Feline Hypertrophic Cardiomyopathy-3 (1)

Study Tips for Cardiac Diseases

When studying cardiac diseases, it is crucial to focus on the following areas for a comprehensive understanding:

  1. Signs/Symptoms: Pay attention to observable signs and symptoms that owners may notice at home, such as changes in activity level, appetite, and breathing patterns which could indicate cardiac distress.

  2. Physical Exam (PE) Findings: Learn to recognize key physical examination findings, such as heart murmurs, pulse quality, and respiratory distress indicators.

  3. Radiographic Changes: Understand the alterations seen on thoracic radiographs, including cardiomegaly, pulmonary edema, and pleural effusion which may indicate heart failure.

  4. Echocardiographic Changes: Familiarize yourself with the significant changes visible on echocardiograms, including wall thickness, chamber size, and heart function metrics that are essential for diagnosing cardiac conditions.

Introduction to Feline Hypertrophic Cardiomyopathy (HCM)

Presented by Sarah M. Cavanaugh, DVM, MS, DACVIM (Cardiology), RUSVM.

Learning Objectives

Key learning objectives for understanding HCM include:

  • Structural & Functional Abnormalities: Recognize the types of structural and functional abnormalities associated with HCM, like left ventricular hypertrophy and impaired diastolic filling.

  • Breed Predisposition: Identify the breeds that are most commonly predisposed to HCM, such as Maine Coons, Ragdolls, and American Shorthairs, and the importance of genetic factors in their development.

  • Physical Exam Findings: Recognize potential findings during physical exams for cats with HCM, which may include heart murmurs, abnormal lung sounds, and signs of congestive heart failure.

  • Echocardiographic Criteria: Review echocardiographic parameters critical for diagnosing HCM, including the measurement of inter-ventricular septum and left ventricle thickness.

  • Systolic Anterior Motion (SAM): Understand the definition of SAM and its significance in the echocardiographic evaluation of HCM.

  • HCM Staging and Treatment Implications: Explore the different stages of HCM, particularly stage B2, and discuss the role of medications like clopidogrel in management.

  • CHF Radiographic Criteria: Learn how to identify left-sided congestive heart failure on radiographs and the common drugs used in CHF management, understanding their mechanisms and potential side effects.

  • Prognosis Variations: Differentiate the prognosis between asymptomatic cats and those that are symptomatic due to CHF or aortic thromboembolism (ATE).

Hypertrophic Cardiomyopathy Overview

HCM involves significant physiological changes:

  • Increased Wall Thickness: Includes concentric hypertrophy predominantly affecting the left ventricle (LV), potentially impacting the right ventricle (RV) in more severe cases.

  • Prevalence: HCM is found in approximately 8-15% of the feline population and is responsible for around 60-70% of cases of feline cardiac disease.

  • Asymptomatic Cats: Many affected cats exhibit no clinical signs and may lack any detectable murmurs or abnormal PE findings, complicating early diagnosis.

Etiology and Pathology of HCM

HCM is known to be hereditary in certain breeds:

  • Genetic Predispositions: Confirmed hereditary factors are notable in breeds like Maine Coon and Ragdoll, with genetic testing available to aid in diagnosis.

  • Non-Genetic Causes: While genetic predispositions exist, ongoing research is warranted to fully understand other potential non-genetic factors contributing to HCM.

  • Characteristics: Pathological features include cardiomyocyte hypertrophy primarily attributed to sarcomeric dysfunction, histopathological changes including myofiber disarray, arteriosclerosis, and fibrosis.

Pathophysiology of HCM

The pathophysiological mechanisms of HCM include:

  • Diastolic Dysfunction: Characterized by impaired myocyte relaxation and increased ventricular wall stiffness, which leads to elevated filling pressures.

  • Consequent Pulmonary Issues: This elevated pressure can result in congestion within the atria and pulmonary venous system, potentially leading to pulmonary edema and CHF.

  • Arrhythmogenesis: Cats with HCM face risks of ventricular arrhythmias arising from myocardial ischemia and fibrosis complications.

Systolic Anterior Motion (SAM)

SAM results from anatomical changes:

  • Hypertrophied Musculature: The displacement caused by papillary muscle hypertrophy plays a critical role in alterations to the left ventricular outflow tract (LVOT), leading to clinical manifestations such as audible heart murmurs due to turbulence.

