A: Atrioventricular Valve Dysplasia

Atrioventricular Valve Dysplasia

BASICS

DEFINITION

A congenital malformation of the mitral or tricuspid valve apparatus.

PATHOPHYSIOLOGY

Atrioventricular valve dysplasia can result in valvular insufficiency, valvular stenosis, or dynamic outflow tract obstruction, depending on the anatomic abnormality. AVVD may occur alone or in association with abnormalities of the ipsilateral outflow tract (e.g., valvular or subvalvular aortic or pulmonic stenosis). It is not uncommon for mitral and tricuspid valve dysplasia to occur together in the same patient.
Valvular insufficiency results in dilation of the ipsilateral atrium, eccentric hypertrophy of the associated ventricle, and, if sufficiently severe, signs of CHF. Cardiomyopathy of chronic volume overload and elevated atrial pressures are the end result culminating in pulmonary congestion if the mitral valve is affected and systemic congestion if the tricuspid valve is affected.
Valvular stenosis results in atrial dilation and hypertrophy and, when severe, hypoplasia of the receiving ventricle. Tricuspid valve stenosis results in elevated right atrial pressure and systemic congestion if pressures exceed 15–20 mm Hg. Right-to-left shunting may occur if there is an atrial septal defect or patent foramen ovale. Mitral valve stenosis results in elevated pulmonary capillary pressure and pulmonary edema if pressures exceed 25–30 mm Hg. Pulmonary hypertension is a common complicating condition in animals with mitral valve stenosis.
Outflow tract obstruction may develop from defects that translocate the anterior leaflet to a position closer to the interventricular septum. Concentric left ventricular hypertrophy develops in proportion to the severity of the obstruction.

SYSTEMS AFFECTED

Cardiovascular—inflow obstruction due to valvular stenosis and chronic volume overload from valvular insufficiency result in elevated pulmonary (left AV valve) or systemic (right AV valve) venous pressures. Signs of low cardiac output develop if the lesion is sufficiently severe. Concentric left ventricular hypertrophy develops secondary to dynamic outflow obstruction.
Respiratory—pulmonary edema may develop secondary to mitral stenosis or mitral valve insufficiency. Pulmonary hypertension is a common complication in animals with mitral stenosis.
Neurologic—collapse and loss of consciousness, most often during physical exertion, may occur with severe disease due to low cardiac output and hypotension. Collapse in animals with dynamic outflow obstruction is most often due to ventricular arrhythmia.

GENETICS

Tricuspid valve dysplasia is inherited as an autosomal recessive trait in Labrador retrievers. Heritability and pattern of inheritance not established in other breeds.

INCIDENCE/PREVALENCE

These are common congenital cardiac anomalies in cats (17% of reported congenital cardiac defects in one study). Less frequently diagnosed in dogs.

SIGNALMENT

Species

Dogs and cats

Breed Predilections

Tricuspid valve dysplasia—increased risk for Labrador retrievers, German shepherd dogs, Great Pyrenees, possibly Old English sheepdogs. Also common in cats.
Mitral valve dysplasia—increased risk in bull terriers, Newfoundlands, Labrador retrievers, Great Danes, golden retrievers, Dalmatians, and Siamese cats. Perhaps the most common congenital heart defects of cats. Mitral valve malformations often are noted in cats with hypertrophic cardiomyopathy.

Mean Age and Range

Variable; signs are most often manifest within the first few years after birth.

Predominant Sex

Males are more likely to evidence heart failure.

SIGNS

Historical Findings

Exercise intolerance is the most common problem in dogs and cats with AV valve dysplasia.
Abdominal distention, weight loss, and stunting may be observed with severe tricuspid valve dysplasia.
Labored respiration is common in dogs or cats with mitral valve dysplasia.
Syncope and collapse if critical mitral or tricuspid valve stenosis, severe outflow tract obstruction, an associated arrhythmia, or heart failure from AV valvular insufficiency.

