Mitral Valve Dysplasia

TL;DR. Mitral valve dysplasia is a rare, congenital heart defect in dogs and cats where the mitral valve develops abnormally, often causing blood to flow backward or struggle to enter the main pumping chamber of the heart.

Mitral valve dysplasia involves a congenital malformation of the mitral valve apparatus, which controls blood flow on the left side of the heart.

What is it?

Mitral valve dysplasia (MVD) is a congenital heart defect, meaning it is a structural abnormality that an animal is born with. The mitral valve is a critical, one-way valve located on the left side of the heart. It sits between the left atrium (the chamber that receives oxygen-rich blood from the lungs) and the left ventricle (the main pumping chamber that sends blood out to the rest of the body).

In a healthy pet, the mitral valve acts as a tight, secure door. When the left ventricle contracts to pump blood to the body, the mitral valve snaps shut, preventing blood from leaking backward into the left atrium. The valve itself is a complex apparatus consisting of thin flaps of tissue (leaflets), anchored by strong, thread-like cords (chordae tendineae) to the heart muscle walls (papillary muscles).

In pets with mitral valve dysplasia, this entire apparatus is malformed. The leaflets may be too thick, too short, or fused together. The supporting cords may be abnormally short, long, or entirely missing. This malformation typically prevents the valve from closing properly, leading to a condition called mitral regurgitation, where blood leaks backward into the left atrium with every heartbeat. Less commonly, the malformed valve may restrict blood flow from entering the ventricle, a condition known as mitral stenosis.

As a leading textbook on veterinary echocardiography notes:

"Mitral valve dysplasia (MVD) is a congenital anomaly of the mitral valve apparatus. It usually leads to MR. Mitral stenosis (MS) and left ventricular outflow obstruction due to systolic an..." — Clinical Echocardiography of the Dog and Cat, p. 297

When blood leaks backward or is blocked from flowing smoothly, the left side of the heart must work much harder to meet the body's demands. Over time, this chronic strain causes the left atrium and left ventricle to stretch and enlarge. Eventually, the pressure backs up into the blood vessels of the lungs, leading to fluid accumulation (pulmonary edema) and left-sided congestive heart failure.

Causes & risk factors

Mitral valve dysplasia is caused by abnormal embryonic development of the heart chambers and valves during pregnancy. Because it is a congenital defect, it is not caused by lifestyle, diet, or environmental factors after birth.

While the condition is relatively rare in both dogs and cats, it is a well-documented genetic concern in several breeds. In cats, congenital heart defects are generally uncommon, and mitral valve dysplasia is rarely diagnosed. When it does occur, it is typically identified in young kittens or young adult cats.

In dogs, certain breeds show a higher predisposition to this congenital malformation. It is important to distinguish congenital mitral valve dysplasia from the much more common "acquired" mitral valve disease (myxomatous mitral valve degeneration) that develops in older, small-breed dogs. While older dogs develop a degenerative form of the disease over time, pets with mitral valve dysplasia are born with the defect.

According to a prominent veterinary internal medicine reference:

"Valvular regurgitation is the predominant functional abnormality, and it may be severe; the pathophysiology and sequelae resemble those of acquired mitral regurgitation... Mitral valve stenosis occurs uncommonly; the ventricular inflow obstruction increases LA pressure and can precipitate the development of pulmonary edema." — Internal Medicine, 5th Edition, p. 141

Signs to watch for

The clinical signs of mitral valve dysplasia depend heavily on the severity of the valve malformation and whether the pet has developed congestive heart failure. In mild cases, pets may show no outward signs of illness for years. In severe cases, signs can appear in early puppyhood or kittenhood.

