Mitral valve dysplasia
In dogs as in people, the heart contains 4 chambers: 2 atria and 2 ventricles. The atrioventricular (AV) valves ensure that the blood flows from the atria to the ventricles when the heart beats. A defect in the mitral valve (the left atrioventricular valve) causes backflow of blood into the left atrium, also called mitral regurgitation or mitral valve insufficiency. This type of "leak" means the workload on the heart is increased to keep up with the demands of the circulation in the body. While mitral valve insufficiency is the most common adult-onset heart problem in older dogs, it may also occur from birth if the heart valve is malformed during embryonic growth. The term "mitral valve dysplasia" refers to this exact situation, where from the moment a pup is born, its mitral valve does not seal properly, and therefore imposes an increased workload on the heart.
Mitral valve dysplasia does occur more commonly in certain breeds, implicating a genetic basis. The exact pattern of inheritance has not been defined.
The importance and impact of mitral valve dysplasia depend mainly on the degree of malformation of the heart valve. A mild degree of mitral valve dysplasia usually means no symptoms and a normal life, whereas severe mitral valve dysplasia can produce life-threatening symptoms even in the first year of life. Therefore, if mitral valve dysplasia is suspected, it is important to neither overreact nor underreact because dogs may do better or worse than expected: many dogs with mitral valve dysplasia do not show any outwardly visible symptoms and a good cardiac evaluation is necessary to determine whether there is cause for concern.
In the vast majority of cases, mitral valve dysplasia first emerges as a consideration based on the detection of a heart murmur with the stethoscope during a visit to the veterinarian. Most dogs show no external symptoms initially. Since may different situations can cause heart murmurs, it is important to investigate heart murmurs in order to be able to confirm or eliminate mitral valve dysplasia as the underlying cause. The best tests for assessing the possibility of mitral valve dysplasia are thoracic radiographs (chest X-rays) and an echocardiogram (also called ultrasound of the heart or cardiac sonogram). These tests are noninvasive and can be done on an outpatient basis. It is often necessary to have a referral to a veterinary cardiologist to have the tests done in the most reliable fashion (directories available at www.acvim.org and www.ecvim-ca.org for North America and Europe, respectively).
Mitral valve dysplasia is treated with medications, given daily at home, if a point is reached where overt symptoms such as laboured breathing start to occur and radiographs (X-rays) confirm that this is due to the heart's dysfunction. These symptoms can happen over time, and the symptoms occur only in moderate or severe cases, when the circulation can become affected to such a degree that pulmonary edema (fluid congestion in the lungs) occurs. Therefore, medications like diuretics help to eliminate retained lung fluid if it is present. Surgical replacement of the heart valve with an artificial, prosthetic valve, as is done in humans, is not feasible currrently in dogs.
- MURMUR: soft to loud, harsh, regurgitant, holosystolic - loudest at left apex (5th to 6th intercostal space) over the mitral valve area.
- ELECTROCARDIOGRAM: commonly shows evidence of left atrial enlargement (wide P wave) with or without evidence of left ventricular enlargement (depends on severity). Atrial arrhythmias, especially atrial fibrillation, are common.
- RADIOGRAPHS: evidence of moderate to marked left atrial enlargement with or without left ventricular enlargement. Pulmonary veins are often enlarged.
- ECHOCARDIOGRAPHY: confirmatory test of choice. Typically shows abnormal location, shape, motion, or attachment of the valve apparatus in moderate or severe cases. Doppler assessment, especially colour-flow Doppler, will show an abnormal flow (regurgitant jet or valvular stenosis or both) in all clinically-significant cases.
Affected individuals and their parents should not be used for breeding. Siblings should only be used after careful screening.
FOR MORE INFORMATION ABOUT THIS DISORDER, PLEASE SEE YOUR VETERINARIAN.
Oyama MA, Sisson DD, Thomas WP, Bonagura JD. Congenital heart disease. In Ettinger SJ, Feldman EC, eds. Textbook of Veterinary Internal Medicine, 7th ed (St. Louis, MO: Saunders Elsevier, 2010) pp. 1250-1298.
Orvalho J. Mitral valve dysplasia. In Cote E, ed. Clinical Veterinary Advisor: Dogs and Cats, 2nd ed (St. Louis, MO: Mosby Elsevier, 2011) pp. 727-728.