Mitral Valve Dysplasia

Mitral valve dysplasia (MVD) is a congenital cardiac malformation characterized by structural and functional abnormalities of the mitral valve apparatus, including the valve leaflets, chordae tendineae, papillary muscles, and annulus. The condition leads to mitral regurgitation (MR) or, less commonly, mitral stenosis, resulting in volume overload of the left atrium and left ventricle, progressive cardiac remodeling, and eventual congestive heart failure (CHF). MVD is one of the most common congenital heart defects in dogs and is also recognized in cats. The severity of the malformation varies widely, from clinically insignificant thickening to severe malformation causing profound hemodynamic compromise. In dogs, MVD is often diagnosed in young animals, but mild forms may remain asymptomatic for years. The disease is distinct from acquired myxomatous mitral valve disease (MMVD), which typically affects older small-breed dogs. MVD can occur as an isolated defect or in association with other congenital anomalies, such as ventricular septal defect, patent ductus arteriosus, or aortic stenosis. The clinical presentation depends on the degree of regurgitation, the presence of concurrent defects, and the chronicity of the disease. Early recognition and appropriate medical management are crucial to delay the onset of heart failure and improve quality of life.

The exact etiology of mitral valve dysplasia is not fully understood, but it is considered a congenital malformation with a strong genetic component. In dogs, certain breeds show a hereditary predisposition, suggesting an autosomal recessive or polygenic mode of inheritance. For example, in Bull Terriers, MVD is inherited as an autosomal dominant trait with variable penetrance. In other breeds, such as Great Danes, German Shepherds, and Golden Retrievers, a familial pattern has been observed. The underlying molecular mechanisms involve abnormal embryonic development of the endocardial cushions, which give rise to the atrioventricular valves. Disruption in the signaling pathways, such as those involving vascular endothelial growth factor (VEGF), bone morphogenetic proteins (BMPs), and Notch, can lead to malformed leaflets, chordae, and papillary muscles. Environmental factors, such as maternal infections, toxins, or nutritional deficiencies during gestation, may also contribute, but no specific teratogen has been definitively identified in veterinary medicine. In cats, MVD is less common but has been reported in Siamese and Persian breeds, suggesting a genetic basis. The condition is present at birth, but clinical signs may not manifest until later in life if the regurgitation is mild. The severity of the malformation determines the hemodynamic consequences, with severe forms leading to early-onset heart failure.

Mitral valve dysplasia is a relatively uncommon congenital heart defect, accounting for approximately 10-15% of all congenital cardiac anomalies in dogs. It is more frequently diagnosed in large and giant breeds, including Great Danes, German Shepherds, Golden Retrievers, Labrador Retrievers, and Bull Terriers. In Bull Terriers, MVD is particularly prevalent and has been extensively studied. The condition is also recognized in cats, with a higher incidence in Siamese and Persian breeds. There is no strong sex predilection, although some studies suggest a slight male predominance in certain breeds. The age at diagnosis varies; severe cases may present with clinical signs in puppies or kittens as young as a few weeks to months, while milder forms may be detected incidentally in adult animals during routine cardiac auscultation or echocardiography. The prevalence of MVD in the general canine population is estimated to be less than 1%, but in high-risk breeds, the prevalence can be significantly higher. For example, in a study of Bull Terriers, the prevalence of MVD was reported to be as high as 30%. Geographic distribution is not a significant factor, but breed popularity influences the number of cases seen in different regions. Early detection through screening programs in predisposed breeds is essential for breeding decisions and early intervention.

