Myxomatous Mitral Valve Disease

TL;DR. Myxomatous mitral valve disease is a common, progressive heart condition in dogs and cats where the main valve on the left side of the heart degenerates and leaks, which can eventually lead to congestive heart failure.

Many dogs with early-stage myxomatous mitral valve disease remain active and show no outward signs of illness.

What is it?

The heart is a highly efficient pump divided into four chambers. The left side of the heart is responsible for receiving oxygen-rich blood from the lungs and pumping it out to the rest of the body. Between the upper chamber (the left atrium) and the lower chamber (the left ventricle) lies the mitral valve. This valve acts as a one-way door, ensuring that blood only flows in one direction: forward.

In myxomatous mitral valve disease (often abbreviated as MMVD or DMVD), this one-way door begins to fail. Over time, the structural proteins within the valve leaflets undergo a degenerative process. Instead of remaining thin, smooth, and flexible, the valve leaflets become thick, lumpy, and weakened. This structural deterioration prevents the valve from closing tightly.

When the left ventricle contracts to pump blood to the body, some of that blood leaks backward through the deformed valve into the left atrium. This backward flow is known as mitral regurgitation. As the disease progresses, the constant backflow of blood causes the left atrium and left ventricle to stretch and enlarge to accommodate the extra volume. If the pressure within the left atrium becomes too high, fluid can back up into the blood vessels of the lungs, leading to a life-threatening condition called congestive heart failure.

While this condition is incredibly common in dogs, it is relatively rare in cats. Cats are far more prone to diseases of the heart muscle itself, such as hypertrophic cardiomyopathy. When myxomatous mitral valve disease does occur in cats, it follows a similar degenerative path, but the clinical signs and diagnostic findings can differ. Because feline cases are less common, much of the clinical guidance for managing this disease in cats is extrapolated from canine medicine.

In myxomatous mitral valve disease, the valve leaflets become thick and deformed, preventing them from closing properly.

Causes & risk factors

MMVD is primarily an age-related, degenerative disease. It is the most common heart disease diagnosed in dogs, accounting for the vast majority of canine cardiac cases. The precise trigger that initiates the degeneration of the valve tissue is not fully understood, but it is highly associated with aging and genetic factors.

According to a leading veterinary internal medicine reference:

"Characteristic valve changes include collagen degeneration and disorganization, fragmentation of valve elastin, and excess deposition of proteoglycan and glucosaminoglycan (mucopolysaccharide), all of which thicken and weaken the valve apparatus. The histologic changes have been described as myxomatous degeneration. Middle-aged and older small to mid-size breeds are most often affected..."

Genetics play a powerful role in who develops this disease. Certain breeds are highly predisposed, with some showing signs of the disease much earlier in life than others. For example, the Cavalier King Charles Spaniel has a well-documented, polygenic genetic predisposition, meaning multiple genes contribute to the development of the disease. In this breed, a significant percentage of dogs will develop a heart murmur caused by MMVD by the time they reach old age. Other small-to-medium breeds, such as Toy Poodles, Chihuahuas, and Dachshunds, are also frequently diagnosed. While less common, large breed dogs can also develop the disease, often presenting with slightly different clinical courses.

Signs to watch for

In the early stages of myxomatous mitral valve disease, most pets show no outward signs of illness. The heart is able to compensate for the mild leak, and your pet may behave completely normally. This is why routine veterinary exams are so critical. As the disease progresses and the heart's ability to compensate declines, signs will begin to emerge.

