Congenital diseases of the dog and cat (Proceedings)

Congenital Disease

Congenital cardiac defects are those present at birth, and are most often from a genetic cause. In addition to genetic causes, they can also be from infectious, environmental, pharmaceutical, or a variety of causes. Most congenital murmurs present as a mid to high range murmur ausculted on physical exam, but others can be harder to detect. Evaluating these congenital defects is important, as some can be corrected or treated, giving the patient a longer lifespan.

The prevalence of congenital cardiac disease in dogs is thought to be about 0.68%. In cats, this number is about 0.2%.

The most common congenital defect in dogs is patent ductus arteriosus (PDA). PDA is detected by ausculting a loud continuous murmur in the left axilla. This murmur is frequently loud enough to be felt as a palpable thrill. The examiner must listen far forward on the thorax to auscult the murmur, as the location of the murmur can be quite focal. Occasionally, the continuous nature of the murmur is missed and only a systolic mitral murmur is detected until chest radiographs or an echocardiogram is performed. Hyperkinetic, or "water-hammer," pulses are another feature of PDA. A variety of breeds, including Border Collies, Shetland Sheepdogs, German Shepherd Dogs, Chihuahuas, Yorkshire Terriers, etc, are reportedly affected.

PDA is a failure of the ductus arteriosus to close after birth. During fetal circulation, oxygenated blood in the pulmonary artery is shunted to the descending aorta to the body, bypassing the lungs. At birth, several factors work to close the ductus, and separate pulmonary from systemic circulation. If the ductus remains open, or patent, blood continually shunts from the higher pressure systemic circulation to the lower pressure pulmonary circulation. The result is an overload of the pulmonary arteries and veins, and subsequent overload of the left side of the heart. Congestive heart failure and occasionally arrhythmias, including atrial fibrillation and ventricular premature contractions, will result if left untreated.

Radiographs and echocardiography are required to evaluate the patient for correction. Typical radiographic features of PDA are a "ductal bump", as well as evidence of left ventricular and left atrial enlargement. Pulmonary overcirculation is often evident on radiographs, as well. Echocardiography generally reveals similar left atrial and ventricular enlargement. The diagnosis of PDA, however, is based on continuous turbulent flow within the pulmonary artery. If electrocardiography is utilized, the most common change is an increase in R-wave amplitude, indicating left ventricular enlargement.

The treatment of PDA involves occluding or ligating the patent ductus. Historically, this has involved open chest surgery to ligate the vessel. Recent advancements in catheter procedures in dogs have allowed veterinary cardiologists to treat PDA more easily and with less peri-operative risk and pain. These include a variety of different devices, including transcatheter embolic coils and nitinol devices. The Amplatz Canine Ductal Occluder, designed by AGA and tested at the University of Minnesota, has become the standard of therapy for PDA. This involves passing a catheter through the femoral artery to the level of the ductus. Angiography allows for visualization of the ductus, and it can be measured to determine appropriate ACDO device size. The device is then delivered at the site of the patent ductus, also through a catheter. ACDO placement has been reported to be more successful at fully occluding the ductus than ligation and coil placement. Patients as small as 3 kg have been treated with the ACDO.

Another complication of a patent ductus arteriosus is right to left shunting. The patient maintains high vascular resistance in the pulmonary circulation, which results in pulmonary hypertension higher than systemic pressures. Because of this pressure differential, deoxygenated blood is shunted from the pulmonary artery to systemic circulation. A murmur is not typically ausculted with R to L PDA, but the second heart sound may be split. Differential cyanosis can be detected by examining the caudal mucus membranes, and polycythemia may be seen on routine bloodwork. On radiographs, an enlarged pulmonary artery as well as right ventricular enlargement are typical findings. On echocardiography, right ventricular hypertrophy is a common feature. However, the patent ductus may be difficult to detect. To confirm the diagnosis, contrast echocardiography can be utilized, where agitated saline is injected into a peripheral vessel, and the abdominal aorta is examined ultrasonagraphically for the presence of microbubbles.

Pulmonic stenosis (PS) is the next most common congenital cardiac defect in dogs. PS is infrequently seen in cats. It is seen as a narrowing in the right ventricular outflow tract. The valve itself may be affected, with the valve leaflets adhered to one another or dysplastic. The annulus, or supporting structure of the valve, may also be narrowed, or hypoplastic. Additionally, another type of PS in which an aberrant coronary artery wraps around the pulmonary artery causing stenosis has been found in English Bulldogs, Boxers, and American Staffordshire Terriers.

Features of physical examination include a moderate to loud systolic murmur at the left heart base, sometimes accompanied by a palpable thrill. Patients with severe PS may have syncopal episodes, exercise intolerance, and right sided congestive heart failure. Sudden death may sometimes result as a consequence of PS.