The pathophysiology of mitral valve dysplasia revolves around the structural abnormalities that lead to mitral regurgitation (MR) or, less commonly, stenosis. In MR, the malformed valve fails to coapt properly during systole, allowing blood to regurgitate from the left ventricle (LV) into the left atrium (LA). This results in volume overload of the LA and LV. The LA dilates to accommodate the regurgitant volume, and the LV undergoes eccentric hypertrophy, with an increase in chamber size and wall thickness to maintain stroke volume. Initially, compensatory mechanisms, including activation of the renin-angiotensin-aldosterone system (RAAS) and sympathetic nervous system, help maintain cardiac output. However, chronic volume overload leads to progressive myocardial dysfunction, fibrosis, and further chamber enlargement. As the LA dilates, the risk of atrial fibrillation and pulmonary hypertension increases. Eventually, the compensatory mechanisms fail, leading to elevated left atrial pressure, pulmonary venous congestion, and pulmonary edema, resulting in congestive heart failure. In cases of mitral stenosis, the malformed valve obstructs diastolic filling, leading to left atrial hypertension, pulmonary congestion, and right-sided heart failure. The severity of the hemodynamic disturbance depends on the degree of regurgitation or stenosis, which is determined by the specific anatomical defects. For example, a cleft in the anterior leaflet may cause severe MR, while thickened, shortened chordae may restrict leaflet motion and cause stenosis. The chronicity of the disease also influences the clinical course, with some animals remaining asymptomatic for years before developing heart failure.

The primary predisposing factor for mitral valve dysplasia is genetic predisposition, with certain breeds having a higher risk due to inherited mutations. In Bull Terriers, the condition is inherited as an autosomal dominant trait, and affected dogs should not be used for breeding. Other breeds, such as Great Danes and German Shepherds, have a suspected polygenic inheritance. Age is a significant factor, as the congenital defect is present from birth, but clinical signs may not appear until later if the regurgitation is mild. Sex may play a minor role, with some studies suggesting a slight male predisposition. Concurrent congenital heart defects, such as ventricular septal defect or patent ductus arteriosus, can exacerbate the hemodynamic burden and accelerate the onset of heart failure. Environmental factors during gestation, such as maternal infections or exposure to toxins, may increase the risk of developmental abnormalities, but specific teratogens have not been identified. Nutritional deficiencies, particularly of taurine in cats, have been associated with dilated cardiomyopathy, but not directly with MVD. However, taurine deficiency can worsen myocardial function in cats with MVD. Other factors that can influence the progression of MVD include systemic hypertension, obesity, and concurrent diseases such as chronic kidney disease, which can complicate management and worsen prognosis.

Clinical signs of mitral valve dysplasia vary depending on the severity of the malformation and the degree of regurgitation or stenosis. In mild cases, animals may be asymptomatic, and the condition is often detected incidentally during routine physical examination when a heart murmur is auscultated. The characteristic murmur of MR is a holosystolic murmur best heard over the left apex, radiating to the left base. In cases of mitral stenosis, a diastolic murmur may be heard, often with a low-pitched rumble. As the disease progresses, clinical signs of left-sided congestive heart failure develop, including exercise intolerance, tachypnea, dyspnea, cough, and in severe cases, syncope. In puppies and kittens with severe MVD, signs may appear as early as 6-8 weeks of age, with failure to thrive, poor growth, and respiratory distress. On physical examination, affected animals may have a palpable precordial thrill, tachycardia, and a soft, irregular pulse if atrial fibrillation develops. Pulmonary crackles may be auscultated if pulmonary edema is present. In advanced cases, signs of right-sided heart failure, such as ascites and jugular venous distension, may occur due to pulmonary hypertension and right-sided volume overload. In cats, clinical signs are similar but may be more subtle, with lethargy and anorexia being prominent. Sudden death can occur in severe cases, particularly if arrhythmias develop.