• Holosystolic murmur (Cardinal): This is a specific type of heart murmur that spans the entire duration of the heart's contraction. It is detected by your veterinarian during a physical exam and is the primary clinical indicator of mitral regurgitation.
• Cough (Common): A persistent cough is a classic sign of progressing heart disease. It often occurs or worsens at night, during the early morning hours, or when your pet first gets up from resting. This cough can be caused by the enlarged left atrium pressing against the main airway (bronchus) or by fluid accumulating in the lungs.
• Decreased exercise tolerance (Common): Your pet may tire out more quickly during walks, show reluctance to play, or sit down frequently during activities they used to enjoy.
• Tachypnea (Common): An increased breathing rate at rest. Monitoring how fast your pet breathes while sound asleep is one of the most sensitive ways to detect early fluid buildup in the lungs.
• Respiratory distress (Common): Labored, heavy, or rapid breathing even when resting. This is a major warning sign that requires immediate emergency care.
• Mid- to late-systolic click (Occasional): A distinct clicking sound heard by your vet through a stethoscope, caused by the abnormal billowing of the degenerated valve leaflets.
• Weakness (Occasional): A general lack of energy, lethargy, or sudden episodes of physical weakness.
• Syncope (Occasional): Fainting or temporary loss of consciousness, which occurs when the heart cannot pump enough oxygenated blood to the brain, especially during exertion or excitement.
• Pleural effusion (Occasional): An accumulation of fluid in the chest cavity outside of the lungs, which further restricts the lungs' ability to expand.
• Ascites (Occasional): Fluid accumulation in the abdomen, leading to a swollen or pot-bellied appearance, typically seen in advanced stages of heart failure.

As detailed in a leading veterinary internal medicine reference:

"In those that do, the signs usually relate to decreased exercise tolerance and manifestations of pulmonary congestion and edema. Diminished exercise capacity and cough or tach ypnea with exertion are common initial owner complaints. As pulmonary congestion and interstitial edema worsen, the resting respiratory rate increases. Coughing tends to occur at night and early morning..."

How vets diagnose it

Diagnosing myxomatous mitral valve disease involves a combination of physical examinations and advanced imaging techniques. Because the disease is progressive, regular diagnostic monitoring is essential for tracking its severity and determining when to start medical therapy.

The diagnostic journey always begins with a thorough physical examination and auscultation (listening to the heart and lungs with a stethoscope). Your veterinarian will listen for the presence of a heart murmur, determine its intensity (graded on a scale of 1 to 6), and identify its location. They will also listen to the lungs to check for abnormal sounds, such as crackles, which can indicate fluid accumulation.

If a murmur is detected, further diagnostic testing is highly recommended:

• Echocardiography (Gold Standard): An ultrasound of the heart is the definitive tool for diagnosing MMVD. It allows a veterinary cardiologist or trained practitioner to view the heart's chambers and valves in real-time. The vet can measure the thickness of the mitral valve leaflets, assess the size of the left atrium and left ventricle, and use color Doppler imaging to visualize the direction and velocity of the leaking blood. This test is crucial for determining the exact stage of the disease.
• Thoracic radiography (Chest X-rays): X-rays are vital for evaluating the overall size and shape of the heart and, most importantly, assessing the lungs. It is the primary tool used to diagnose pulmonary edema (fluid in the lungs) and pleural effusion, helping your vet determine if your pet has transitioned into active congestive heart failure.
• Electrocardiography (ECG): An ECG records the electrical activity of the heart. It is used to detect arrhythmias (abnormal heart rhythms) that can occur when the heart muscle becomes stretched or strained by chronic valve disease.

Echocardiography is the gold standard for assessing the severity of mitral valve regurgitation.

Treatment options

Treatment for myxomatous mitral valve disease is tailored to the specific stage of the disease. Veterinary medicine categorizes MMVD into stages (typically Stages A through D) based on the severity of the valve changes and whether the pet has developed heart failure.

In the early stages (Stages A and B1), where a murmur is present but there is no evidence of heart enlargement on X-rays or echocardiography, drug treatment is generally not recommended. Instead, management focuses on regular monitoring and maintaining a healthy body weight.

Once the disease progresses to Stage B2, where the heart has significantly enlarged due to the volume of leaking blood, medical intervention may be initiated. Your veterinarian may prescribe a class of medication known as positive inotropes. These drugs help the heart muscle contract more efficiently and dilate blood vessels, which has been clinically proven to delay the onset of congestive heart failure and extend a pet's comfortable life.

If your pet enters congestive heart failure (Stage C), active and aggressive medical management is required to manage fluid accumulation and support heart function. This multi-drug protocol typically includes:

• Diuretics: These medications are the cornerstone of heart failure therapy. They prompt the kidneys to remove excess salt and water from the body, helping to clear fluid from the lungs (pulmonary edema) and chest cavity.
• ACE inhibitors: These drugs help relax blood vessels, lowering blood pressure and reducing the resistance the heart must pump against, making it easier for the heart to move blood forward.
• Positive inotropes: Continued use of these medications helps support the pumping action of the weakened heart muscle.
• Dietary management: Your vet may recommend a diet moderately restricted in sodium to help prevent fluid retention, along with omega-3 fatty acid supplementation to support heart muscle health.

