Myxomatous mitral valve disease

What it is

Myxomatous mitral valve disease (MMVD), also called degenerative mitral valve disease or endocardiosis, is a chronic degeneration of the atrioventricular valve apparatus. Glycosaminoglycans accumulate in the leaflets, which thicken, become nodular and lose their ability to coapt; the chordae tendineae lengthen and may rupture. The tricuspid valve is often affected as well, usually to a lesser degree. The leak (mitral regurgitation) overloads the left atrium and ventricle with volume, and both chambers dilate progressively.

Who it affects

Small and medium-sized breeds, mainly in middle-aged and older dogs: Cavalier King Charles spaniel (early onset and strong heritability), dachshund, Chihuahua, miniature poodle, Yorkshire terrier and similar breeds. Males tend to develop it earlier. Many affected dogs never reach heart failure.

Anatomy involved

• Mitral valve (valva atrioventricularis sinistra): thickened, shortened leaflets with prolapse into the left atrium.
• Left atrium and ventricle: eccentric dilation from volume overload. A severely enlarged left atrium lifts the trachea and can compress the left principal bronchus.
• Lungs: when left atrial pressure rises enough, pulmonary venous congestion leads to cardiogenic pulmonary oedema, initially perihilar.

Clinical signs

A left apical systolic murmur (5th intercostal space, costochondral level) is the first finding and may precede signs by years. With progression: exercise intolerance, tachypnoea and raised sleeping respiratory rate, cough, dyspnoea from pulmonary oedema and occasionally syncope. Rupture of a chorda tendinea or of the left atrium can cause sudden decompensation.

Diagnosis

• Auscultation: left apical systolic murmur; its intensity roughly parallels severity.
• Thoracic radiographs: left atrial and ventricular enlargement, increased vertebral heart score, dorsal displacement of the trachea and, in heart failure, a perihilar to caudodorsal interstitial-alveolar pattern with distended pulmonary veins.
• Echocardiography: confirms thickened, prolapsing leaflets and regurgitation and measures chamber size (left atrium to aorta ratio, normalised left ventricular diameter), which defines stage.
• ACVIM staging: A (at risk), B1 and B2 (preclinical, without and with significant cardiac remodelling), C (current or past heart failure) and D (refractory heart failure).

Treatment

Stage B1 needs only periodic re-examination. In stage B2, pimobendan delays the onset of heart failure. In stage C, treatment combines furosemide, pimobendan and an ACE inhibitor, often with spironolactone; acute pulmonary oedema needs oxygen and parenteral furosemide. Owners can monitor the sleeping respiratory rate at home. Surgical mitral valve repair is available in a few specialist centres.