The differential diagnoses for mitral valve dysplasia include other congenital heart defects that cause similar clinical signs and murmurs, as well as acquired heart diseases. Key differentials include: 1) Myxomatous mitral valve disease (MMVD) - typically occurs in older small-breed dogs, with progressive thickening of the valve leaflets; echocardiography shows nodular thickening and prolapse, whereas MVD has more severe structural abnormalities. 2) Dilated cardiomyopathy (DCM) - characterized by severe LV dilation and systolic dysfunction, with no primary valve malformation; echocardiography reveals poor fractional shortening. 3) Endocarditis - bacterial infection of the valve, often with fever, leukocytosis, and positive blood cultures; echocardiography may show vegetative lesions. 4) Ventricular septal defect (VSD) - a defect in the interventricular septum, causing a harsh holosystolic murmur over the right sternal border; color Doppler shows a jet across the septum. 5) Patent ductus arteriosus (PDA) - a continuous murmur, bounding pulses, and characteristic echocardiographic findings of a patent ductus. 6) Tricuspid valve dysplasia - similar to MVD but affecting the right side, with a murmur over the right apex and signs of right-sided heart failure. 7) Pulmonic stenosis - a systolic ejection murmur over the left base, with right ventricular concentric hypertrophy on echocardiography. 8) Aortic stenosis - a systolic ejection murmur over the left base, with LV concentric hypertrophy and post-stenotic dilation of the aorta. 9) Atrial septal defect (ASD) - a systolic murmur over the left base, with right atrial and ventricular dilation; echocardiography shows the defect. 10) Mitral stenosis (isolated) - rare, but can be differentiated by echocardiographic findings of thickened, fused leaflets and Doppler evidence of diastolic flow obstruction. Definitive diagnosis is made by echocardiography, which allows direct visualization of the valve morphology and assessment of regurgitation or stenosis.

The diagnostic algorithm for mitral valve dysplasia begins with a thorough history and physical examination, with particular attention to cardiac auscultation. If a heart murmur is detected, especially in a young animal of a predisposed breed, further diagnostic testing is warranted. The next step is thoracic radiography to evaluate cardiac size and pulmonary vasculature. Radiographic findings may include left atrial and left ventricular enlargement, as well as pulmonary edema in cases of CHF. However, radiography is not definitive for MVD. The gold standard for diagnosis is echocardiography, which should be performed by a veterinary cardiologist. Echocardiography allows detailed assessment of the mitral valve apparatus, including leaflet morphology, thickness, mobility, and the presence of clefts, prolapse, or shortened chordae. Color flow Doppler is used to evaluate the severity of mitral regurgitation or stenosis. Additional measurements, such as left atrial to aortic root ratio (LA/Ao), left ventricular internal diameter in diastole (LVIDd), and fractional shortening (FS), help assess the hemodynamic impact. In cases where echocardiography is not available, electrocardiography (ECG) may show evidence of left atrial enlargement (P mitrale) or arrhythmias, but it is not diagnostic. Advanced imaging, such as cardiac MRI or CT, is rarely needed but may be useful in complex cases. Genetic testing is available for some breeds, such as Bull Terriers, and can aid in breeding decisions. The diagnostic algorithm should also include screening for concurrent congenital defects, as MVD can be associated with other anomalies. In asymptomatic animals with a murmur, echocardiography is recommended to confirm the diagnosis and establish a baseline for monitoring.

Laboratory findings in mitral valve dysplasia are generally non-specific but can help assess the severity of the disease and the presence of complications. Complete blood count (CBC) is usually within normal limits, but may show stress leukogram in animals with heart failure. Serum biochemistry may reveal mild elevations in liver enzymes (ALT, ALP) due to hepatic congestion, and renal parameters (BUN, creatinine) may be elevated in cases of reduced cardiac output or concurrent kidney disease. Electrolyte imbalances, particularly hyponatremia and hyperkalemia, can occur in advanced heart failure due to RAAS activation and diuretic therapy. Blood gas analysis may show respiratory alkalosis in animals with tachypnea, or metabolic acidosis in severe cases. Cardiac biomarkers are valuable in assessing the severity and prognosis. N-terminal pro-B-type natriuretic peptide (NT-proBNP) is elevated in animals with cardiac disease and can help differentiate cardiac from respiratory causes of dyspnea. Troponin I may be elevated in cases of myocardial injury. In cats, taurine levels should be measured, as deficiency can exacerbate myocardial dysfunction. Urinalysis is important to assess renal function, especially before initiating ACE inhibitors or diuretics. Urine protein-to-creatinine ratio (UPC) may be indicated if proteinuria is present. In animals with suspected endocarditis, blood cultures and serology for infectious agents should be performed. Overall, laboratory findings are supportive but not diagnostic for MVD; echocardiography remains the definitive diagnostic tool.

