Myxomatous mitral valve disease (MMVD) is the most common form of heart disease in dogs, representing 70–80% of acquired cardiac conditions in canines. In senior dogs over the age of 13, approximately 30% have clinical evidence of the disease and postmortem studies suggest up to 90% may be affected. Known by various names—including endocardiosis, mitral valvular degeneration, and degenerative valve disease—all describe the same progressive pathology affecting the mitral valve apparatus.

Anatomy and Pathology of the Mitral Valve

The mitral valve plays a crucial role in regulating blood flow between the left atrium and ventricle. Its apparatus consists of two valve leaflets (anterior and posterior), a fibrous annulus (where the leaflet edges attach to the heart muscle), the chordae tendineae (which attach the leaflets to the papillary muscles), and the papillary muscles themselves. When functioning properly, the valve ensures that blood flows forward from the left ventricle into the aorta without leaking back into the atrium.

On a microscopic level, the mitral valve is composed of four layers: the atrialis, fibrosa, spongiosa, and ventricularis. In MMVD, structural degeneration is most pronounced in the spongiosa layer, where glycosaminoglycans and proteoglycans accumulate. The fibrosa becomes disorganized and the atrialis may show fibroblastic proliferation. These microscopic changes translate into thickened, nodular valve leaflets and potentially ruptured chordae tendineae, leading to valve incompetence and regurgitation.

Hemodynamic Consequences

Mitral regurgitation reduces forward stroke volume because some of the blood leaks backward into the left atrium during systole (contraction of the heart). While mild regurgitation may be compensated for without noticeable clinical signs, moderate to severe and chronic leakage can lead to volume overload, chamber dilation, and eventually congestive heart failure.

ACVIM Staging System: A Framework for Diagnosis and Management

To help clinicians guide treatment and communication with pet owners, the American College of Veterinary Internal Medicine (ACVIM) developed a staging system that categorizes the disease from early risk (preclinical disease) through clinical heart failure:

• Stage A: Dogs at genetic risk but with no evidence of structural heart changes. Common predisposed breeds include Cavalier King Charles Spaniels, Dachshunds, Miniature Poodles, Yorkshire Terriers, and Chihuahuas. Dogs with a genetic predisposition for MMVD typically develop disease earlier in life (as early as 4 years of age, with an average of 6.5 years). No treatment or referral to a cardiologist is necessary however an exam, every 6 to 12 months (by the primary care veterinarian), that includes auscultation of the heart is recommended. 

• Stage B: Dogs have a detectable heart murmur but no clinical signs of heart failure.

  • B1: Dogs have no radiographic or echocardiographic evidence of heart enlargement. A majority of dogs remain in stage B1 and do not require treatment; approximately 30% to 40% will progress to stage B2.

  • B2: Cardiac enlargement is present (e.g., vertebral heart score [VHS] >10.5 or vertebral left atrial size [VLAS] >3.0 on radiographs). These dogs benefit from the early introduction of pimobendan, as was shown by the EPIC study - treatment at this stage delays the onset of heart failure and improves survival.

• Stage C: Dogs have clinical signs of heart failure (e.g., tachypnea, dyspnea, pulmonary edema) that resolve with treatment. Prompt recognition and differentiation from other causes of cough or respiratory distress—such as bronchomalacia or pulmonary fibrosis—are essential for accurate staging and treatment.

• Stage D: Dogs have refractory heart failure that require advanced interventions and higher doses of diuretics. These cases are typically managed with the help of a veterinary cardiologist.

Importantly, progression is one-way: once a dog enters a higher stage, even with effective treatment, they cannot be downgraded to a lower stage.

Diagnostic Tools for Primary Care

While echocardiography remains the gold standard for diagnosing MMVD and assessing disease severity, primary care veterinarians can use several accessible tools to stage patients effectively:

• Auscultation: A left apical systolic murmur of grade 3 or higher warrants further investigation.

• Thoracic radiography: Used to assess heart size and identify pulmonary edema. The vertebral heart score (VHS) and vertebral left atrial score [VLAS] provide objective criteria to distinguish B1 from B2.

• Cardiac biomarkers: NT-proBNP and troponin I levels can help differentiate cardiac vs. non-cardiac causes of respiratory symptoms and predict disease progression.

• Resting respiratory rate monitoring: Instructing owners to track their dog’s resting respiratory rate at home can help detect early signs of decompensation in Stage B2 and C dogs.

Clinical Importance of Early Detection

Identifying the transition from Stage B1 to B2 is critical. While dogs in B1 are monitored, those in B2 should receive medical therapy to slow disease progression. Accurate staging enables timely intervention, improves quality of life, and can extend survival.

Looking Ahead

While this article focuses on the diagnosis and staging of mitral valve disease, medical and interventional treatments—including drug protocols and potential surgical options—will be discussed in detail in a future installment. For now, recognizing clinical signs, understanding the pathophysiology, and applying appropriate staging tools are essential skills for every veterinary professional involved in the care of dogs with this common cardiac disease.

Learn more:
- Degenerative Mitral Valve Disease in Dogs: Diagnosis and staging
- Managing Mitral Valve Disease
- Degenerative Mitral Valve Disease - Non-medical (interventional and surgical) treatment