Key symptoms to watch for include:

• Holosystolic murmur (Cardinal): A distinct, continuous heart murmur heard by your vet through a stethoscope during the contraction phase of the heartbeat. This is often the very first sign detected, sometimes during a routine puppy or kitten exam.
• Coughing (Common): A persistent cough, which occurs as the left atrium enlarges and presses against the airways, or as fluid begins to accumulate in the lungs.
• Tachypnea (Common): An abnormally fast breathing rate, especially when the pet is resting or sleeping.
• Dyspnea (Common): Labored, heavy, or difficult breathing. This is a sign of respiratory distress.
• Reduced exercise tolerance (Common): Tiring out quickly during play, walks, or normal activities.
• Inappetence (Common): A loss of appetite or reluctance to eat, often associated with the increased effort required to breathe.
• Atrial fibrillation (Common): An irregular, rapid heart rhythm that occurs as the left atrium becomes severely stretched and electrical signals in the heart become disorganized.
• Ascites (Occasional): Fluid accumulation in the abdomen, causing a pot-bellied appearance. This occurs if the heart failure progresses to affect the right side of the heart as well.
• Syncope (Occasional): Fainting or temporary loss of consciousness, caused by a sudden drop in blood flow and oxygen delivery to the brain.
• Pulmonary edema (Occasional): Fluid buildup inside the lungs, which is a life-threatening emergency.

Reduced exercise tolerance and rapid or heavy breathing are common signs of progressive heart disease in dogs.

How vets diagnose it

Diagnosing mitral valve dysplasia requires a systematic veterinary evaluation, starting with a physical exam and progressing to advanced cardiac imaging.

• Physical Examination: Your vet will use a stethoscope to listen to your pet's heart and lungs. The presence of a loud, holosystolic murmur on the left side of the chest is a strong indicator of mitral valve disease. Your vet will also assess your pet's pulse quality, gum color, and breathing effort.
• Echocardiography (Gold Standard): An ultrasound of the heart is the definitive test for mitral valve dysplasia. It allows a veterinary cardiologist to visualize the physical structure of the mitral valve, the chordae tendineae, and the papillary muscles. The vet can see exactly how the valve is malformed and measure the size of the heart chambers.
• Doppler Echocardiography: This specialized ultrasound technique color-codes the direction and speed of blood flow through the heart. It allows the vet to confirm and quantify the severity of mitral regurgitation (leaking backward) or mitral stenosis (narrowed flow).
• Thoracic Radiography (Chest X-rays): X-rays are essential for evaluating the overall size and shape of the heart and, crucially, for checking the lungs. X-rays are the primary tool used to determine if fluid has begun to accumulate in the lungs (pulmonary edema).
• Electrocardiography (ECG): An ECG records the electrical activity of the heart. It is used to detect abnormal heart rhythms, such as atrial fibrillation, which can develop as a consequence of severe left atrial enlargement.
• Cardiac Catheterization: In rare cases, specialized referral centers may perform cardiac catheterization. This invasive procedure involves threading a tiny catheter into the heart chambers to measure pressures directly, though it is rarely required given the accuracy of modern echocardiography.

Echocardiography with Doppler is the gold standard diagnostic tool for evaluating congenital mitral valve malformations.

Treatment options

Because mitral valve dysplasia is a structural birth defect, there is no medical cure. Treatment is focused on managing the symptoms, supporting heart function, reducing the workload on the heart, and treating congestive heart failure if it develops.

Your vet may prescribe medications from several different classes, depending on your pet's specific symptoms and the stage of their disease:

Diuretics and Fluid Management

• Spironolactone: This is an aldosterone antagonist and potassium-sparing diuretic. It helps prevent the body from retaining excess sodium and water, and it also helps protect the heart muscle from progressive scarring (fibrosis) caused by chronic heart disease.
• Hydrochlorothiazide: A thiazide diuretic that may be added to the treatment plan if your pet's fluid retention becomes difficult to manage with standard medications alone.

Vasodilators (Reducing Heart Workload)

• Amlodipine: A calcium channel blocker that dilates systemic blood vessels. By lowering the resistance in the blood vessels throughout the body, it makes it easier for the left ventricle to pump blood forward, reducing the amount of blood that leaks backward through the malformed mitral valve.
• Hydralazine: Another potent vasodilator that works directly on the blood vessels to reduce the workload (afterload) on the failing left ventricle.
• Sildenafil: A selective vasodilator (phosphodiesterase type 5 inhibitor) used specifically if your pet develops pulmonary hypertension (high blood pressure in the blood vessels of the lungs) as a consequence of chronic left-sided heart failure.

