Hypertrophic Cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is a primary myocardial disease characterized by concentric hypertrophy of the left ventricular wall and interventricular septum in the absence of identifiable hemodynamic causes such as systemic hypertension, aortic stenosis, or hyperthyroidism. The hypertrophy is typically asymmetric, with the interventricular septum often more affected than the free wall, and may involve the papillary muscles. The disease is most commonly diagnosed in cats, where it is the most prevalent cardiac disorder, but it is rare in dogs. The condition leads to diastolic dysfunction, increased left ventricular filling pressures, and potentially dynamic left ventricular outflow tract obstruction (LVOTO) due to systolic anterior motion (SAM) of the mitral valve. Clinical manifestations range from asymptomatic disease to congestive heart failure, arterial thromboembolism (ATE), and sudden cardiac death. The diagnosis is based on echocardiographic detection of left ventricular wall thickness ≥6 mm in cats, with exclusion of secondary causes of hypertrophy. Management focuses on controlling clinical signs, preventing thromboembolism, and managing comorbidities, as no curative therapy exists.

The etiology of HCM is primarily genetic, with autosomal dominant inheritance patterns identified in some cat breeds. In Maine Coon cats, a mutation in the MYBPC3 gene (A31P) has been associated with HCM, while in Ragdoll cats, a different MYBPC3 mutation (R820W) is implicated. These mutations lead to abnormal myosin-binding protein C, affecting sarcomere function and causing myocyte hypertrophy and disarray. In other breeds and in many cats, the genetic basis remains unknown, suggesting polygenic or sporadic mutations. Secondary causes of myocardial hypertrophy must be excluded, including systemic hypertension, hyperthyroidism, aortic stenosis, and acromegaly. In dogs, HCM is extremely rare and often mimics other diseases such as subaortic stenosis or systemic hypertension. No infectious, toxic, or nutritional etiologies have been identified for primary HCM. The molecular pathophysiology involves dysregulation of calcium handling, increased oxidative stress, and altered signaling pathways (e.g., MAPK, TGF-β) that promote myocyte hypertrophy and interstitial fibrosis.

HCM is the most common heart disease in cats, with a prevalence estimated between 15% and 34% in apparently healthy cats, depending on the population studied. It affects both sexes, but males may be overrepresented in some studies. The disease is typically diagnosed in middle-aged to older cats (median age 6-7 years), but can occur in kittens as young as 4 months. Breed predispositions include Maine Coon, Ragdoll, Persian, British Shorthair, Sphynx, and American Shorthair. In Maine Coons, the A31P mutation has a prevalence of about 30-40% in some populations, and affected cats may develop HCM at a younger age. In dogs, HCM is exceedingly rare, with isolated case reports in breeds such as the English Bulldog and Boxer, but it is not a well-recognized clinical entity. Geographic distribution is worldwide, with no seasonal variation. The disease is often subclinical, and many affected cats are diagnosed incidentally during screening or pre-anesthetic evaluation.

The primary defect in HCM is diastolic dysfunction due to increased left ventricular wall thickness, myocardial fibrosis, and myocyte disarray. The hypertrophied myocardium has reduced compliance, leading to impaired relaxation and increased left ventricular end-diastolic pressure. This results in elevated left atrial pressure, which can progress to left atrial enlargement and pulmonary venous congestion, culminating in congestive heart failure (pulmonary edema and pleural effusion). Dynamic left ventricular outflow tract obstruction (LVOTO) occurs in about 30-50% of cats with HCM due to systolic anterior motion (SAM) of the mitral valve, where the anterior mitral valve leaflet is drawn into the outflow tract during systole, causing obstruction and mitral regurgitation. This exacerbates left atrial pressure and may contribute to clinical signs. Myocardial ischemia can occur due to increased oxygen demand and reduced capillary density, leading to myocyte death and fibrosis. Arterial thromboembolism (ATE) is a devastating complication, arising from blood stasis in the enlarged left atrium, particularly the left atrial appendage, leading to thrombus formation. Emboli typically lodge at the aortic trifurcation, causing hindlimb paresis or paralysis. The exact trigger for thrombus formation is multifactorial, involving endothelial dysfunction, abnormal blood flow, and hypercoagulability. In advanced stages, systolic dysfunction may develop, leading to end-stage HCM with left ventricular dilation and reduced ejection fraction.