Radiographic evidence of PS typically shows right ventricular enlargement and dilation of the pulmonary artery. Echocardiographically, right ventricular hypertrophy is commonly seen, as well as septal flattening of the left ventricle in cross-section. High velocity, turbulent flow can be seen, typically arising at the level of the valve. Severity of the disease is graded by using the Modified Bernoulli equation (pressure=4 x velocity^2) to determine the pressure difference between the right ventricle and pulmonary artery. Mild stenosis is below 50 mmHg; moderate is 50-80 mmHg; severe is greater than 80 mmHg. Evidence of right ventricular enlargement may also be detected on ECG.

Treatment of PS may be surgical, medical, or per-catheter. Surgery involves creating a conduit using a tubular graft, or placing a patch graft to create an aneurysm around a bisected valve. Per-catheter treatment of PS is through balloon valvuloplasty. This catheter procedure is most favored if the stenosis is purely valvular, but may also be utilized with some success with annular hypoplasia. Historically, balloon valvuloplasty has been contraindicated in patients with an aberrant coronary artery, as it could rupture the coronary artery, causing death. However, this has been attempted in a few instances in the hands of experienced cardiologists without disastrous results.

Aortic stenosis is another common congenital defect, especially in certain large breed dogs, including Newfoundlands, Boxers, Golden Retrievers, Rottweilers, and other large breeds. Because there seems to be breed predisposition, AS is believed to also be genetic. The feature of AS is a narrowed left ventricular outflow tract, either valvular or subvalvular. Left ventricular hypertrophy commonly results, and patients with severe disease may develop congestive heart failure or ventricular arrhythmias. Sudden death, likely as a result of these arrhythmias, occurs commonly with severe AS. These patients are also prone to infectious endocarditis.

On physical exam, the most common finding is a systolic murmur at the left heart base. With severe AS, a loud murmur may be also be ausculted at the right cranial location, and it may even radiate to the carotid arteries and to the dorsum of the skull. With significant AS, femoral pulses feel hypokinetic. On radiographs, the cardiac silhouette loses its cranial waist on lateral projection, and the aorta is widened on the VD or DV projection. Echocardiography reveals turbulent high pressure flow arising at the lesion in the LVOT. Aortic insufficiency may also been seen. To most accurately determine the velocity of flow through the stenotic lesion, and therefore the severity, the aorta can be evaluated from a subcostal view, which allows the probe to be as parallel to the aorta as possible.

While many cardiologists medically manage AS with beta blockers, there is no known successful surgical intervention. Researchers continue to attempt to find a correction for AS.

Valvular dysplasia affecting the mitral or tricuspid valves are also seen in both dogs and cats, though not commonly. Labrador Retrievers, in particular, are prone to tricuspid valve dysplasia (TVD), and Bull Terriers are prone to mitral valve dysplasia (MVD). Dysplasia refers to an abnormality in the formation of the valve, which results in regurgitation or stenosis. The consequence of valve dysplasia is generally congestive heart failure.

On physical examination, the most common finding is a systolic murmur. However, tricuspid valve dysplasia may not cause any murmur at all, which makes screening breeding dogs for TVD especially difficult. Medical treatment is necessary once CHF has developed, and though attempted, surgical correction of MVD or TVD is not typically successful.

Ventricular septal defects affect both dogs and cats. The septum fails to form correctly, which results in a communication between right and left ventricles. Because of the difference in pressure between the ventricles, blood passes from the left into the right.

On physical exam, small defects cause loud murmurs on the right side of the chest, often with a palpable thrill. Smaller defects actually have softer murmurs because the difference in pressure is lower. Large defects may even cause equilibration of pressures between each side of the heart, leading to cyanosis due to deoxygenated blood from the right side of the heart entering systemic circulation. Therefore, smaller defects causing loud murmurs have the best prognosis. Large hemodynamic defects cause left heart volume overload and congestive heart failure. Treating the CHF as a result of the VSD is generally the only therapy available. The location of a VSD may allow it to be corrected via a catheter procedure, but these types of VSDs are extremely rare.

Atrial septal defects are relatively uncommon in dogs and cats, but these defects have been found in certain families of dogs, especially Standard Poodles. ASDs result from a failure of the atrial septum to form properly. There is a variety of different defects that arise from the abnormal formation of the septum. The most common physical exam finding is a soft basilar murmur that is generally from relative pulmonic stenosis due to a large volume of blood passing through the RVOT. As with VSDs, the right morphology and location of ASD may allow for correction, but this is rare. Typically, no treatment is necessary. ASDs are often a sequela of TVD or PS due to right sided volume or pressure overload.

There are a large variety of other more rare defects, which may occur alone or in combination with other congenital anomalies. Tetralogy of Fallot, for instance, is a four part defect with a large over-riding aorta, VSD, right ventricular hypertrophy, and PS. Other congenital defects include endocardial cushion defects, transposition of the great vessels of the heart, and cor triatriatum dexter or sinister.