Physical Examination Findings

Mitral Valve Dysplasia

A holosystolic murmur is heard over the cardiac apex on the left. With severe disease the murmur is accompanied by a thrill or gallop heart sounds. A soft diastolic murmur may be present in the same location in animals with mitral stenosis but many affected animals have no audible murmur. A systolic ejection murmur that intensifies with exercise or excitement is audible in animals with dynamic outflow tract obstructions.
Evidence of left heart failure—tachypnea, increased respiratory efforts, pulmonary crackles, and cyanosis in animals with severe defects.

Tricuspid Valve Dysplasia

A holosystolic murmur is heard over the cardiac apex on the right. With severe disease the murmur is accompanied by a thrill or gallop heart sounds. Silent tricuspid regurgitation is well documented in cats and is attributable to a large regurgitant orifice and laminar regurgitant flow. Distention and pulsation of the external jugular veins may be evident.
Evidence of right heart failure—ascites and, more rarely, peripheral edema with severe malformations.

DIAGNOSIS

DIFFERENTIAL DIAGNOSIS

With the noted exception of the age of onset, congenital AV valvular insufficiency resembles acquired degenerative AV valve insufficiency with respect to historical findings, physical examination abnormalities, and clinical sequelae.
The right-sided murmur of tricuspid insufficiency is sometimes confused with the right-sided murmur of a ventricular septal defect.
Ascites caused by silent tricuspid regurgitation or tricuspid valve stenosis is often attributed to pericardial effusion, hepatic disease, or obstruction of the caudal vena cava.
Dogs and cats with cor triatriatum share many of the clinical features of AV valve stenosis.
There is no certain way to distinguish mitral valve dysplasia producing outflow tract obstruction and the obstructive form of cardiomyopathy. If the obstruction can be abolished with a beta blocker and left ventricular hypertrophy resolves, it is likely that the primary abnormality was mitral valve dysplasia.

CBC/BIOCHEMISTRY/URINALYSIS

Usually normal

IMAGING

Radiographic Findings

Mitral Valve Dysplasia

Left atrial and left ventricular enlargement with valvular insufficiency. Isolated left atrial enlargement with valvular stenosis. Mild left atrial enlargement with dynamic outflow obstruction.
Evidence of left heart failure—distended pulmonary veins, interstitial or alveolar edema in severe cases.

Tricuspid Valve Dysplasia

Right atrial and right ventricular enlargement with valvular insufficiency. Cardiac silhouette may appear globoid with pronounced enlargement. Isolated right atrial enlargement with valvular stenosis.
Evidence of right heart failure—dilated caudal vena cava, hepatosplenomegaly, or ascites in severe cases.

Echocardiography

Mitral Valve Dysplasia

Valvular insufficiency results in left atrial dilation and eccentric hypertrophy of the left ventricle. The papillary muscles are typically flattened and displaced dorsally. Chordae tendineae are often short and thickened. Doppler echocardiography demonstrates a high velocity retrograde systolic transmitral jet and modestly increased transmitral inflow velocities.
Mitral stenosis results in left atrial dilation while the left ventricular dimensions are normal or small. The valve leaflets are often thickened, relatively immobile, and often fused. Doppler echocardiography demonstrates a high velocity transmitral diastolic jet with a reduced EF slope. There may also be evidence of concurrent mitral insufficiency and/or secondary pulmonary hypertension. Exclude the possibility of cor triatriatum sinister.
Dynamic left ventricular outflow obstruction is characterized by systolic motion of the anterior mitral valve leaflet toward the interventricular septum, increased LV outflow tract velocities, and concentric left ventricular hypertrophy.

Tricuspid Valve Dysplasia

Valvular insufficiency results in right atrial dilation and eccentric hypertrophy of the right ventricle. The papillary muscles and chordae tendineae may be fused, creating a curtain-like appearance of the tricuspid valve. Doppler echocardiography demonstrates a high velocity retrograde systolic trans- tricuspid jet and modestly increased trans-tricuspid inflow velocities.
Tricuspid stenosis results in right atrial dilation with normal or small right ventricular dimensions. The valve leaflets do not open completely. Doppler echocardiography demonstrates a high velocity diastolic trans-tricuspid jet with a reduced EF slope. There may be evidence of concurrent tricuspid valve insufficiency and/or right-to-left shunting across a patent foramen ovale or associated atrial septal defect. Exclude the possibility of cor triatriatum dexter.

