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Basics ⬇

Basics

Definition

A rare, primary heart muscle disease characterized functionally by severe diastolic dysfunction with restrictive left ventricular filling and normal to near normal systolic function, morphologically by a non-dilated non-hypertrophied left ventricle with increased endocardial and/or myocardial fibrosis and severe atrial enlargement, and clinically by advanced heart failure, thromboembolic disease, and cardiac death.

Pathophysiology

  • Increased cardiomyofilament calcium sensitivity leading to severely impaired relaxation and high myocardial stiffness due to endomyocardial fibrosis (endomyocardial type) and/or interstitial fibrosis (myocardial type), and disorganized myofiber architecture (disarray; both types) are main characteristics of primary RCM. RCM-like myocardial changes and clinical syndromes can result from myocardial remodeling and dysfunction secondary to other causes (e.g., endomyocarditis, immune-mediated disease, or end-stage hypertrophic cardiomyopathy).
  • Diastolic heart failure and arterial thromboembolism lead to high mortality.

Systems Affected

  • Cardiovascular
  • Respiratory

Genetics

Primary RCM can be a spontaneous or familial disease but is generally considered of genetic cause in humans with an autosomal dominant pattern of inheritance. Several genes encoding -actin, -myosin heavy chain, cTnI, and cTnT can be affected. RCM-causing mutations have not been identified in cats.

Incidence/Prevalence

Primary feline RCM is rare. Prevalence ranging from present 1–15% of all myocardial diseases in cats has been reported.

Signalment

  • Cats
  • No breed predilection
  • Middle-aged to older cats
  • Male predisposition

Signs

Historical Findings

  • If cat does not have CHF:
    • Lethargy, weakness, weight loss
    • Syncope (usually indicates relevant arrhythmia)
    • Paresis or paralysis (i.e., signs of arterial thromboembolism)
  • If cat has CHF:
    • Labored breathing
    • Tachypnea
    • Ascites
    • Jugular venous distension
    • Cyanosis

Physical Examination Findings

  • If not in CHF:
    • Depression
    • Tachycardia
    • Arrhythmias
    • Prominent gallop sounds
    • Heart murmur uncommon
  • If in CHF: above signs plus the following:
    • Tachypnea
    • Labored breathing
    • Panting
    • Cyanosis
    • Hepatomegaly or ascites with jugular venous distention
    • Pulmonary crackles
    • Muffled cardiac or respiratory sounds if cat has pleural effusion
    • Paralysis or paresis with loss of femoral pulses; one or more extremities cold and painful (arterial thromboembolism).

Causes

  • Primary RCM: Currently unknown; genetic cause documented in humans.
  • Secondary RCM: Late or end stage of underlying disease (e.g., hypertrophic cardiomyopathy), link between prior interstitial pneumonia and feline endomyocarditis suspected in one study.

Diagnosis ⬆ ⬇

Diagnosis

Differential Diagnosis

  • Advanced stages of other feline cardiomyopathies:
    • Hypertrophic, dilated, arrhythmogenic right ventricular, unclassified, and tachycardia-induced cardiomyopathy
    • Myocardial infarct
    • CHF secondary to thyrotoxicosis or hypertensive heart disease.

CBC/Biochemistry/Urinalysis

Routine chemistry panel and urinalysis helpful to document concurrent or complicating conditions (e.g., prerenal azotemia and potassium depletion).

Other

Laboratory Tests

  • Plasma T4 concentration in cats 6 years old
  • Plasma cardiac troponin I concentration (more specific if ischemic heart disease or myocarditis suspected).

Imaging

Thoracic Radiography

  • Cardiomegaly with severe bi-atrial enlargement (“valentine” heart on v/d projections).
  • Interstitial or alveolar infiltrates or pleural effusion with pulmonary venous distention if in CHF.