Prognosis

The prognosis for pets with myxomatous mitral valve disease is highly variable. Many dogs diagnosed with a mild heart murmur live for several years without ever showing clinical signs of heart disease or requiring medication. They can enjoy a normal lifespan and an excellent quality of life.

However, for dogs that develop severe mitral regurgitation, the prognosis becomes more guarded. A sudden worsening of the condition can occur if the chordae tendineae (the fibrous cords that anchor the valve leaflets, often compared to the strings of a parachute) rupture. As noted in a leading veterinary echocardiography reference:

"The consequence of these abnormalities is a more or less severe MR, depending on the deformation, the degree of valvular leaflet retraction, and the condition of the chordae tendineae. Although this process affects mostly the mitral valve, the tricuspid valve and more rarely the aortic and pulmonic valves can be affected."

If these cords snap, the valve leaflet can swing freely, leading to massive, acute mitral regurgitation and rapid-onset, life-threatening congestive heart failure.

Once a pet has developed congestive heart failure, the prognosis is generally guarded to poor, requiring lifelong, diligent medical management. However, with modern veterinary therapies, many dogs can still enjoy several months to over a year of good quality life after their first episode of heart failure.

As noted, myxomatous mitral valve disease is rare in cats. Because of this, long-term prognosis data in this species is limited, and feline cases must be monitored closely on an individual basis, as their response to standard therapies can be less predictable than in dogs.

Prevention

Because myxomatous mitral valve disease is a degenerative, genetically influenced condition, there is no known way to prevent its onset. You cannot prevent the valve from undergoing these structural changes through diet, exercise, or lifestyle modifications.

However, proactive management is highly effective. For predisposed breeds, annual veterinary examinations starting in young adulthood are essential for early detection. If you are purchasing a puppy from a breed prone to MMVD, ask the breeder for proof of cardiac clearance (such as OFA heart certifications) for both parents. Responsible breeders screen their breeding dogs annually using echocardiography or veterinary auscultation to reduce the transmission of the genetic factors responsible for this disease.

When to call your vet

If your pet has been diagnosed with a heart murmur or MMVD, you must monitor them closely at home. One of the most effective ways to do this is by measuring their Resting Respiratory Rate (RRR). While your pet is sound asleep, count the number of breaths they take in one minute (one breath is one rise and fall of the chest). A normal resting respiratory rate is typically under 30 breaths per minute. If you notice a steady increase in this rate over several days, contact your veterinarian.

You should contact your veterinarian immediately or seek emergency veterinary care if you observe any of the following red flags:

• Your pet's resting respiratory rate is consistently above 30 to 40 breaths per minute.
• Your pet is struggling to breathe, breathing with an open mouth, or stretching their neck out to breathe.
• Your pet's gums or tongue appear blue, purple, or pale gray instead of a healthy pink.
• Your pet experiences a fainting spell, collapses, or suddenly becomes too weak to stand.
• Your pet has a new or worsening cough, especially if it is moist or productive.
• Your pet is extremely restless at night, unable to find a comfortable sleeping position.

For specific breeds

While any dog can develop myxomatous mitral valve disease, certain breeds carry a significantly higher risk. If you own one of these breeds, you should be particularly vigilant about routine veterinary checkups and cardiac screening:

• Cavalier King Charles Spaniel (highest risk, polygenic inheritance)
• Doberman Pinscher
• Boston Terrier
• Airedale Terrier
• Whippet
• Newfoundland
• Pekingese
• Pomeranian
• Poodle (including Miniature and Toy Poodles)
• [Wire Fox Terrier](/p/breeds/wirefoxterrier_dog) and Fox Terrier
• Chihuahua
• Dachshund
• [English Cocker Spaniel](/p/breeds/englishcockerspaniel_dog) and Cocker Spaniel
• German Shepherd Dog
• Great Dane
• Irish Wolfhound
• Miniature Pinscher
• Miniature Schnauzer
• Shih Tzu
• Lhasa Apso

Sources

• Clinical Echocardiography of the Dog and Cat, page 155
• Internal Medicine (5th Edition), pages 149, 151