Jonathan A. Abbott,     Blacksburg, Virginia

Causes

Studies performed during the 1970s confirmed that SAS in Newfoundland has a genetic basis (Pyle et al, 1976). The mode of inheritance was not defined completely, but the results of planned breeding experiments suggest that SAS is either a polygenic trait or an autosomal-dominant trait that is subject to modification by other genes or environmental factors. Recently published data provide evidence that SAS in golden retrievers is familial, although the mode of inheritance was not determined (Stern et al, 2012). There is also evidence that SAS has a genetic basis in Dogue de Bordeaux (Ohad et al, 2013). Because inheritance of SAS is not, apparently, explained by simple Mendelian genetics, dogs with mild SAS may have severely affected offspring. As a result, detection of mild SAS, especially in breeding dogs, is important despite the fact that mild outflow tract obstruction usually does not have clinical consequences.

SAS is observed most often in large- and giant-breed dogs. The German shepherd, boxer, golden retriever, Newfoundland, and rottweiler all are predisposed to the development of SAS. Mongrels are less likely to develop SAS than are purebred dogs, which further suggests that canine SAS is a genetic disorder. In recent years SAS has been identified in many other breeds, including smaller breeds of dogs.

Pathologic Features

The subvalvular lesion is fibrous or fibrocartilaginous (Web Figure 66-1). In the mildest form of SAS the lesion consists of subvalvular nodules, and these cases may be clinically silent. More severe forms of SAS result in outflow tract obstruction caused by a ridge that partially or completely encircles the subvalvular left ventricular outflow tract (LVOT). The most severe forms consist of plaque or tunnel lesions that involve a large area of the LVOT.

In pathologic studies of SAS in the Newfoundland, subvalvular lesions were detected only in pups that were older than 3 weeks. Based on this finding, the lesion responsible for SAS in that breed was judged to be not strictly congenital, but rather acquired, early in life. This observation also has been made in human patients. It has been speculated that the lesion arises because congenital morphologic abnormalities of the LVOT increase shear stress and induce proliferation of cells in the LVOT (Freedom et al, 2005). Recently it was reported that the echocardiographically determined aortoseptal angle in canine patients with SAS exceeds that in unaffected dogs (Quintavalla et al, 2010). A statistical relationship between aortoseptal angle and severity of SAS also was identified. These data are consistent with findings in human beings with SAS but, in the absence of longitudinal studies, it is not certain that the relationship is causal. It is possible that the abnormal outflow tract geometry instead reflects a concurrent anomaly of aortoventricular connection. In the dog, the obstruction may progress for the first 12 months of life; this has important implications for diagnostic screening, genetic counseling, and patient prognosis.

Pathophysiology

Outflow tract stenosis impedes ventricular ejection. To maintain normal perfusion pressure distal to the stenosis, the left ventricle must generate supraphysiologic systolic pressures. As a result there is a systolic pressure difference, or gradient, across the obstruction. This pressure gradient can be measured by selective catheterization but more often is estimated from Doppler echocardiographic studies. The pressure load that results from high resistance to ventricular ejection is a stimulus for the development of left ventricular concentric hypertrophy. Concentric hypertrophy is an increase in myocardial mass that is associated with a normal end-diastolic chamber volume. As a result, the ventricular walls are thick relative to lumen size. However, a broad spectrum of ventricular morphology is observed in SAS. In some patients, wall thickness is increased in association with reduced end-diastolic ventricular volume—a geometry described as concentric remodeling—and occasionally an increase in ventricular volume results from chamber dilation associated with the development of systolic myocardial dysfunction. SAS is associated with arteriosclerotic lesions of the intramural coronary arteries. Abnormally narrow coronary vessels, large myocardial mass, and left ventricular systolic hypertension contribute to the development of myocardial ischemia, which likely is the predisposition for the ventricular arrhythmias that sometimes complicate SAS. Although a chronic left ventricular pressure load can result in the development of systolic myocardial dysfunction, this is relatively uncommon in dogs with SAS. Evaluation of echocardiographic indices of diastolic function discloses abnormalities in some patients with SAS. These abnormalities are associated with the severity of obstruction and hypertrophy and likely also the extent to which coronary perfusion is impaired.

In general, dogs with mild or moderately severe obstructions are at low risk of premature death. However, a few dogs in this category develop complications of long-standing SAS, including infective endocarditis and congestive heart failure. Dogs with severe obstruction are at much greater risk of serious sequelae. Sudden unexpected death, presumably the result of lethal ventricular tachyarrhythmia, occurs in the first 2 to 3 years of life in a substantial number of cases (Kienle et al, 1994).

Diagnostic Approach

There are two distinct goals in the evaluation of dogs suspected to have SAS: (1) identification and characterization of LVOT obstruction to provide prognostic information and guide therapy, and (2) elimination of affected dogs from the breeding population.

At the time of initial diagnosis most dogs with SAS are asymptomatic; this is true regardless of the severity of obstruction. The history in mildly or moderately affected dogs is unremarkable and reflects normal behavior and activity. In severely affected dogs exercise intolerance, weakness, and syncope are noted occasionally. When left-sided heart failure is present, signs can include coughing, dyspnea, and tachypnea.

In mildly or moderately affected dogs, a grade 1 to 4 (on a scale of 6) systolic murmur can be heard best in the left fourth to fifth intercostal spaces near the costochondral junction. In mild cases the murmur intensity typically peaks in early to mid systole and has a relatively brief duration. Severely affected dogs usually have a grade 4 to 6 systolic murmur that is of longer duration. Typically the murmur radiates widely and can be heard over the right hemithorax and in the thoracic inlet. In patients with severe SAS the arterial pulse usually is late rising and weak.

The diagnostic value of electrocardiography and radiography in the evaluation of SAS is low. Dogs with mild or moderate SAS usually have a normal electrocardiogram, but evidence of left ventricular hypertrophy may be detected in severely affected patients. Ventricular arrhythmias, including ventricular premature complexes and ventricular tachycardia, sometimes are recorded. Depression of the ST segment suggests myocardial ischemia and is most evident on exercise or ambulatory electrocardiograms. Thoracic radiographs are of limited value in identifying SAS. Left ventricular enlargement usually is not obvious; however, poststenotic dilatation of the ascending aorta may be recognized in severely affected patients. Radiography is indicated in affected dogs showing tachypnea or respiratory distress.