Degenerative Mitral Valve Disease

Also known as: MVD — Mitral Valve Disease — Mitral Insufficiency — Mitral Endocardiosis — Myxomatous Mitral Valve Disease (MMVD) — Endocardiosis — Mitral Regurgitation — Chronic Valvular Disease

Introduction

Myxomatous degeneration of the mitral valve is the most common acquired cardiac disease in the dog. Degenerative mitral valve disease (DMVD) is a progressive disease and subtle changes in valve structure precede the development of clinically significant disease. The aetiology of DMVD is unknown. Genetic predisposition for development of the disease is likely, however the inheritance is complex.

The mitral apparatus consists of the mitral valve leaflets, valve annulus, chordae tendinae and papillary muscles. The mitral valve leaflets are known as anterior and posterior leaflets. In the normal dog, these are thin, translucent structures that are anchored to the papillary muscles by chordae tendinae. Both papillary muscles (anterior and posterior) arise from the left ventricular free wall. The mitral valve prevents the backflow of blood from the left ventricle to the left atrium during systole. In early systole, when left ventricular pressure exceeds left atrial pressure, the mitral valve leaflets close. In normal dogs, the chordae tendinae tether the leaflets to prevent them prolapsing into the left atrium. When the mitral valve is incompetent, there is regurgitation of blood from the left ventricle to the left atrium. Mitral regurgitation may be mild, with no clinical consequence, or may be severe. The severity of mitral regurgitation is determined primarily by the size of the orifice, that results from incomplete apposition of the mitral valve leaflets, and the relationship between left ventricular and left atrial systolic pressures. Mitral regurgitation causes an increase in left atrial pressure, which over time can lead to left atrial dilation. In diastole, the left ventricle is filled by both pulmonary venous return and blood that has been regurgitated into the left atrium. Therefore, both the left atrium and left ventricle become volume overloaded. This may result in ventricular dilation and eccentric hypertrophy. In severe cases, increased left ventricular and left atrial filling pressures may result. This leads to an increase in pulmonary venous pressure and may result in left-sided congestive heart failure.

Signalment

Degenerative mitral valve disease tends to affect middle-aged and older dogs, particularly males. The disease more commonly affects small breed dogs, with Cavalier King Charles Spaniels, Chihuahuas, Boston Terriers, Poodles, Pomeranians and Bull Terriers being predisposed. The disease is also recognized in large breed dogs.

History and Clinical Signs

Animals may remain asymptomatic for years, the disease is usually clinically silent until it is advanced.

In most affected dogs, DMVD does not cause clinical signs and the disease is detected by the auscultation of a cardiac murmur at routine health checks.

In cases where DMVD becomes clinically significant, a cough is usually the first clinical sign noticed by the owner. The coughing is likely of multifactorial aetiology and may be related to pulmonary oedema, stimulation of the juxtapulmonary (J) receptors that are associated with pulmonary capillaries and detect increases in pulmonary venous pressure, compression of a mainstem bronchi by an enlarged left atrium and concurrent airway disease. Occasionally, syncope is the first sign of clinically significant DMVD. This may occur due to arrhythmias or on exertion where mitral regurgitation limits stroke volume and therefore cardiac output.

Diagnosis

Physical Examination

• Systolic murmur with point of maximal intensity over the left apex. Murmur grade is usually correlated with severity of mitral regurgitation, severe regurgitation causes a loud murmur.
• Mid-systolic click, associated with mitral prolapse. In many dogs, clicks are a precursor to mitral regurgitation.

Other findings will depend on the stage of disease. Crackles may be detected on thoracic auscultation in patients with pulmonary oedema, resulting from left-sided congestive heart failure. Abdominal palpation is usually normal, but ascites and hepatomegaly may be present when there is concurrent right-sided congestive heart failure.

Primary respiratory disease, such as chronic bronchitis, is also common in older small breed dogs. It is important to distinguish between the patient with clinically significant respiratory disease and incidental DMVD from the patient with clinically significant DMVD. Respiratory sinus arrhythmia, indicating vagal influence on heart rate and rhythm, is usually not present in severe cardiac disease. In contrast, sinus arrhythmia is usually preserved or accentuated when respiratory disease is the cause of clinical signs.

Diagnostic Imaging

Radiography

Early in the course of DMVD, thoracic radiographs will be normal. As the disease progresses, cardiomegaly will become apparent. There may be evidence of left atrial enlargement, with or without dorsal displacement of the trachea and narrowing of the mainstem bronchus. Pulmonary venous distension may be observed if there is increased pulmonary venous pressure. Interstitial pulmonary oedema may precede alveolar pulmonary oedema. Evidence of right-sided congestive heart failure may be present in severe cases, radiographic findings include distension of the caudal vena cava, hepatomegaly, ascites and pleural effusion.

