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, with males being predisposed. 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

Left lateral, right lateral and ventrodorsal views of the thorax are needed. The key radiographic signs associated with mitral valve dysplasia and resulting left sided congestive heart failure are cardiomegaly leading to possible dorsal displacement of trachea. There will possibly be none, some or all of the following radiographic signs - left atrial enlargement (DV view appears at 2-3 o'clock position), left ventricular enlargement, bronchial compression, pulmonary venous congestion (enlarged pulmonary arteries and veins) and/or pulmonary oedema.

Evidence of right sided congestive heart failure maybe present in severe cases e.g. distended caudal vena cava, hepatomegaly, ascites, pleural effusions.

Echocardiography

Evidence of left atrial and left ventricular enlargement is visible on echocardiography. The 'fractional shortening' on M wave echography is also increased which is measured as the percentage change in the left ventricular diameter during systole and is used as a measure of systolic function. It is also possible to see structural changes in the valve leaflets in some cases. It may detail irregularities of the valves affected (e.g. thickening, shortening, and/or prolapse of the valve leaflets), abnormal valve movements & valve regurgitation, left atrial enlargement (wide P wave) and left ventricular dilation (tall R wave, wide QRS complex). The regurgitant jet of blood can be detected using colour doppler and evidence of turbulent flow.

Electrocardiogram (ECG)

A resting ECG trace may show evidence of an enlarge left atrium (wide P wave), an enlarged left ventricle (tall R wave, wide QRS complex, shift of mean electrical axis to the left) and rhythm disturbances such as sinus tachycardia, atrial fibrillation, atrial premature complexes and atrial tachycardia.

Laboratory Tests

Pro-brain natriuretic peptide (N-BNP) is a newly described cardiac hormone considered to be an effective marker of severity and prognosis of acute coronary syndromes and congestive heart failure. Circulating levels of the hormone increase in peripheral blood with increased myocardial stress. Commercial assays are not currently available.

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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