Approaching the Canine Myxomatous Mitral Valve Disease (MMVD) in General Practice

Introduction

Myxomatous mitral valve disease (MMVD) is the most common acquired heart disease and the leading cause of congestive heart failure (CHF) in dogs. The disease is characterized by progressive mitral valve thickening and poor coaptation, leading to mitral regurgitation (MR), left atrial (LA) and left ventricular (LV) dilation. While disease progression varies, many dogs remain asymptomatic for years. Management strategies depend on accurate staging, making early detection, monitoring, and referral guidance by general practitioners (GPs) essential.

Pathophysiology and Natural History

MMVD involves degenerative changes to the mitral valve leaflets and chordae tendineae. These changes include collagen disruption, extracellular matrix dysregulation, and activation of valvular interstitial cells, with a likely polygenic genetic predisposition. Mitral regurgitation causes LA and LV volume overload and structural remodeling. Prognosis varies; more than 70% of dogs with preclinical MMVD survive beyond six years, whereas dogs with CHF have an average survival of nine to 15 months.

Staging and Recommendations

The American College of Veterinary Internal Medicine (ACVIM) consensus provides a practical framework for staging MMVD. Stage A includes at-risk breeds with no structural disease – such as Cavalier King Charles Spaniels, Dachshunds, Chihuahuas, and many other small-breed dogs – which only requires annual auscultation. Stage B1 dogs have a murmur without cardiac enlargement. In these cases, no therapy is needed, with recheck recommended in six to 12 months. Stage B2 dogs exhibit a heart murmur plus enough LA and LV enlargement that warrants initiation of pimobendan. Stage C encompasses dogs with active or historic CHF. Once a dog is diagnosed with CHF, treatment involves some combination of a loop diuretic (typically furosemide), pimobendan, ACE inhibitors, and spironolactone. Stage D dogs have CHF that is refractory to standard therapies and may benefit from advanced medical therapies and/or surgical/transcatheter interventions.

Diagnosis in General Practice

Recognition begins with signalment and auscultation, with older, small-breed males at higher risk. The hallmark is a left apical systolic murmur, sometimes with a palpable thrill or mid-systolic click. For asymptomatic dogs, the key decision is differentiating Stage B1 from B2, which may be challenging in the general practice setting. Thoracic radiographs are practical for assessing cardiomegaly, with vertebral heart score (VHS) >11.5 or vertebral left atrial size (VLAS) >3.0 being more specific to stage B2 disease. Natriuretic Peptide (NT-proBNP) testing and focused cardiac ultrasound (TFAST/POCUS) are useful adjuncts when imaging is equivocal. Symptomatic dogs should be evaluated for CHF, with tachypnea, dyspnea, exercise intolerance, syncope, and cough as common signs. Thoracic radiographs remain the gold standard for CHF diagnosis, and careful interpretation is crucial.

Treatment Strategies

Acute CHF management focuses on relieving pulmonary edema and improving hemodynamics, primarily with furosemide, supplemental oxygen, and, when possible, pimobendan. Advanced therapies, such as nitroprusside or dobutamine, may be considered. Chronic management relies on multimodal therapy, ideally combining furosemide, pimobendan, an ACE inhibitor, and spironolactone. Dietary adjustments, including moderate sodium restriction and Omega-3 supplementation, may provide additional benefit.

Anesthesia Risk in MMVD Patients

Many dogs with MMVD require anesthesia for various procedures. Risk assessment should be guided by the stage of disease and the urgency and nature of the procedure. Stage A and B1 dogs are generally low-risk, and even Stage B2 dogs generally tolerate anesthesia well, as long as volume overload is carefully managed. Stage C and D dogs pose greater anesthetic risk, and elective procedures should generally be avoided. Key concerns include exacerbation of MR, fluid overload, hypotension, and arrhythmias. Preoperative thoracic radiographs provide baseline information, and certain drugs — especially dexmedetomidine — should be avoided or used cautiously. Postoperative monitoring is essential to detect precipitated CHF.

Future Directions

Interventional therapies for MMVD, such as transcatheter edge-to-edge repair (TEER), surgical mitral valve repair, and left atrial decompression (LAD), are emerging but currently limited to select referral centers. While medical therapy remains the cornerstone of management, awareness of these advances can guide client discussions and long-term planning.

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