Echocardiography

  • Note: Definitive diagnostic criteria are poorly defined and remain controversial. Early (non-congestive) RCM has rarely been documented in cats.
  • Anatomical findings:
    • Severe bi-atrial enlargement
    • Non-hypertrophied, non-dilated left ventricle (normal chamber dimension, normal wall thickness)
    • Severe enlargement of the left atrium with spontaneous echocardiographic contrast or thrombi frequently seen
    • Prominent, often diffuse echogenic scar (“moderator bands”, false tendons) leading to a small left ventricular lumen size and narrowing of the mid-ventricle (endomyocardial fibrosis)
    • Focal areas of highly echogenic and often thin myocardium indicative of ischemia or scarring
    • Myocardium can appear normal with pure myocardial form of RCM
    • Pleural effusion and mild to moderate pericardial effusion may be present.
  • Functional findings:
    • Severe left ventricular diastolic dysfunction (restrictive left ventricular filling with an E:A ratio >2.0, short isovolumic relaxation time (<37 msec), shortened deceleration time of E (<50 msec), and E:E'; >15)
    • Normal to low normal left ventricular systolic function (in some cases LV systolic dysfunction is present)
    • Regional wall motion abnormalities possible.
    • Severe left atrial appendage enlargement with evidence of blood stasis
    • Midventricular obstruction with flow turbulence in cats with bridging endomyocardial fibrosis
    • In cats with secondary RCM, characteristics of the underlying disease can predominate; however, severe atrial enlargement and restrictive LV filling will be present in nearly all cats.

Diagnostic Procedures

Electrocardiography

  • Note: ECG findings are neither sensitive nor specific.
  • Sinus tachycardia is common, but cats with severe CHF and hypothermia may be bradycardic.
  • Ventricular or supraventricular ectopic beats, paroxysmal or sustained supraventricular or ventricular tachycardia, or atrial fibrillation
  • Atrial or ventricular enlargement patterns
  • ST segment elevation or depression.

Pathology

  • Note: Histopathologic confirmation is needed in the definitive diagnosis of RCM.
  • Increased heart weight (>19 g)
  • Severe bi-atrial dilatation
  • Locally or diffusely thickened opaque endocardium
  • False tendons (“moderator bands”) present in some cats
  • Normal luminal size of the left and right ventricle (enlargement possible with secondary RCM or CHF)
  • Diffuse or focal cardiomyocyte disarray
  • Increased interstitial and replacement fibrosis
  • Abnormal intramural coronary arterioles with medial hypertrophy and narrowed lumen
  • Increased number of inflammatory cells seen only in cats with acute endomyocarditis-this finding is commonly absent in cats with endocardial fibrosis.

Treatment ⬆ ⬇

Treatment

Appropriate Health Care

  • Patients with acute, severe CHF are hospitalized for emergency care.
  • Mildly symptomatic animals can be treated with outpatient medical management.

Nursing Care

  • Dyspneic animals should receive oxygen.
  • Sedation and preload-reducing drugs are mandatory.
  • Thoracocentesis if relevant pleural effusion.
  • Low-sodium fluids are only administered if dehydration present and kidney function is compromised.
  • Maintain a low-stress environment (e.g., cage rest, minimize handling).
  • Heating pad for hypothermic patients.
  • Respiratory rate should be used to monitor the immediate success of treatment.

Activity

  • Cage rest is suggested for CHF patients.

Diet

  • In acute heart failure, maintain intake with hand feeding if necessary.

Client Education

  • The owner should be counseled regarding the technique of pill administration in cats, possible adverse effects of medications, the importance of maintaining stable food and water intake, and monitoring their cat's resting respiratory rate at home.

Medications ⬆ ⬇

Medications

Drug(s) Of Choice

Acute Congestive Heart Failure

  • Parenteral administration of furosemide (0.5–2 mg/kg IV, IM, SC q1–6h). CRI may be considered.
  • Dermal application of nitroglycerin ointment (2%, one eighth–one fourth inch q12h).
  • Oxygen delivered by cage, mask, nasal tube.
  • Thoracocentesis as necessary to reduce or eliminate pleural effusion.
  • Dobutamine only if cats are hypotensive (systolic blood pressure <90 mmHg); 1–5 µg/kg/minute as continuous rate infusion, start a lower dose and increase over 0.5 to 1 hour).
  • Severe supraventricular tachyarrhythmias can be treated with regular diltiazem (1.5–2.5 mg/kg PO q8h) or long-acting diltiazem (10 mg/kg PO q24h).
  • Ventricular tachycardia may resolve with resolution of CHF.
  • Acute therapy of ventricular tachycardia may include lidocaine (0.25–0.5 mg/kg IV SLOWLY); monitor closely for neurologic signs of toxicity.
  • Pimobendan (1.25 mg/cat PO q12h) may be helpful to increase cardiac performance in acute heart failure but is only used in animals that cannot be stabilized and systemic hypotension cannot be corrected. Note: Pimobendan is not approved for clinical use in cats and clinical safety and efficacy data are limited. Antiplatelet medication (Clopidogrel bisulfate, 18.75 mg PO q24h) or anticoagulants (e.g., unfractionated heparin, 150–250 IU/kg SC q6h) may be administered, in particular in cats with severe left atrial enlargement and spontaneous echocardiographic contrast.