Echocardiography

• Thickened mitral valve leaflets
• Prolapse of mitral valve leaflets into the left atrium during systole
• Tricuspid leaflets may also be affected, though usually not as severely as the mitral valve
• Increased diastolic left ventricular diameter
• Hyperdynamic left ventricle
• Colour Doppler jet of mitral regurgitation
• (Flail leaflet)

Thickening of the mitral valve leaflets is usually diffuse, but most pronounced at the leaflet edges. With myxomatous degeneration, the mitral valve becomes stiffer and distorted. The conformation of the valve remains constant throughout the cardiac cycle. Normally, the mitral valve leaflets do not extend beyond a line across the mitral annulus in systole. In dogs with DMVD, the mitral leaflets prolapse towards the left atrium during systole. Colour Doppler can be used to demonstrate the jet of mitral regurgitation. The size of the jet is related to the severity of mitral regurgitation. Most mitral regurgitation jets in DMVD are eccentric.

The more severe the DMVD, the greater the degree of left ventricular and left atrial dilation.

Fractional shortening may be increased (hyperynamic left ventricle). This is because, in the setting of mitral regurgitation, impedance to ventricular emptying is reduced (blood can be ejected into the low pressure left atrium)and end-diastolic ventricular stretch is increased by the addition of the regurgitant fraction, increasing the force of contraction.

A serious complication of DMVD is chordae tendinae rupture, resulting in a 'flail leaflet' and acute worsening of mitral regurgitation. A leaflet segment typically 'flails' back into the left atrium during systole.

Electrocardiogram (ECG)

Electrocardiography is primarily used to diagnose arrhythmias, but can provide evidence of chamber enlargement. Most arrhythmias in DMVD are supraventricular in origin and occur secondary to left atrial stretch. Ventricular arrhythmias may develop in association with left ventricular dilation and fibrosis.

• P-mitrale: wide P waves in leads II, III and aVF, indicates left atrial enlargement

Laboratory Tests

Pro-brain natriuretic peptide (NT-proBNP) concentration is associated with severity of DMVD. Elevated NT-proBNP levels are useful in discriminating patients with respiratory distress caused by heart failure from those with primary respiratory tract disease.

Treatment

If animal is presented in left or right sided heart failure treatment is given at the onset of clinical signs. Such treatments include ACE inhibitors and diuretics.

If the disease is detected but the animal is not yet in heart failure then no treatment is required. Exercise must also be restricted and special formulated sodium reduced cardiac diets recommended.

Symptomatic treatments are also given if clinical signs persist while the animal is on heart failure medications.

No treatment is recommended prior to the onset of heart failure. Once there is evidence of congestive heart failure, treatment is aimed at its management through a combination of drugs.

The aims of treatment are to:

1. Reduce Preload

Diuretics to reduce circulating fluid volume (Frusemide, Benzofluazide, Spironolactone, Amiloride)

Vasodilators to reduce venous return (Nitrates, ACE inhibitors, Alpha antagonists)

2. Reduce Afterload

Vasodilators to decrease systemic vascular resistance

ACE inhibitors e.g. Enalapril, Benzapril, Imidopril

Pimobendan

Calcium channel blockers e.g. Amlodipine

Nitrates e.g. Nitroprusside

3. Enhance Systolic function

Positive inotropes to increase cardiac contractility and increase cardiac output (Pimobendan, Digoxin, Dobutamine, Xanthines)

4. Improve Diastolic function

Negative chronotropes to increase the length of diastole (Digoxin, Atenolol)

Calcium channel blockers to improve relaxation (Amlodipine)

5. Control cardiac arrhythmias using anti-arrhythmic drugs

Prognosis

Asymptomatic patients may live for many years. Once heart failure occurs, life expectancy is usually around one year although some patients remain stable for years on heart failure medications.

Mitral Valve Dysplasia can remain asymptomatic for many years (average 4 years). Once congestive heart failure has developed, the progression of the diseae can be monitored by the severity of the clinical signs (cough, exercise intolerance) and radiographically looking at cardiac size, the degree of pulmonary oedema and the size of the left atrium. Cardiac size can be measured objectively using the Vertebral Heart Score method. Mean survival is 200-300 days once in overt cardiac failure with standard treatment protocols.

References

• Ettinger, S.J. and Feldman, E. C. (2000) Textbook of Veterinary Internal Medicine Diseases of the Dog and Cat Volume 2 (Fifth Edition), W.B. Saunders Company
• Ettinger, S.J, Feldman, E.C. (2005) Textbook of Veterinary Internal Medicine (6th edition, volume 2), W.B. Saunders Company
• Fossum, T. W. et. al. (2007) Small Animal Surgery (Third Edition), Mosby Elsevier * Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition) Merial
• Nelson, R.W. and Couto, C.G. (2009) Small Animal Internal Medicine (Fourth Edition) Mosby Elsevier.
• Tilley, L.P. and Smith, F.W.K.(2004) The 5-minute Veterinary Consult (Third edition) Lippincott, Williams & Wilkins.
• Tilley,L.P., Smith, F.W.K, Oyama, M., Sleeper, M. (2007) Manual of Canine and Feline Cardiology Saunders.

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