An anomalous left main coronary artery with its origin from the pulmonary artery (ALCAPA) is one of the few clinically significant coronary anomalies. While the majority of congenital coronary artery abnormalities involve various origins of the vessels from the aorta and are usually benign, ALCAPA can be fatal either during infancy or result in sudden death in adulthood. The incidence of ALCAPA is between 1:30,000 and 1:300,000 infants, and accounts for 0.25-0.5% of congenital cardiac defects. Albeit rare, it is the most common cause of myocardial infarction in pediatric patients. Collaterals originating from the right coronary artery perfuse the left coronary artery (LCA), which subsequently drain into the pulmonary artery (PA). Less commonly, the defect involves the right, rather than the main pulmonary artery. Rarely the defect includes the circumflex, right coronary, or both coronary arteries. Perfusion of the anomalous left coronary is maintained by the elevated pulmonary artery pressures after birth. Without clinical appreciation and correction, this major defect is fatal.
Approximately 15% of patients will have an associated anomaly. The literature describes ALCAPA in combination with ventricular septal defects (VSD), patent ductus arteriosus (PDA), tetralogy of Fallot (TOF), pulmonary valvular stenosis, coarctation of the great vessels, and one case of hypoplastic left heart syndrome. Should the presence of one of these defects result in an increase in the pulmonary artery pressure, there is a resultant decline in the degree of myocardial ischemia. Associated anomalies are more likely to be present in defects involving the right pulmonary artery rather than the main pulmonary artery.
Symptoms usually occur within the first two months of life–the time the ductus arteriosus closes, as the clinical effect of ALCAPA is dependent on the pressure difference between the systemic and pulmonary circulatory systems. Decreased resistance in the pulmonary vasculature results in a left-to-right shunt from the right coronary artery to the PA–coronary steal phenomenon. The elevated pulmonary vascular resistance in utero results in systemic pulmonary arterial pressures. Consequently, the perfusion through the anomalous LCA is sufficient, albeit desaturated. Thus, symptoms rarely present early during infancy. Antegrade flow through the LCA decreases with the decline in PA pressure and pulmonary vascular resistance, and becomes more dependent on the collateral network between the right and left coronary arteries. The maturity of this collateral network will determine the clinical course after the ductal closure, as it ultimately determines the myocardial viability. A child with a well-developed collateral network may survive past infancy, albeit with progressive LV dysfunction. The lack of adequate collateral vessels from the RCA causes myocardial ischemia and ventricular dysfunction from insufficient perfusion, with eventual myocardial infarct, aneurysmal changes, and ultimately death. Early ischemic changes are usually located within the subendocardium with a patchy distribution.
The typical clinical symptoms appreciated at 4-6 weeks include poor feeding, tachypnea, failure to thrive, and pallor. With an enlarged cardiac silhouette seen on chest x-ray, children are commonly diagnosed with heart failure due to a dilated cardiomyopathy. Clinicians have confused a syndrome of severe pallor, diaphoresis, and irritability as infantile colic, in which the term "anginal equivalent" was coined. It is pertinent to suspect an anomalous coronary defect in infants presenting with heart failure and dilated cardiomyopathy. In older children, due to mature collateral networks or associated defects maintaining elevated pulmonary vascular resistance, complaints include angina, dyspnea, and fatigue. The defect may result in arrhythmias or even sudden death.
Physical exam findings include tachypnea, tachycardia, a gallop rhythm, cardiomegaly, hepatomegaly, and commonly a systolic ejection murmur due to mitral regurgitation. The murmur is a result of either papillary muscle dysfunction or the ventricular remodeling. The enlarged cardiac silhouette on chest x-ray is nonspecific; however, pulmonary congestion is also common in infants. Lateral or anterolateral infarction with Q waves and elevated ST segments on electrocardiogram (ECG) are classic findings in infants. In older children and adults, the Q waves are less prominent; however, hypertrophy of the posterobasal left ventricular wall may be appreciated.
Diagnostic studies include echocardiogram and cardiac catheterization. A dilated, poorly contracting left ventricle is common on echocardiography, with mitral regurgitation due to a poorly mobile posterior leaflet. It is possible to visualize the coronary ostia on color-flow Dopper echocardiography; however, coronary angiography remains the best diagnostic tool and should be obtained if color-flow Doppler does not identify both ostia with certainty. While the anomalous LMCA may originate anywhere along the main PA or one of the branches, it most commonly originates from the rightward posterior sinus. Coronary arteriogram will also show the collaterals originating from the RCA, late filling of the PA, and the left-to-right shunt. The cardiac catheterization will also document elevated LV end-diastolic, left atrial and pulmonary artery pressures. Pulmonary angiography may be performed if doubt remains about the diagnosis, or the origin of the LCA ostium is uncertain. Magnetic resonance angiography (MRA) has been reported to be useful in defining the proximal course of the vessels.
Without surgical correction, this anomaly is fatal. Simple ligation of the anomalous LCA was the first successful operation described for ALCAPA. Ligation of the LMCA from the PA excluded the left-to-right shunt, thus allowing the collaterals from the RCA to perfuse the ventricle. This procedure has since fallen out of favor due to the significant risk of sudden death. Additional surgical corrections include bypass, reimplantation, and in situ conduit procedures, with the ultimate goal of developing a dual coronary artery system. Bypass grafting has used the left subclavian artery, internal mammary artery, and saphenous vein graft, but the results have been disappointing. Ultimately, reimplantation, if possible, is the ideal technique. This approach depends on the left coronary artery to be sufficient length for mobilization to the left sinus of Valsalva. If arterial mobilization is an issue, an ostium can be created in another part of the aorta for reimplantation. Takeuchi fashioned a conduit to direct flow from the aorta to the anomalous artery by creating an aortopulmonary window and intrapulmonary baffle with a flap of the pulmonary artery. Dehiscence of the baffle–a cited complication–results in the redevelopment of the left-to-right shunt. The outcome depends on the degree of irreversible ventricular dysfunction. However, neither severe ventricular dysfunction nor mitral insufficiency is a contraindication to revascularization, as significant recovery is the norm. Management of the mitral insufficiency remains controversial. While the regurgitation resolves in most cases with revascularization, the extremely rare case of substantial myocardial ischemia with infarcted papillary muscles exists. In this case, valvular repair has shown good results.