Cardiac Catheterization

Indicated only in those cases in which the diagnosis cannot be confirmed by echocardiography or if surgical correction is anticipated.
Mitral dysplasia— hemodynamic measurements should include left ventricular pressures, pulmonary capillary wedge pressure or direct measurement of LA pressure, pulmonary artery pressures, and, in cases of dynamic obstruction, simultaneous recording of aortic and left ventricular pressures with medical provocation. Contrast studies are best accomplished with a left ventricular injection in cases of valvular insufficiency, and direct left atrial injection via trans-septal catheterization in cases of valvular stenosis.
Tricuspid dysplasia— hemodynamic measurements should include right ventricular and right atrial pressures. Contrast studies are best accomplished with a right ventricular injection in cases of valvular insufficiency, and right atrial injection in cases of valvular stenosis.

DIAGNOSTIC PROCEDURES

Electrocardiographic Findings

Usually reflect pattern of chamber enlargement.

Severe defects may be accompanied by a variety of arrhythmias, particularly atrial premature beats, supraventricular tachycardia, or atrial fibrillation.

TREATMENT

APPROPRIATE HEALTH CARE

Inpatient treatment required for CHF.

CLIENT EDUCATION

Owners should be informed of heritability and advised against breeding.

ACTIVITY

Restricted in accordance with severity.

DIET

Sodium-restricted if overt or pending CHF.

SURGICAL CONSIDERATIONS

Valve repair or replacement is available in a few centers.
Balloon valvuloplasty is sometimes effective for valvular stenosis.

MEDICATIONS

DRUG(S) OF CHOICE

Mitral or tricuspid dysplasia with insufficiency—diuretics, angiotensin converting enzyme inhibitors, and pimobendan (0.3 mg/kg q12h) for patients with imminent or overt congestive heart failure. Furosemide (2–4 mg/kg q12–24h), enalapril (0.5 mg/kg q12h) are used to control congestion. Digoxin (0.002–0.004 mg/kg q12h) is used to control supraventricular tachyarrhythmias.
Mitral or tricuspid stenosis—diuretics to control edema. Furosemide (2–4 mg/kg q12–24h) dose adjusted to resolve congestion. Heart rate should be maintained near 150 bpm using digoxin (0.002–0.004 mg/kg q12h), a calcium channel blocker such as diltiazem (1–1.5 mg/kg q8h), or a beta-receptor blocking drug, such as atenolol (0.5–1.5 mg/kg q12–24h).
Dynamic outflow tract obstruction—titrate a beta-receptor blocking drug, such as atenolol (0.5–1.5 mg/kg q12–24h), to abolish or diminish severity of outflow obstruction.

PRECAUTIONS

Standard patient monitoring for cardiac medication side effects (e.g., digitalis toxicity, azotemia).

FOLLOW-UP

PATIENT MONITORING

Recheck yearly if no signs of heart failure.
Recheck at a minimum of every 3 months if signs of CHF (thoracic radiographs, ECG, and echocardiography advisable).

PREVENTION/AVOIDANCE

Do not breed affected animals.

POSSIBLE COMPLICATIONS

Congestive heart failure—left-sided with mitral valve dysplasia; right-sided with tricuspid valve dysplasia.
Collapse or syncope with exercise.
Paroxysmal supraventricular tachycardia or atrial fibrillation with severe disease.

EXPECTED COURSE AND PROGNOSIS

Depends on severity of underlying defect
Guarded to poor with serious defects

MISCELLANEOUS

ASSOCIATED CONDITIONS

Mitral valve dysplasia commonly accompanies valvular or subvalvular aortic stenosis as well as TVD.
Tricuspid valve dysplasia commonly accompanies pulmonic stenosis as well as MVD.

PREGNANCY/FERTILITY/BREEDING

Should be avoided—heritable defect and possibility of causing decompensated or worsening heart failure.