs and Cats, Merck Veterinary Manual, accessed 2026-11-05.
- Coughing in Cats, VCA, accessed 2026-11-05.
- Home Breathing Rate Evaluation, VCA Animal Hospitals, accessed 2026-11-05.
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