Genetic predisposition is the most significant risk factor, with specific mutations identified in Maine Coon and Ragdoll cats. Breed-related risk is well-documented, with purebred cats having a higher prevalence than mixed-breed cats. Age is a risk factor, as the disease typically manifests in middle age, but can occur in young cats. Male sex may be a risk factor in some studies, but this is not consistent. Concurrent diseases such as systemic hypertension and hyperthyroidism can exacerbate or mimic HCM, and their presence must be ruled out. Obesity and physical inactivity may worsen clinical signs but are not direct causes. Stress and anesthesia can precipitate congestive heart failure or ATE in cats with subclinical HCM. In dogs, no specific predisposing factors are known due to the rarity of the disease.

Clinical signs of HCM vary widely depending on the stage of disease. Many cats are asymptomatic (subclinical) and are diagnosed incidentally. When clinical signs occur, they are often related to congestive heart failure or ATE. Acute signs of congestive heart failure include tachypnea, dyspnea, open-mouth breathing, and lethargy. Cats may present with a history of reduced appetite and hiding behavior. On physical examination, a systolic heart murmur is heard in about 50-70% of affected cats, often due to LVOTO or mitral regurgitation. A gallop rhythm (S3 or S4) may be auscultated, indicating diastolic dysfunction. In cats with ATE, acute hindlimb paresis or paralysis is the hallmark, with cold, painful extremities, absent femoral pulses, and cyanotic nail beds. Other signs include vocalization, pain, and distress. In chronic heart failure, pleural effusion may cause muffled heart and lung sounds. Sudden cardiac death can occur without prior signs. In dogs, clinical signs are similar but may include exercise intolerance and syncope.

Differential diagnoses for HCM include: (1) Systemic hypertension: causes concentric left ventricular hypertrophy; blood pressure measurement is essential to rule out. (2) Hyperthyroidism: causes myocardial hypertrophy due to increased thyroid hormone; serum T4 levels should be measured. (3) Aortic stenosis (subaortic or valvular): causes left ventricular hypertrophy; echocardiography with Doppler can identify outflow tract obstruction and valve abnormalities. (4) Acromegaly (hypersomatotropism): causes myocardial hypertrophy due to growth hormone excess; insulin-like growth factor-1 (IGF-1) levels may be elevated. (5) Restrictive cardiomyopathy (RCM): characterized by severe atrial enlargement and normal or mildly increased wall thickness; echocardiography shows restrictive filling patterns. (6) Unclassified cardiomyopathy: includes cases that do not fit classic phenotypes. (7) Myocardial fibrosis or infiltrative diseases (e.g., amyloidosis) can cause increased wall thickness. (8) In dogs, subaortic stenosis and systemic hypertension are more common causes of left ventricular hypertrophy. Definitive diagnosis of HCM requires echocardiographic evidence of increased wall thickness (≥6 mm in cats) with no identifiable secondary cause.

The diagnostic approach to HCM begins with a thorough history and physical examination. If a heart murmur, gallop rhythm, or clinical signs suggestive of heart disease are present, further evaluation is warranted. The first step is to rule out secondary causes of hypertrophy: measure systemic blood pressure (using Doppler or oscillometric methods) to exclude hypertension, and measure serum total T4 to exclude hyperthyroidism in cats over 6 years of age. If these are normal, echocardiography is the gold standard for diagnosis. Echocardiography should include two-dimensional (2D) and M-mode measurements of left ventricular wall thickness (interventricular septum and left ventricular free wall) in diastole, left atrial size, and assessment of systolic function. A wall thickness ≥6 mm in diastole is diagnostic for HCM in cats. Additional echocardiographic findings include left atrial enlargement, SAM of the mitral valve, and Doppler evidence of LVOTO (peak outflow velocity >2.5 m/s). If echocardiography is not available, thoracic radiography and NT-proBNP testing can support a diagnosis of heart disease but are not definitive. In cases where HCM is suspected but echocardiography is equivocal, cardiac biomarkers such as NT-proBNP and cardiac troponin I may be helpful. Genetic testing for MYBPC3 mutations is available for Maine Coon and Ragdoll cats and can identify at-risk individuals, but a negative test does not exclude HCM.