Imaging plays a crucial role in the diagnosis and management of mitral valve dysplasia. Thoracic radiography is typically the first imaging modality performed. In animals with MVD, radiographs may show generalized cardiomegaly, with specific enlargement of the left atrium and left ventricle. The cardiac silhouette may appear globoid, and the trachea may be elevated due to left atrial enlargement. In cases of congestive heart failure, interstitial to alveolar pulmonary patterns, consistent with pulmonary edema, are seen, often in the perihilar region in dogs and diffuse in cats. Radiography is also useful to rule out other thoracic diseases, such as pneumonia or neoplasia. Echocardiography is the gold standard for diagnosis. Two-dimensional (2D) echocardiography allows direct visualization of the mitral valve leaflets, chordae tendineae, and papillary muscles. Abnormalities include thickened, dysplastic leaflets, clefts, prolapse, and shortened or fused chordae. The left atrium and left ventricle are typically dilated. M-mode echocardiography provides measurements of chamber dimensions and systolic function. Doppler echocardiography, including color flow, pulsed-wave, and continuous-wave, is used to assess the severity of mitral regurgitation or stenosis. Color flow Doppler shows a high-velocity jet of regurgitant flow into the left atrium during systole. The severity of MR can be graded based on the jet area relative to the left atrial area. In cases of mitral stenosis, Doppler shows elevated diastolic flow velocities across the valve. Advanced imaging, such as cardiac computed tomography (CT) or magnetic resonance imaging (MRI), is rarely needed but may be useful in complex cases to assess the anatomy in detail. These modalities can provide three-dimensional reconstructions of the valve apparatus and help plan surgical interventions, if considered.

Cytology and histopathology are not typically used for the diagnosis of mitral valve dysplasia, as the condition is a structural malformation rather than an inflammatory or neoplastic process. However, histopathological examination of the valve may be performed post-mortem or if surgical biopsy is obtained. Grossly, the mitral valve leaflets may be thickened, nodular, or cleft, with abnormal chordae tendineae. Histologically, the valve tissue shows disorganized collagen and elastin fibers, myxomatous degeneration, and fibrosis. In severe cases, there may be calcification. The chordae tendineae may be thickened, shortened, or fused, and the papillary muscles may be hypertrophied or malpositioned. These findings are consistent with a congenital malformation. In cases where endocarditis is suspected, cytology of blood cultures or valve tissue may reveal bacterial organisms. However, in the absence of infection, cytology and histopathology are not indicated for diagnosis. The diagnosis of MVD is primarily based on echocardiographic findings, and histopathology is reserved for research or post-mortem confirmation.

The treatment of mitral valve dysplasia depends on the severity of the disease and the presence of clinical signs. In asymptomatic animals with mild regurgitation, no treatment is required, but regular monitoring is recommended. In animals with moderate to severe regurgitation or clinical signs of heart failure, medical therapy is indicated. The goal of treatment is to manage congestive heart failure, reduce volume overload, and slow the progression of cardiac remodeling. The standard therapy for heart failure due to MVD includes: 1) Diuretics: Furosemide is the first-line diuretic for pulmonary edema. The dose is 1-2 mg/kg IV or IM initially, followed by 1-2 mg/kg PO q8-12h, titrated to the lowest effective dose. In severe cases, a continuous rate infusion (CRI) of furosemide at 0.66-1 mg/kg/hr may be used. 2) ACE inhibitors: Enalapril or benazepril at 0.5 mg/kg PO q12-24h is recommended to reduce afterload and attenuate remodeling. 3) Pimobendan: This positive inotrope and vasodilator is recommended in dogs with CHF due to MVD. The dose is 0.25-0.3 mg/kg PO q12h. It has been shown to improve quality of life and survival. 4) Spironolactone: An aldosterone antagonist at 2 mg/kg PO q24h may be added for its anti-fibrotic effects. 5) In cases of atrial fibrillation, digoxin (0.005-0.01 mg/kg PO q12h) or diltiazem (0.5-1.5 mg/kg PO q8h) may be used to control ventricular rate. 6) In severe cases, oxygen supplementation and cage rest are essential. Surgical intervention, such as mitral valve repair or replacement, is rarely performed in veterinary medicine due to the complexity and high cost, but may be considered in select cases. In cats, treatment is similar, but doses may differ, and taurine supplementation is recommended if deficient. Dietary management includes a low-sodium diet to reduce fluid retention. Regular monitoring of renal function and electrolytes is essential, especially during diuretic therapy.