Chronic Therapy

  • Furosemide is gradually decreased to lowest effective dose.
  • Angiotensin-converting enzyme inhibitors (ACEIs) may reduce fluid retention, decrease the need for diuretics, and counterbalance adverse effects of diuretics (e.g., enalapril 0.25–0.5 mg/kg PO q12–25h).
  • Diltiazem decreases heart rate and improves supraventricular arrhythmias in affected cats. The addition of digoxin (0.007 mg/kg PO q48h) may allow better control of ventricular response rate in cats with atrial fibrillation. Cats with hemodynamically important ventricular and supraventricular ectopy can also benefit from sotalol (0.5–2.0 mg/kg PO q12h).
  • Pimobendan (0.625–1.25 mg/cat PO q12h) may be helpful in the management of chronic heart failure. Note: Pimobendan is not approved for clinical use in cats.
  • Treat associated conditions (e.g., dehydration, hypothermia, hypokalemia).
  • Clopidogrel (one fourth of a 75-mg tablet PO q24h) to inhibit platelet chronically. Aspirin (25 mg/kg PO q72h) may also be considered but efficacy is questionable. In cases of echogenic smoke or prior thromboembolism, both drugs (clopidogrel and aspirin) may be used concurrently.
  • Treatment of cats with preclinical RCM has rarely been reported but includes ACEIs and antiplatelet drugs. There is currently no specific treatment for left ventricular diastolic dysfunction available.

Contraindications

  • For beta-blocking drugs-should not be administered in cats with RCM.
  • For diltiazem-bradycardia, atrioventricular block, myocardial failure, and hypotension.
  • For furosemide-severe dehydration, severe hypokalemia, and moderate to severe azotemia.
  • For ACEIs-moderate to severe azotemia, hypotension, and hyperkalemia.

Possible Interactions

  • Use of ACEIs in cats on high doses of furosemide may result in hypotension, azotemia, and hyperkalemia.
  • Chronic aspirin therapy may increase risk of renal side effects of ACEIs and may lead to inappetence and gastrointestinal upset.

Follow-Up ⬆ ⬇

Follow-Up

Patient Monitoring

  • Frequent physical re-examinations to assess response to treatment and resolution of CHF.
  • Frequent re-evaluation of hydration status and renal function is important in the first few days of therapy to avoid dehydration, hypokalemia, and azotemia.
  • Repeated thoracocentesis if necessary.
  • “Hands-off” hourly assessment of respiratory rate in first 12–24 hours can be used to monitor efficacy of CHF therapy.
  • Radiographs may be repeated in 12–24 hours to monitor pulmonary infiltrate resolution.
  • Repeat physical examination and analysis of blood biochemistries after 3 to 7 days of treatment of acute CHF.
  • ECG and radiographs repeated at clinician's discretion.
  • Stable patients are reevaluated every 2–4 months or more frequently if problems occur. Repeat echocardiograms are indicated every 6 to 9 months.

Prevention/Avoidance

  • No known preventative measures for RCM.

Possible Complications

Tissue necrosis or loss of function in limbs affected by thromboembolic complications, adverse effects of medications, sudden death, and euthanasia due to refractory heart failure.

Expected Course and Prognosis

  • Somehow variable, but most cats have a grave prognosis.

Miscellaneous ⬆

Miscellaneous

Associated Conditions

Aortic thromboembolism

Age-Related Factors

Hyperthyroidism should be ruled out with appropriate testing in feline cardiovascular patients 6 years of age.

Synonyms

  • Intermediate cardiomyopathy
  • Unclassified cardiomyopathy

Abbreviations

  • •A = peak velocity of late transmitral flow
  • ACEI = angiotensin converting enzyme inhibitor
  • CHF = congestive heart failure
  • cTnI = cardiac troponin I
  • cTnT = cardiac troponin T
  • E = peak velocity of early transmitral flow
  • E'; = peak velocity of mitral annular motion
  • RCM = restrictive cardiomyopathy

Author Karsten E. Schober

Consulting Editors Larry P. Tilley and Francis W.K. Smith, Jr.

Client Education Handout Available Online

Suggested Reading

Charles PY, Li YJ, Nan CL, Huang XP. Insights into restrictive cardiomyopathy from clinical and animal studies. J Geriatr Cardiol 2011, 8:168–183.

Fox PR. Endomyocardial fibrosis and restrictive cardiomyopathy: pathologic and clinical features. J Vet Cardiol 2004, 6:25–31.