Routine laboratory findings in HCM are often unremarkable. Complete blood count (CBC) may show stress leukogram in cats with acute heart failure. Serum biochemistry may reveal elevated liver enzymes (ALT, ALP) due to passive congestion, and renal parameters (BUN, creatinine) may be elevated in cases of reduced cardiac output or concurrent kidney disease. Electrolyte imbalances, particularly hypokalemia, may occur with diuretic therapy. Urinalysis may show proteinuria or casts if concurrent hypertension or kidney disease is present. Blood gas analysis may indicate respiratory alkalosis in dyspneic cats. Cardiac biomarkers: NT-proBNP is elevated in cats with HCM and heart failure, with a cutoff of >100 pmol/L suggesting heart disease; levels >270 pmol/L are associated with heart failure. Cardiac troponin I (cTnI) is elevated in HCM, indicating myocardial damage. In cats with ATE, serum potassium may be elevated due to muscle ischemia. Thyroid hormone (T4) should be measured to rule out hyperthyroidism. Genetic testing for MYBPC3 mutations can be performed in specific breeds.

Thoracic radiography is useful for assessing cardiomegaly and pulmonary edema. In cats with HCM, the cardiac silhouette may appear normal in mild cases, but in advanced disease, there is left atrial enlargement (seen as a bulge in the cranial left cardiac border on the dorsoventral view) and generalized cardiomegaly. Pulmonary edema may be present, often with a patchy, perihilar distribution, and pleural effusion may be seen. Echocardiography is the primary imaging modality for diagnosis. 2D and M-mode echocardiography reveal concentric hypertrophy of the left ventricular free wall and interventricular septum (≥6 mm in diastole). The hypertrophy may be asymmetric, with the septum thicker than the free wall. Left atrial enlargement is common, and the left atrial to aortic root ratio (LA/Ao) is often >1.5. Doppler echocardiography can detect LVOTO (peak systolic outflow velocity >2.5 m/s) and mitral regurgitation. Tissue Doppler imaging can assess diastolic dysfunction. In dogs, echocardiography is also used, but HCM is rare. Advanced imaging such as cardiac MRI is rarely needed but can quantify fibrosis. CT is not routinely used for HCM diagnosis.

Cytology and histopathology are not typically used for antemortem diagnosis of HCM, as echocardiography is sufficient. However, if a myocardial biopsy is obtained (rarely), histopathology would show myocyte hypertrophy, myofiber disarray, and interstitial fibrosis. Special stains such as Masson's trichrome can highlight fibrosis. In post-mortem examination, the heart shows concentric left ventricular hypertrophy, often with a small left ventricular cavity. Microscopic findings include myocyte hypertrophy with enlarged nuclei, myofiber disarray, and variable degrees of fibrosis. In cats with ATE, thrombi may be found in the left atrium. Genetic testing on tissue samples can identify mutations.

Treatment of HCM is aimed at managing clinical signs, preventing complications, and improving quality of life. For asymptomatic cats with mild to moderate hypertrophy and no left atrial enlargement, no specific therapy is required, but regular monitoring is recommended. In cats with severe hypertrophy, left atrial enlargement, or prior episodes of heart failure, treatment is indicated. For cats with congestive heart failure, initial stabilization includes oxygen therapy, furosemide (1-2 mg/kg IV or IM, repeated as needed, then 1-2 mg/kg PO q8-12h), and, if pleural effusion is present, thoracocentesis. Pimobendan (0.25-0.3 mg/kg PO q12h) is often used in cats with heart failure, although its role in HCM is debated; it may be beneficial in cats with systolic dysfunction or those with concurrent DCM. Beta-blockers such as atenolol (6.25-12.5 mg/cat PO q12h) are used to reduce heart rate and improve diastolic filling, particularly in cats with LVOTO or tachycardia. Diltiazem (1.75-2.5 mg/kg PO q8h) is an alternative to beta-blockers, but atenolol is preferred for LVOTO. For cats with ATE, aggressive treatment includes pain management (e.g., buprenorphine 0.01-0.02 mg/kg IV or IM q6-8h), anticoagulation with clopidogrel (18.75 mg/cat PO q24h) or low-molecular-weight heparin (e.g., enoxaparin 1 mg/kg SC q6-12h), and supportive care. In acute ATE, thrombolytic therapy (e.g., tissue plasminogen activator) is rarely used due to risks. Long-term anticoagulation with clopidogrel is recommended for cats with left atrial enlargement or prior ATE. Aspirin (5-10 mg/cat PO q72h) is less effective but may be used if clopidogrel is not tolerated. In dogs, treatment is similar but less evidence-based. Dietary modifications include sodium restriction in heart failure. Exercise restriction is advised for cats with LVOTO or heart failure.

