Feline Hypertrophic Cardiomyopathy: ACVIM Staging, Diagnosis, and Evidence-Based Management

Bottom line.

• Hypertrophic cardiomyopathy (HCM) is the most common cardiac disease in adult cats, affecting approximately 1 in 7 cats (prevalence ~14-15%), and is the leading cause of cardiac morbidity and mortality in the species.
• The hallmark lesion is concentric left ventricular hypertrophy, most often asymmetric, with consequent diastolic dysfunction; systolic function is typically preserved or hyperdynamic until late-stage disease.
• Clinical staging by the ACVIM 2020 consensus (Stage A through D) guides management: most cats present in preclinical (B1/B2) disease and may remain there for years; ~15-25% will progress to congestive heart failure (CHF) and ~10% will develop arterial thromboembolism (ATE).
• Echocardiography remains the gold standard for diagnosis; NT-proBNP and cTnI are useful screening and monitoring adjuncts.
• Therapeutic targets in preclinical HCM are limited; clopidogrel is recommended for thromboprophylaxis in cats with moderate-to-severe left atrial enlargement, spontaneous echocardiographic contrast, or low left atrial appendage velocity.
• This is a clinician-facing evidence summary. It is not a treatment protocol; confirm drug dosing, staging criteria, and monitoring intervals against current formularies and cardiologist guidance.

Clinical facts

• Prevalence: HCM affects approximately 14-15% of the general cat population; prevalence rises steeply with age and in certain breeds (Maine Coon, Ragdoll, British Shorthair, Scottish Fold, Sphynx, and Persian — some with known sarcomeric gene mutations: MYBPC3 A31P in Maine Coon; MYBPC3 R820W in Ragdoll).
• Pathophysiology: Sarcomeric gene mutations (autosomal dominant, incomplete penetrance, variable expressivity) drive cardiomyocyte disarray, increased collagen deposition, and myocardial wall thickening. The result is impaired ventricular relaxation (diastolic dysfunction), elevated filling pressures, and — when left atrial pressure rises sufficiently — pulmonary edema or pleural effusion.
• Dynamic LVOTO: Left ventricular outflow tract obstruction (LVOTO) occurs in a subset of HCM cats; it is dynamic (worsened by tachycardia, dehydration, sympathetic tone) and is associated with systolic anterior motion (SAM) of the mitral valve leaflet.
• ATE risk: Atrial remodeling with left atrial enlargement, spontaneous echocardiographic contrast (SEC/"smoke"), and reduced left atrial appendage velocity are the principal echocardiographic predictors of ATE risk.
• Diagnosis: Echocardiography (maximal LV wall thickness ≥6 mm in diastole in a cat of appropriate body size, after excluding secondary causes) remains the diagnostic standard.
• Biomarkers: NT-proBNP (point-of-care SNAP or laboratory assay) and cTnI are useful for screening apparently healthy cats in primary care and for monitoring progression; they do not replace echocardiography.

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What the evidence shows

ACVIM 2020 consensus framework

The foundational reference for clinical practice is the 2020 ACVIM consensus statement by Luis Fuentes and colleagues, which established a phenotypic classification of feline cardiomyopathies aligned with the European Society of Cardiology scheme, and proposed a staging system (A through D) for clinical management decisions.¹ Stage A cats have breed risk but no structural disease; Stage B1 cats have echocardiographic HCM without hemodynamic compromise; Stage B2 cats have HCM with left atrial enlargement (LA:Ao ≥ 1.5 by echocardiography); Stage C represents cats with current or prior CHF; Stage D represents refractory CHF.

The consensus endorses clopidogrel for thromboprophylaxis in cats at increased ATE risk (moderate-to-severe LA enlargement, low LA fractional shortening, low LA appendage velocity, or SEC) at a level of evidence classed as medium — based primarily on the FAT CAT trial demonstrating clopidogrel superiority over aspirin for secondary ATE prevention.¹ Treatment of preclinical HCM without these risk features is generally not recommended, as no survival benefit from beta-blockers, ACE inhibitors, or diltiazem has been demonstrated in controlled studies.

Clinical-diagnostic advances

A 2025 comprehensive review by Beier in Veterinary Sciences confirmed that HCM is characterized by variable but often asymmetric concentric hypertrophy of the left ventricle, diastolic dysfunction, reduced intracavitary internal diameter, cardiomyocyte disarray, and excessive collagen deposition.² Genetic mutations in sarcomeric genes are well-evidenced in breed-associated HCM, and approximately 15-25% of cats progress to CHF and 10% develop ATE over a 10-year follow-up. Emerging therapeutic targets include cardiac myosin inhibitors (mavacamten, aficamten — active in cats but no approved veterinary product available) and rapamycin formulations in ongoing trials.

Practical echocardiographic criteria

LV wall thickening ≥6 mm (interventricular septum or LV free wall in diastole) in a cat of appropriate size defines HCM on echocardiography after excluding secondary causes. Secondary causes to exclude before diagnosing HCM: systemic hypertension (direct Doppler blood pressure), hyperthyroidism (total T4), and significant aortic stenosis. Regular follow-up echocardiography at 6-12 month intervals is appropriate for cats with identified HCM; the REVEAL Study (Fox et al., 2018) established that LA:Ao ≥ 1.5, LA enlargement, and elevated NT-proBNP values predict risk of cardiac events.

How this fits clinical practice

HCM is a disease most practitioners encounter regularly. Practical priorities are: (1) confirm the diagnosis echocardiographically and exclude secondary causes; (2) stage the disease using the ACVIM framework; (3) identify cats at ATE risk by LA size and function metrics; (4) initiate clopidogrel-based thromboprophylaxis for those at increased risk; and (5) establish a monitoring cadence (q 6-12 months for stable preclinical disease; more frequent as disease advances). Referral to a veterinary cardiologist is appropriate for initial staging, cases with symptomatic LVOTO, and cats with systolic dysfunction.

No drug has been shown to delay progression from preclinical to clinical HCM; set client expectations accordingly while emphasizing that many HCM cats will remain in preclinical stages for years.

Always confirm specific drug dosing, diagnostic thresholds, and monitoring intervals against current formularies and cardiologist guidance.

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References

1. Luis Fuentes V, Abbott J, Chetboul V, et al. 2020. ACVIM consensus statement guidelines for the classification, diagnosis, and management of cardiomyopathies in cats. J Vet Intern Med 34(3):1062-1077. https://onlinelibrary.wiley.com/doi/10.1111/jvim.15745
2. Beier SL. 2025. Clinical-Diagnostic and Therapeutic Advances in Feline Hypertrophic Cardiomyopathy. Vet Sci 12(3):289. https://www.mdpi.com/2306-7381/12/3/289

Changelog

• 2026-07-03: First published.