Heart Rate and Rhythm Control

• Digoxin: A cardiac glycoside that helps slow and control the heart rate, particularly in pets that have developed atrial fibrillation or other rapid heart arrhythmias due to severe atrial enlargement.

Cough Suppressants and Airway Support

• Hydrocodone & Butorphanol: These are opiate-based cough suppressants (antitussives). They are used to manage chronic, harsh coughing caused by an enlarged heart pressing physically against the main airways (bronchi).
• Tramadol: An opiate-like medication that may be used for mild sedation or comfort if a pet is highly anxious due to breathing difficulties.
• Theophylline & Aminophylline: These are bronchodilators (methylxanthines) that help relax and open the airways, making it easier for pets with heart-related airway compression to breathe.

Prognosis

The prognosis for pets with mitral valve dysplasia is highly variable and depends entirely on the severity of the valve malformation and how rapidly the heart chambers enlarge.

Pets with mild to moderate mitral valve dysfunction can often live comfortable, active lives for many years, sometimes reaching a normal lifespan with appropriate monitoring and medical management.

However, for pets diagnosed with severe mitral regurgitation or significant mitral stenosis, the prognosis is generally poor. These animals often develop progressive, severe left-sided congestive heart failure early in life, which can be difficult to manage long-term.

Because this condition is exceptionally rare in cats, long-term prognosis data for feline patients is limited, and much of the clinical guidance is extrapolated from canine cases. Regular follow-up appointments with a veterinary cardiologist are essential to monitor the progression of the disease and adjust medications as needed.

Prevention

Because mitral valve dysplasia is a congenital defect, it cannot be prevented through lifestyle changes, diet, vaccines, or environmental management.

The only effective means of prevention is responsible breeding practices. Dogs and cats diagnosed with congenital mitral valve dysplasia should never be bred.

For breeds known to be predisposed to the condition, pre-breeding screenings—including thorough veterinary physical exams and, ideally, screening echocardiograms by a board-certified veterinary cardiologist—are highly recommended to ensure that affected animals do not pass the genetic predisposition on to future generations.

When to call your vet

If your pet has been diagnosed with mitral valve dysplasia, you must monitor their resting respiratory rate at home. Count your pet's breaths for one full minute while they are sound asleep. A normal resting respiratory rate is typically under 30 breaths per minute.

Seek emergency veterinary care immediately if you observe any of the following red flags:

• Your pet is struggling to breathe, breathing with an open mouth, or stretching their neck out to breathe.
• Your pet's resting respiratory rate consistently exceeds 40 breaths per minute.
• Your pet faints, collapses, or suddenly loses consciousness (syncope).
• Your pet's gums appear pale, blue, or purple instead of a healthy pink.
• Your pet is extremely weak, lethargic, or unable to stand.

Contact your vet during regular business hours if you notice a gradual increase in coughing, a progressive decline in their ability to exercise, or a sudden loss of appetite.

For specific breeds

Several breeds have a documented predisposition to congenital mitral valve dysplasia. If you own one of these breeds, it is highly recommended to have your puppy or kitten's heart thoroughly evaluated by a veterinarian during their initial checkups.

Predisposed breeds include:

• Large and Giant Breeds: Mastiff, Rottweiler, German Shepherd Dog, Golden Retriever, Newfoundland, Great Dane, Dalmatian, and Bull Terrier.
• Small and Medium Breeds: Poodle, Shih Tzu, Cocker Spaniel, Cavalier King Charles Spaniel, Chihuahua, Dachshund, Lhasa Apso, Miniature Pinscher, Miniature Schnauzer, and Whippet.

Sources

• Small Animal Critical Care Medicine, p. 264
• Internal Medicine, 5th Edition, p. 141
• Clinical Echocardiography of the Dog and Cat, p. 297

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