Hypertrophic Obstructive Cardiomyopathy (HOCM)

HOCM is noted for its specific features:

  • Echocardiographic Findings: This condition is characterized by significant obstructive hypertrophy observable on echocardiography, warranting a thorough assessment.

Left Atrial Dilation

Chronic increases in filling pressures can lead to:

  • Consequences: Chronic filling pressure increases can result in dilation of the left atrium, significantly raising the risk of thrombus formation, which is observed in 12-17% of affected cats.

Classification System of HCM Stages

The classification system includes:

  • Stage A: Predisposed individuals.

  • Stage B1: Subclinical cats at low risk of complication.

  • Stage B2: Subclinical cats with a higher risk due to detectable structural changes.

  • Stage C: Cats that exhibit current or refractory CHF.

  • Stage D: Cats with a history of CHF with significant clinical sequelae.

Signalment for HCM

  • Age and Gender Prevalence: The average age for an HCM diagnosis is approximately 6-7 years, but cats of any age may be affected. Male cats are generally diagnosed more frequently than females.

  • Common Breeds: Domestic Shorthair cats are most commonly identified, but HCM is also observed in Maine Coons and Persians.

Clinical History

  • Symptomatic Presentation: About 50% of cats diagnosed with HCM are asymptomatic. Symptomatic cats may present with dyspnea due to CHF, while non-specific signs such as lethargy or hiding can also be significant.

Physical Exam Findings

  • Auscultation Results: Even asymptomatic cats may exhibit normal findings upon auscultation. However, possible findings during PE include:

    • Systolic heart murmurs in 40-70% of cases.

    • Gallop sounds present in approximately 30% of instances.

    • Abnormal lung sounds attributing to CHF presence.

Definitive Diagnosis - Echocardiogram

An echocardiogram is essential for precise HCM diagnosis:

  • Diagnostic Techniques: Utilizes advanced techniques like 2D, M-mode, and Doppler echocardiography, with a wall thickness measurement threshold of ≥6mm indicating HCM irrespective of body weight.

Differential Diagnoses for LV Concentric Hypertrophy

Consideration of potential underlying causes is essential, including:

  • Hypertension and Hyperthyroidism: Other conditions like acromegaly and aortic stenosis may mimic findings observed in HCM. Specific infiltrative diseases such as lymphoma should be ruled out.

CHF Diagnosis - Stage C

CHF diagnosis is supported by findings that may include:

  • Signs and Symptoms: Evidence of pulmonary edema and left atrial enlargement is critical.

Other/Adjunct Diagnostics

The following diagnostics can be beneficial:

  • Biomarkers: Vital biomarkers, including natriuretic peptides and cardiac troponin I, assist in differentiating between cardiac and non-cardiac respiratory signs.

  • Electrocardiogram: An electrocardiogram may be useful to evaluate arrhythmias present in HCM patients.

Treatment - Asymptomatic - Stage B1

  • Beta-Blockers: While no consensus exists, beta-blockers may be utilized to potentially reduce left ventricular outflow tract obstruction (LVOTO).

Treatment - Asymptomatic - Stage B2

Management is advised for:

  • Moderate to Severe Left Atrial Enlargement: Interventions focused on addressing risk factors emerging from significant structural changes.

Monitoring – Stages B and C

  • Home Care Monitoring: Essential monitoring at home includes tracking the cat's respiratory rate and being alert for signs of distress. A respiratory rate exceeding 30 breaths per minute should prompt immediate evaluation by a veterinarian.

Treatment - CHF - Stage C

For acute CHF treatments:

  • Medication Protocols: Initial treatment may involve furosemide, with possible thoracocentesis, while outpatient management may include continuous oral furosemide and considerations for additional medications as necessary.

Treatment – ATE – Stage C

  • Management Approach: Strategies include implementing analgesia, antithrombotic therapies, and intensive monitoring of affected cats.

Prognosis

  • Variations Based on Symptoms: Asymptomatic cases have a fair prognosis, although symptomatic cases tend to exhibit poorer long-term outcomes, with risks such as sudden death and events of ATE drastically influencing survival rates.

Monitoring the Heart Failure Patient

Effective monitoring must address:

  • Signs of Progression and Side Effects: Both at home and in clinical settings, monitoring for signs of disease progression and potential side effects from medications is critical. Reevaluation schedules should align appropriately with any treatment alterations.