The prognosis for mitral valve dysplasia varies widely depending on the severity of the malformation and the response to therapy. In animals with mild regurgitation and no clinical signs, the prognosis is good, and they may live a normal lifespan. In animals with moderate regurgitation, the disease may progress to heart failure over months to years, but with appropriate medical management, survival times of 1-3 years after the onset of heart failure are possible. In severe cases, especially those presenting with heart failure at a young age, the prognosis is poor, with survival times of weeks to months despite aggressive therapy. Negative prognostic indicators include early onset of clinical signs, severe left atrial and ventricular enlargement, poor systolic function, and the presence of arrhythmias. In Bull Terriers, the prognosis is particularly guarded, as the disease is often progressive. In cats, the prognosis is also variable, but severe cases may have a poor outcome. Regular monitoring and early intervention can improve the prognosis. The response to therapy, particularly the resolution of pulmonary edema and improvement in exercise tolerance, is a key indicator of prognosis. Overall, the prognosis is better for animals with mild disease and those that respond well to medical management.

Follow-up care for animals with mitral valve dysplasia is essential to monitor disease progression and adjust therapy. Asymptomatic animals with mild regurgitation should be re-examined every 6-12 months, including physical examination, thoracic radiography, and echocardiography to assess changes in cardiac size and function. Animals with moderate to severe disease or those receiving treatment for heart failure should be re-evaluated more frequently, typically every 1-3 months initially, then every 3-6 months once stabilized. At each recheck, the owner should be questioned about exercise tolerance, appetite, and respiratory rate at rest. The resting respiratory rate is a valuable indicator of pulmonary edema; an increase above 30-40 breaths per minute in dogs or 40-50 in cats warrants immediate evaluation. Physical examination should include assessment of heart rate, rhythm, murmur intensity, and lung auscultation. Thoracic radiography is used to evaluate pulmonary edema and cardiac size. Echocardiography is performed to assess left atrial size, left ventricular dimensions, and systolic function. Serum biochemistry and electrolytes should be monitored, especially in animals on diuretics and ACE inhibitors, to detect azotemia or electrolyte imbalances. The dosage of furosemide should be titrated to the lowest effective dose to maintain euvolemia. In animals with atrial fibrillation, heart rate control should be assessed. Long-term management includes dietary sodium restriction, weight management, and regular exercise. Owners should be educated on the signs of heart failure and when to seek emergency care. In breeding animals, genetic counseling is recommended to prevent transmission of the disease.

