Chapter 62: Mitral Valve Dysplasia

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Mitral Valve Dysplasia



Appropriate opening and closing of the mitral valve allows unimpeded left ventricular filling and prevents valvular regurgitation. These functions depend on the integrated activity of all anatomic components of the mitral valve apparatus. Disruption or malformation of any of these components, including the mitral leaflets, the chordae tendineae, the mitral annulus, the left atrial wall, the papillary muscles, or the left ventricular wall, may produce valve dysfunction. Although acquired degenerative valve disease is the most common cause of mitral valve dysfunction in dogs, it should be recognized that mitral valve dysplasia (MVD) is a common form of congenital heart disease in both dogs and cats. Currently MVD may have surpassed ventricular septal defect as the most common congenital anomaly in cats. Alterations in cases of MVD include annular enlargement; short, thick leaflets with an occasional cleft; short and stout or long and thin chordae tendineae; upward malposition of atrophic or hypertrophic papillary muscles; and insertion of one papillary muscle directly into one or both leaflets (Liu and Tilley, 1975). Animals that appear to be overrepresented include cats of all breeds, Great Danes, German shepherds, bull terriers, golden retrievers, Newfoundlands, dalmatians, and Mastiffs (Oyama et al, 2005). A recent study suggests that male cats as well as Siamese and Siamese-cross cats are more commonly affected with supravalvular mitral stenosis (Campbell and Thomas, 2012).



Pathophysiology


The pathophysiologic consequences of MVD relate to (1) systolic regurgitation of blood from the left ventricle into the left atrium, (2) impaired left ventricular diastolic filling across a stenotic mitral valve, or (3) obstruction to left ventricular ejection in cases of inappropriate systolic displacement of the mitral valve into the left ventricular outflow tract. More than one functional disturbance can occur in the same animal. Like acquired degenerative mitral valve disease, congenital insufficiency of the mitral valve produces volume overload of the left atrium and left ventricle. The two primary determinants of the volume of insufficiency are the regurgitant orifice area and the left ventricular–to–left atrial pressure gradient. Therefore animals with a large regurgitant orifice area and those that must generate greater left ventricular pressures (e.g., due to MVD complicated by subaortic stenosis) experience greater hemodynamic consequences than animals with small leaks and unimpeded left ventricular ejection. As the left atrial, pulmonary venous, and pulmonary capillary pressures rise secondary to the volume overload, left-sided heart failure develops with accumulation of fluid within the pulmonary interstitium and alveoli.


If the valvular dysfunction is manifested primarily as stenosis, it produces a different range of pathophysiologic consequences. Lesions that restrict pulmonary venous return and left atrial emptying, including mitral stenosis, supravalvular mitral stenosis, and cor triatriatum sinister, are accompanied by elevated left atrial and pulmonary venous pressures. The severity of this pressure increase depends on the resistance of the stenotic valve and the volume of transmitral flow, which increases during exercise. In the initial stages of the disease the increased pressure maintains an adequate gradient for left ventricular filling. This elevated pressure gradient contributes to left atrial enlargement and hypertrophy, pulmonary venous congestion, and pulmonary edema because of increased pulmonary capillary hydrostatic pressure. The chronic hypoxia associated with lack of alveolar ventilation, and perhaps responses related to elevated left atrial pressures, may produce reactive pulmonary arterial vasoconstriction in an attempt to diminish ventilation-perfusion mismatch. Ultimately, right ventricular dysfunction and right-sided heart failure may develop from the pressure overload associated with long-standing pulmonary hypertension.


The third alteration that may accompany MVD is systolic displacement of the anterior mitral valve leaflet, a chordae tendineae, or a papillary muscle into the left ventricular outflow tract. Although systolic anterior motion (SAM) or dynamic left ventricular outflow tract obstruction is most frequently associated with hypertrophic cardiomyopathy, it also may stem from mitral dysplasia independent of septal hypertrophy. Several mechanisms have been hypothesized for this association, including (1) a decrease in the ability of the papillary muscles to restrain the valve posteriorly, and (2) an interposition of the leaflets anteriorly into the outflow stream, which then propels them anteriorly into the outflow tract, and a geometry for mitral valve coaptation that favors SAM (Levine et al, 1995). Consequences of SAM include increases in systolic left ventricular pressure, wall tension, and myocardial work (promoting concentric hypertrophy); increased myocardial oxygen demand; reduced coronary perfusion pressure as aortic diastolic pressure falls and left ventricular diastolic pressure rises; and mitral regurgitation caused by incomplete valve closure (Sherrid, 1998).



Diagnosis



History and Physical Examination


As is true for many forms of congenital heart disease, animals with MVD commonly are identified before the development of clinical signs because of abnormal findings on cardiac auscultation during the initial veterinary examination. The absence of clinical signs often continues for a variable period of time until owners begin to recognize evidence of left-sided heart failure (e.g., coughing, tachypnea, restlessness, and exercise intolerance). Uncommonly, animals with mitral stenosis also may display evidence of right-sided heart failure (e.g., abdominal distention secondary to ascites formation and tachypnea resulting from pleural effusion). Severe mitral stenosis can be associated with recurrent bouts of flash pulmonary edema or hemoptysis. Less common historical complaints may include syncope, and on occasion animals may die suddenly without any history of clinical signs.


The wide range of phenotypic expressions and pathophysiologic consequences of MVD produce a variety of abnormal physical examination findings. Regardless of whether the valve primarily is insufficient or stenotic or displays SAM, the most common auscultatory abnormality is a left apical systolic murmur of mitral valve regurgitation. Careful auscultation in animals with mitral stenosis may identify a left apical diastolic rumbling murmur of mitral valve stenosis, whereas patients with SAM often have a left-sided systolic murmur that varies in intensity directly with the heart rate (and sympathetic tone) and some exercise or other provocation may be required for the abnormality to be identified clearly. Additional abnormal cardiac auscultatory findings may include S3 or S4 gallops, a variety of supraventricular or ventricular arrhythmias, and on very rare occasions an audible systolic click or opening snap. Animals with pulmonary edema may have increased bronchovesicular sounds or crackles, whereas those with pleural effusion may have muffled heart and lung sounds. Jugular venous distention or pulsation, hepatojugular reflux, ascites, and hepatomegaly may signify the presence of right-sided heart failure.

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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Chapter 62: Mitral Valve Dysplasia
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