The prognosis for HCM is highly variable. Cats with mild to moderate hypertrophy and no clinical signs may live for years with a good quality of life. The median survival time for cats with HCM and congestive heart failure is approximately 1-2 years, but some cats survive longer with appropriate therapy. Cats that experience ATE have a guarded prognosis, with a median survival time of 6-12 months, and many are euthanized due to severe pain and poor recovery. Negative prognostic indicators include left atrial enlargement (LA/Ao >2), severe hypertrophy (wall thickness >8 mm), presence of LVOTO, and elevated NT-proBNP levels. Sudden cardiac death can occur at any stage. In dogs, the prognosis is poor due to the rarity and often advanced presentation.

Follow-up for cats with HCM should be tailored to the severity of disease. Asymptomatic cats with mild hypertrophy should be re-examined every 6-12 months, including echocardiography to monitor progression. Cats with moderate to severe hypertrophy or left atrial enlargement should be re-evaluated every 3-6 months. Cats with heart failure require more frequent monitoring, initially weekly until stabilized, then every 1-3 months. Monitoring should include physical examination, body weight, blood pressure, and serum biochemistry (especially renal parameters and electrolytes) if on diuretics. Echocardiography should be repeated to assess changes in wall thickness, left atrial size, and systolic function. NT-proBNP can be measured serially to assess response to therapy. Owners should be educated to monitor resting respiratory rate at home; an increase >30 breaths per minute may indicate impending heart failure. Dose adjustments of furosemide, pimobendan, and beta-blockers should be based on clinical status and blood pressure.

Pearls: (1) A heart murmur is not always present in HCM; absence of a murmur does not rule out the disease. (2) In cats with acute dyspnea, perform thoracic radiography and echocardiography after stabilization, but do not delay oxygen and furosemide. (3) Always measure blood pressure and T4 in any cat with suspected HCM to rule out secondary causes. (4) In cats with ATE, treat pain aggressively and consider anticoagulation early. (5) Clopidogrel is preferred over aspirin for ATE prevention. Pitfalls: (1) Do not use pimobendan in cats with LVOTO, as it may worsen obstruction. (2) Avoid overuse of furosemide, which can cause dehydration and renal failure. (3) Do not administer beta-blockers to cats with acute heart failure, as they can worsen congestion. (4) Do not assume a cat with a murmur has HCM; other diseases like mitral valve dysplasia or hyperthyroidism can cause murmurs. (5) In dogs, do not diagnose HCM without ruling out subaortic stenosis and hypertension.

Based on Plumb's Veterinary Drug Handbook, the following protocols are recommended: Furosemide: 1-2 mg/kg IV, IM, or SC initially, repeated every 1-2 hours until respiratory rate improves, then 1-2 mg/kg PO q8-12h. Pimobendan: 0.25-0.3 mg/kg PO q12h, given on an empty stomach. Atenolol: 6.25-12.5 mg/cat PO q12h, starting at low dose and titrating to effect (heart rate <160 bpm). Diltiazem: 1.75-2.5 mg/kg PO q8h (or sustained-release 10 mg/kg q24h). Clopidogrel: 18.75 mg/cat PO q24h. Enoxaparin: 1 mg/kg SC q6-12h for acute ATE. Aspirin: 5-10 mg/cat PO q72h (less preferred). Buprenorphine: 0.01-0.02 mg/kg IV or IM q6-8h for pain. In dogs, dosages are similar but adjusted for weight; atenolol 0.25-1 mg/kg PO q12h, pimobendan 0.25 mg/kg PO q12h. All dosages should be adjusted based on renal and hepatic function, and drug interactions monitored (e.g., beta-blockers with calcium channel blockers can cause bradycardia).

Key studies and guidelines: The ACVIM consensus statement on the diagnosis and treatment of feline HCM (2015) provides evidence-based recommendations. The REVEAL study (2012) demonstrated that NT-proBNP is useful for diagnosing heart disease in cats. The FATCAT study (2014) showed that clopidogrel is more effective than aspirin in preventing ATE in cats. Studies on Maine Coon cats have identified the MYBPC3 A31P mutation and its association with HCM. Research on pimobendan in cats with HCM is limited, but a study by Gordon et al. (2012) suggested potential benefits in cats with systolic dysfunction. Beta-blockers have been shown to reduce LVOTO in some cats, but their effect on survival is unclear. The use of diltiazem has been studied in older trials, but atenolol is now preferred. Overall, evidence is limited for many therapies, and treatment is often based on extrapolation from human medicine and expert opinion.