Myxomatous Mitral Valve Disease in Dogs: ACVIM Stages and When to Treat

Myxomatous mitral valve disease (MMVD), also called myxomatous degeneration, endocardiosis or chronic valvular disease, is the most common acquired heart disease in dogs. It is a progressive degeneration of the mitral valve leaflets, which lose their ability to seal the opening between the left atrium and left ventricle. This leads to mitral regurgitation, the backflow of blood that produces a murmur and, over years, can cause overload of the left-sided chambers and, in some cases, congestive heart failure. Diagnosis and staging follow the ACVIM consensus (Keene et al., 2019), which organizes the disease into stages A, B1, B2, C and D to guide when to investigate and when to treat. The decisions described here are clinical and must always be made by a veterinarian.

What it is and why it happens

In MMVD, the mitral valve leaflets undergo gradual thickening, with deposition of myxomatous material and lengthening of the chordae tendineae. The valve no longer closes completely during systole, and part of the blood that should be ejected through the aorta flows back into the left atrium. This regurgitation is what is heard as a murmur and, when significant and chronic, dilates the left atrium and left ventricle.

The disease usually progresses slowly: many dogs spend years with mild regurgitation and no clinical signs, and some never develop heart failure, dying from other causes or old age. In others, the overload progresses and the heart loses its ability to compensate, producing pulmonary congestion. Understanding this natural history helps calibrate the frequency of monitoring without unnecessary alarm (Häggström et al., 2009).

Predisposed breeds and age

MMVD mainly affects small- and medium-sized dogs in middle to older age, with prevalence that rises with age. Classically associated breeds include the Cavalier King Charles Spaniel, in which the disease tends to appear earlier, as well as the Dachshund, Poodle, Chihuahua, Pinscher, Miniature Schnauzer and King Charles Spaniel, among others. The ACVIM consensus classifies these predisposed animals, still without a murmur, as stage A: they are at increased risk, but the heart is still structurally normal.

Breed predisposition and age guide surveillance: a dog of an at-risk breed from middle age onward deserves careful auscultation at every visit, even without owner complaints. It is worth noting that sex, advanced age and breed characteristics also appear among the variables associated with prognosis in natural-history studies, reinforcing the value of follow-up (Borgarelli et al., 2008).

The left apical systolic murmur: the first sign

In most dogs, the first sign of MMVD is a systolic murmur best heard over the left apex, directly over the mitral valve. It is usually detected by the veterinarian during routine auscultation, often before any symptoms. Murmur intensity tends to relate to the severity of regurgitation, although it does not replace it: a soft murmur does not rule out progressing disease, and confirming severity depends on imaging.

Coughing, exercise intolerance, a fast resting respiratory rate or fainting are later signs and warrant prompt evaluation. A new murmur, or one that has changed in intensity, is the classic trigger to investigate the heart in a structured way and define the ACVIM stage.

ACVIM staging: A, B1, B2, C and D

The ACVIM consensus (Keene et al., 2019) divides MMVD into stages that combine risk, structural change and clinical signs. Stage A: dogs of a predisposed breed, at increased risk, but with no murmur and no identifiable structural change. Stage B1: a murmur and regurgitation are present, but without significant cardiac remodeling, meaning no relevant enlargement of the left-sided chambers on imaging.

Stage B2: the regurgitation is severe enough to cause enlargement of the left atrium and left ventricle, but the dog has not yet experienced heart failure, the preclinical phase with cardiomegaly, which is decisive for deciding on treatment. Stage C: signs of congestive heart failure (such as pulmonary edema) are present now or have occurred in the past. Stage D: heart failure refractory to conventional treatment, requiring advanced management. Stages B2, C and D depend on objective imaging criteria defined in the consensus and should not be assumed from auscultation alone.

When to treat in the preclinical B2 phase: the EPIC study

The major change brought by the 2019 consensus was to recommend treatment in the preclinical phase for selected stage B2 dogs, before any sign of heart failure. This recommendation is based on the EPIC study (Boswood et al., 2016), a randomized, placebo-controlled, multicenter clinical trial of 360 dogs with preclinical MMVD and cardiomegaly.

In EPIC, eligible dogs had cardiac enlargement defined by objective criteria: a left atrium-to-aorta ratio of at least 1.6, a normalized left ventricular internal diameter in diastole of at least 1.7, and a vertebral heart sum greater than 10.5. Administration of pimobendan (0.4 to 0.6 mg/kg/day, in divided doses) significantly prolonged the time to the composite endpoint of onset of heart failure, cardiac death or euthanasia: the median was 1228 days in the pimobendan group versus 766 days in the placebo group, with a hazard ratio of 0.64. In practical terms, treatment delayed the preclinical phase by about 15 months and was well tolerated. For this reason, the consensus reserves preclinical pimobendan for dogs that meet these cardiomegaly criteria, not for every dog with a murmur.

The role of each test: ECG, echocardiography, radiography and blood pressure

Confirming the stage depends on combining tests, and each answers a different question. Echocardiography is the central tool for measuring chamber enlargement and applying the B2 criteria, and is decisive for placing a dog within the EPIC parameters. Thoracic radiography assesses heart size via the vertebral heart sum and, above all, identifies congestion and pulmonary edema, which distinguishes stage B from stage C.

The electrocardiogram (ECG) does not diagnose valve degeneration itself, but it is irreplaceable for detecting and characterizing arrhythmias, which are common with atrial overload and advanced disease, and for tracking rate and rhythm over time. Measuring systemic blood pressure helps identify concurrent hypertension, which can worsen regurgitation and influence the treatment plan. Within the INpulse connected ecosystem, tools such as INcardio X and INcardio Agile support recording and sharing the ECG, and BPScan supports blood-pressure measurement, always as an aid to the veterinarian's decision, never a replacement for it.

When to refer to a cardiologist

Referral to a veterinary cardiologist, or specialized echocardiography, is particularly indicated when there is a new or relevant murmur in a predisposed breed, when stage B2 is suspected and cardiomegaly must be confirmed to decide on preclinical treatment, or when signs suggestive of heart failure appear, such as persistent coughing, exercise intolerance, a fast resting respiratory rate or syncope.

Further evaluation is also warranted by arrhythmias on the ECG, suspected pulmonary hypertension, and stage C or D cases that do not respond as expected. Periodic follow-up, with reassessment of imaging and tests according to the stage, allows treatment to be adjusted at the right time. All of these decisions, from the indication to treat to the frequency of reassessment, rest with the attending veterinarian, ideally with support from a cardiology specialist.

Frequently asked questions

Does every dog with a murmur need medication? No. A murmur indicates regurgitation, but preclinical treatment with pimobendan is, according to the ACVIM consensus, reserved for stage B2 dogs that demonstrably have left-sided chamber enlargement by the imaging criteria of the EPIC study. Dogs in B1, with a murmur but no cardiomegaly, are generally monitored rather than medicated.

Can MMVD be cured? There is no medical cure for valve degeneration with drug management; the goal is to slow progression, delay heart failure and preserve quality of life. Some centers offer valve surgery, but the indication is specific and must be discussed with a specialist. Will my dog necessarily develop heart failure? Not necessarily: some dogs with mild regurgitation live for years without progressing and may die from other causes, which reinforces why the stage, and not the murmur alone, guides decisions.