Restrictive Cardiomyopathy-1 (1)

Feline Restrictive Cardiomyopathy (RCM)

Understand structural and functional abnormalities associated with RCM.
Recognize potential physical exam findings in cats diagnosed with RCM.
Identify echocardiographic criteria essential for diagnosing RCM.
Familiarize with common medications used for the treatment of CHF (Congestive Heart Failure) and/or ATE (Arterial Thromboembolism) due to RCM, including their mechanisms of action and potential adverse effects.
Discuss prognosis for cats exhibiting symptoms related to RCM (CHF/ATE).

Defined as a primary myocardial disease that is characterized by diastolic dysfunction (with the potential of systolic dysfunction), all while maintaining a normal thickness of the ventricular walls.
The left ventricle (LV) is typically the most affected, but the right ventricle (RV) may also be involved.
RCM is recognized as the second most common cardiomyopathy found in cats, following Hypertrophic Cardiomyopathy (HCM).
Many cats diagnosed with RCM exhibit no auscultable abnormalities upon examination, complicating initial diagnosis.

The exact etiology of RCM remains largely unknown. There is speculation around a potential progression from HCM, suggesting RCM may represent an end-stage of this condition.
Although there has been a historical consideration of viral causation, most viral links have been ruled out following extensive research.
Anatomical observations in RCM may reveal notable fibrotic changes, both focal and diffuse, in the endocardium and/or myocardium upon post-mortem examination.
Commonly observed is the dilation of the atria, which can lead to further complications like thromboembolic events.

Diastolic abnormalities inherent in RCM lead to delayed myocardial relaxation, causing impaired filling of the ventricles.
The initial phase of diastole, referred to as isovolumetric relaxation, is significantly prolonged in cats with RCM.
This non-compliant ventricle results in restrictive filling, where the first phase of diastole is shortened while the second phase presents high velocity flow due to elevated atrial pressures, leading to clinical manifestations.

RCM typically presents in adult cats, with a mean diagnosis age of around 7 years.
There is no identifiable sex or breed predisposition for this condition.
Many cats can remain asymptomatic until they exhibit complications related to CHF or ATE.
Clinical signs may include:
- Dyspnea and tachypnea, often presenting as labored or rapid breathing.
- Signs characteristic of right-sided congestive heart failure, such as abdominal distension due to ascites.
- It is common for cats to show at least one clinical sign at the time of diagnosis.
- ATE may lead to paralysis, weakness, lameness, or acute pain in affected limbs.
- Sudden death may unexpectedly be the first observable clinical sign, emphasizing the need for regular veterinary check-ups in at-risk cats.

A significant number of cats with RCM show no audible abnormalities during auscultation; thus, clinical examination may yield subtle findings.
Possible cardiac exam findings include:
- The presence of gallop sounds or low-grade systolic murmurs, indicating abnormal heart dynamics.
- Heart rate may exhibit variability, presenting as normal, bradycardic (low), or tachycardic (high) regardless of the underlying condition.
- Potential for premature beats or arrhythmias that could be identified during examination.
Femoral pulse quality generally remains normal unless compromised by ATE, rhythm abnormalities, or cases of low-output heart failure.
Respiratory auscultation may reveal abnormal lung sounds, including increased breath sounds, inspiratory crackles, wheezes, or muffled sounds, indicative of CHF.
Hypothermia may occur in the presence of distal aortic thromboembolism or in cases of profound low-output heart failure.
If ATE is a concern, assessment of limb perfusion and neurological status is critical, as it has direct implications for treatment outcomes.

Echocardiography is a crucial tool in the diagnostic workup, necessary for assessing cardiac structure and function.
Diagnostics should evaluate transmitral flows and note the distinct differences between HCM and RCM, particularly with features suggestive of endomyocardial fibrosis, evident during echocardiographic studies.

Further diagnostic capabilities may include:
- Thoracic radiographs or Point-of-Care Ultrasound (POCUS) to assess for CHF signs such as pulmonary congestion.
- Comprehensive physical exams and possibly a TFAST ultrasound to evaluate for pleural effusions.
- Electrocardiography (ECG) to recognize arrhythmias that can range from supraventricular to ventricular in origin.
Advanced cases of RCM may show a combination of systolic and diastolic dysfunction, potentially complicating the clinical picture and contributing to progressive low output heart failure alongside signs of CHF.

There is a conspicuous lack of clinical trials investigating the long-term effects of therapy in asymptomatic cats with RCM, highlighting the need for more focused research.
Treatment strategies for asymptomatic cats with:
- Moderate to severe atrial enlargement and intact systolic function may include the adoption of antithrombotic therapy such as clopidogrel, possibly supplemented with ACE inhibitors or spironolactone.
- If signs of systolic dysfunction are detected, veterinarians may consider incorporating pimobendan into management plans.
For symptomatic patients presenting with CHF or ATE, it is vital to refer to comprehensive guidelines concerning HCM and heart failure management.
Notably, ATE incidence may be more prevalent in cats with RCM as compared to those with HCM, establishing a unique characteristic of RCM that requires careful monitoring.
In cases where arrhythmias are present, cross-reference treatment protocols with HCM and heart failure management guidelines to address these complications effectively.

Research indicates a reported median survival time (MST) for cats experiencing symptoms due to RCM of approximately 9.1 months, as noted in a study by Spalla I et al (2016).
For asymptomatic cats diagnosed with RCM, prognostic outcomes remain uncertain, necessitating continued observation and possible intervention.
A 2019 study (Chetboul V et al) suggested an MST of around 1.8 years for all cats included in the study, comprising both symptomatic and asymptomatic cases, highlighting a need for individualized approaches based on the clinical status of each cat.