Clinical Pearls: 1) In young animals with a left apical holosystolic murmur, MVD should be a primary differential, especially in predisposed breeds. 2) Echocardiography is essential for diagnosis; do not rely solely on radiography. 3) In Bull Terriers, MVD is inherited as an autosomal dominant trait; affected dogs should not be bred. 4) Pimobendan has been shown to improve survival in dogs with CHF due to MVD; consider early use. 5) Resting respiratory rate is a simple and effective tool for owners to monitor for pulmonary edema. 6) In cats, taurine deficiency can exacerbate myocardial dysfunction; measure taurine levels and supplement if low. 7) Concurrent congenital defects are common; perform a thorough echocardiographic examination to rule out other anomalies. Pitfalls: 1) Mistaking MVD for myxomatous mitral valve disease in older dogs; MVD is congenital and typically presents in younger animals. 2) Failing to recognize that mild MVD can be asymptomatic and may not require treatment, but still needs monitoring. 3) Overdosing furosemide, leading to dehydration and azotemia; always titrate to the lowest effective dose. 4) Neglecting to monitor renal function and electrolytes during ACE inhibitor and diuretic therapy. 5) Delaying echocardiography in animals with a murmur, leading to missed diagnosis and progression of disease. 6) In cats, using dog-specific drug dosages without adjustment; always consult a veterinary pharmacopeia for feline doses. 7) Assuming that a soft murmur indicates mild disease; severity of regurgitation can be significant even with a soft murmur. 8) Failing to consider surgical options in severe cases, although they are limited.

The following drug protocols are based on Plumb's Veterinary Drug Handbook and current ACVIM consensus guidelines for the management of congestive heart failure due to mitral valve dysplasia. 1) Furosemide: For acute pulmonary edema, administer 1-2 mg/kg IV or IM, repeated as needed. For maintenance, 1-2 mg/kg PO q8-12h, titrated to the lowest effective dose. In severe cases, a CRI of 0.66-1 mg/kg/hr IV may be used. 2) Enalapril: 0.5 mg/kg PO q12-24h. Benazepril: 0.25-0.5 mg/kg PO q24h. 3) Pimobendan: 0.25-0.3 mg/kg PO q12h, administered at least 1 hour before or 2 hours after food. 4) Spironolactone: 2 mg/kg PO q24h. 5) Digoxin: 0.005-0.01 mg/kg PO q12h, with monitoring of serum levels (therapeutic range 0.8-2.4 ng/mL). 6) Diltiazem: 0.5-1.5 mg/kg PO q8h for rate control in atrial fibrillation. 7) Sildenafil: 1-2 mg/kg PO q8-12h for pulmonary hypertension. 8) In cats, furosemide: 1-2 mg/kg IV or IM initially, then 1-2 mg/kg PO q12-24h. Enalapril: 0.25-0.5 mg/kg PO q24h. Pimobendan: 1.25 mg/cat PO q12h. Spironolactone: 1-2 mg/kg PO q24h. Taurine: 250-500 mg PO q12h if deficient. All doses should be adjusted based on renal function, and serum electrolytes and renal parameters should be monitored regularly. Contraindications: ACE inhibitors should be used with caution in animals with renal disease or hyperkalemia. Pimobendan should not be used in animals with hypertrophic cardiomyopathy. Digoxin should be used with caution in animals with renal impairment or electrolyte imbalances. Drug interactions: Furosemide may increase the risk of digoxin toxicity. ACE inhibitors may increase serum potassium levels. Spironolactone may potentiate the effects of other diuretics.

The evidence base for the management of mitral valve dysplasia is largely extrapolated from studies on acquired myxomatous mitral valve disease (MMVD) and other causes of congestive heart failure. The EPIC study (2016) demonstrated that pimobendan delays the onset of congestive heart failure in dogs with MMVD and cardiomegaly, and it is now recommended for use in dogs with MVD and significant regurgitation. The ACVIM consensus statement on the diagnosis and treatment of myxomatous mitral valve disease (2019) provides guidelines that are often applied to MVD, including the use of ACE inhibitors and diuretics. However, there are few studies specifically on MVD. A study by Chetboul et al. (2004) described the echocardiographic features of MVD in Bull Terriers and found that the disease is progressive. Another study by Tidholm et al. (2001) reported the clinical findings and outcome in dogs with congenital mitral valve defects. In cats, a study by Nakamura et al. (2011) evaluated the use of pimobendan in cats with heart failure, but the evidence is limited. Overall, the management of MVD is based on the principles of treating heart failure, and the prognosis is guarded for severe cases. Further research is needed to establish breed-specific guidelines and to evaluate the efficacy of surgical interventions.