Overview

• A degenerative disorder of the left A-V (atrio-ventricular) valve, or mitral valve.
• Common in older small breed dogs, accounting for 75-80% of canine cardiac disease.
• Also known as chronic mitral valvular disease (CMVD), chronic degenerative mitral valve disease (MVD), or endocardiosis.
• Myxomatous degeneration causes thickening and contraction of the valve leaflets, and thickening with lengthening (earlier) or shortening (later) of the chordae tendinae. Myxomatous degeneration is characterized by an increase in the spongiosa layer of the valve and accumulation of glycosaminoglycans. Other features include disruption and fragmentation of collagen fibrils and denudation of overlying endothelium. Inflammation is absent.
• Most dogs with mitral regurgitation also have some degree of tricuspid regurgitation. The tricuspid valve disorder is believed to be the same degenerative process that is affecting the mitral valve that also affects simultaneously the tricuspid valve.
• Inherited in the Cavalier King Charles Spaniel and the Dachshund breeds (polygenetic threshold trait).

Results of MMVD:

1. Mitral regurgitation (MR).
2. Left atrial enlargement with MR.
   • Left atrial enlargement and left ventricular enlargement (eccentric hypertrophy) occur due to volume overload. Studying the re-circulation circuit (left atrium and left ventricle in the case of MR) identifies the cardiac chambers that undergo volume overload and eccentric hypertrophy.
3. Right atrial enlargement due to tricuspid regurgitation.
4. Left mainstem bronchial compression due to left atrial enlargement
   • Causes cough
5. Supraventricular arrhythmias
   • Due to atrial enlargement
   • Produces signs of or exacerbation of fluid accumulation
     • Pulmonary edema
     • Pleural effusion
     • Ascites
   • Produces syncope/weakness
   • Atrial fibrillation
     • A common supraventricular arrhythmia
     • Often pushes a subclinical case into congestive heart failure
     • Often heralds the onset of ascites
6. May cause a tear in the left atrium which usually produces immediate cardiac tamponade
7. Left ventricular (LV) enlargement occurs due to volume overload and pathologic LV remodeling occurs (loss of cardiomyocytes and inadequate hypertrophy)
   • Causes eventual myocardial failure and pulmonary edema (congestive heart failure)
   • May promote ventricular arrhythmias
8. Rupture of chordae tendinae
   • Induces severe and acute pulmonary edema related to sudden increase in left atrial and pulmonary venous pressure
   • Chordal rupture occurs in 16% of dogs with CMVI. It increases in frequency in dogs with more advanced disease with the highest prevalence (70%) in dogs with advanced CMVI. (Serres et al. JVIM 2007;21:258-264).
9. Pulmonary artery hypertension secondary to MR can develop and lead to right heart failure
   • Pulmonary hypertension occurs in about 30% of dogs with an average systolic pulmonary artery pressure of about 50 mmHg. Note that this included dogs with pre-clinical CMVI. (Borgarelli et al. JVC 2004;6:27-34).