Coronary artery anomalies include anomalies of origin, termination, structure or course. Coronary artery fistulae (CAF) are classified as abnormalities of termination and are considered a major congenital anomaly.
A coronary artery fistula involves a sizable communication between a coronary artery, bypassing the myocardial capillary bed and entering either a chamber of the heart (coronary-cameral fistula)  or any segment of the systemic or pulmonary circulation (coronary arteriovenous fistula). The pathophysiology of these lesions is identical, and they are often collectively termed coronary arterial-venous fistulae (CAVFs). A coronary artery connection to the pulmonary artery (coronary-pulmonary artery fistula) may also be considered under this grouping; however, if a named coronary artery arises directly from the pulmonary trunk with absence of a direct aortic connection, this is classified as an anomalous origin of the coronary artery from the pulmonary artery. 
Maude Abbott published the first pathological account of this condition in 1908. The first successful surgical closure of a coronary fistula was performed in 1947 by Bjork and Crafoord in a patient with a preoperative diagnosis of patent ductus arteriosus.
Most coronary artery fistulas are small, do not cause any symptoms, and are clinically undetectable until echocardiography or coronary arteriography is performed for an unrelated cause; they usually do not cause any complications and can spontaneously resolve. However, larger fistulae are usually 3 times the size of a normal caliber of a coronary artery and may or may not cause symptoms or complications.
Small fistulas usually do not cause any hemodynamic compromise. However, the larger fistulae can cause coronary artery steal phenomenon, which leads to ischemia of the segment of the myocardium perfused by the coronary artery. The pathophysiologic mechanism of coronary artery fistula is myocardial stealing or reduction in myocardial blood flow distal to the site of the coronary artery fistula connection. The mechanism is related to the diastolic pressure gradient and runoff from the coronary vasculature to a low-pressure receiving cavity. If the fistula is large, the intracoronary diastolic perfusion pressure progressively diminishes. 
The coronary vessel attempts to compensate by progressive enlargement of the ostia and feeding artery. Eventually, myocardium beyond the site of the fistula's origin is at risk for ischemia, which is most frequently evident in association with increased myocardial oxygen demand during exercise or activity. Over time, the coronary artery leading to the fistulous tract progressively dilates, which, in turn, may progress to frank aneurysm formation, intimal ulceration, medial degeneration, intimal rupture, atherosclerotic deposition, calcification, side-branch obstruction, mural thrombosis, and, rarely, rupture.
The factors that determine the hemodynamic significance of the fistulous connection include the size of the communication, the resistance of the recipient chamber, and the potential for development of myocardial ischemia. Occasionally, high-output congestive heart failure has been described.
Coronary artery fistulae may mimic the physiology of various heart lesions. Fistulae that drain (1) to the systemic veins or right atrium have a physiology similar to an atrial septal defect; (2) to the pulmonary arteries have physiology similar to a patent ductus arteriosus, (3) to the left atrium do not cause a left to right shunt but do cause a volume load similar to mitral regurgitation; and (4) to the left ventricle have physiology similar to that of aortic insufficiency.
Normally, 2 coronary arteries arise from the root of the aorta and taper progressively as they branch to supply the myocardium. A fistula occurs if a substantive communication arises that bypasses the myocardial capillary phase and communicates with a low-pressure cardiac cavity (atria or ventricle) or a branch of the systemic or pulmonary systems. Direct communication between a coronary artery and one of the cardiac chambers is noted. The origin of a fistula is rarely bilateral, involving both left and right coronary artery systems. Fistulous opening into a chamber or the drainage is mostly single or, rarely, double if both coronary artery systems are involved.
Normal thin-walled vessels present at the arteriolar level may drain into the cardiac cavity (arteriosinusoidal vessels) and venous communications (thebesian veins) to the right atrium. These small vessels do not steal significant nutrient flow and do not constitute fistulous connections. Fistulae can be small or large, dilated or ectatic, and tend to enlarge over time. Often, the limits of what constitutes a fistula and what constitutes a normal vessel are debated.
Major sites of origin of the fistulae are from the right coronary artery (40-60%), left anterior descending (30-60%), circumflex and a combination thereof. Most fistulae terminate in a venous chamber or vessel and, only rarely, into the left ventricle or the pericardium. The major sites of termination include the right side of the heart (90%), left ventricle, left atrium and the coronary sinus. The most frequent sites of termination in the right side of the heart, in descending order, are the right ventricle, right atrium, and pulmonary vasculature.
In the setting of cardiac outflow obstruction such as pulmonary atresia with intact septum, the term coronary-sinusoidal connections is preferred. In this setting, epicardial coronary blood may flow to and fro during the cardiac cycle. In systole, right ventricular flow decompresses via coronary-sinusoidal connections to the aorta in a reverse direction, while in diastole, the aorta perfuses the coronary artery in a normal antegrade fashion. This contrasts with coronary arteriovenous fistulae in the absence of outflow obstruction, in which coronary steal is the primary pathophysiologic problem. In pulmonary atresia and coronary-sinusoidal connections, myocardial ischemia, necrosis, fibrosis, and systemic desaturation may occur. Areas of coronary stenosis and/or interruption of the coronary system may complicate this abnormality.
Coronary fistula communications can be congenital and acquired. Congenital coronary artery fistulae may occur as an isolated finding or may appear in the context of other congenital cardiac anomalies or structural heart defects, most frequently in critical pulmonary stenosis or atresia with an intact interventricular septum and in pulmonary artery branch stenosis, tetralogy of Fallot, coarctation of the aorta, hypoplastic left heart syndrome, and aortic atresia.
Acquired coronary artery fistula may rarely arise as a consequence of trauma such as a gun shot wound or a stab wound. They can also occur after cardiac surgery or invasive cardiac catheterization with percutaneous transluminal coronary angioplasty, pacemaker implantation, or endomyocardial biopsy.
Coronary artery fistulae are thought to arise as a persistence of sinusoidal connections between the lumens of the primitive tubular heart that supply myocardial blood flow in the early embryologic period. Coronary artery fistulae occur in the absence of any outflow obstruction. Another explanation may be faulty development of the distal branches of the coronary artery rectiform vascular network.
When these channels persist in association with outflow obstruction (eg, pulmonary atresia), they are a variant form of fistulae termed coronary-sinusoidal connections. Associated syndromes most often associated with coronary-sinusoidal connections include pulmonary atresia or stenosis with an intact ventricular septum.
Coronary artery fistula accounts for 0.2-0.4% of congenital cardiac anomalies. Approximately 50% of pediatric coronary vasculature anomalies are coronary artery fistulae.
Fistula-related complications are present in 11% of patients younger than 20 years and in 35% of patients older than 20 years.
Fistulae can be associated with the following complications:
Mitral valve papillary muscle rupture from chronic ischemia
Congestive heart failure from volume overload
Sudden cardiac death
Secondary aortic valve disease
Secondary mitral valve disease
Small fistulas remain clinically silent and are recognized at routine echocardiography and autopsy. In the small fistulas, the myocardial blood supply is not compromised enough to cause symptoms. Spontaneous closure usually occurs; however, some can dilate over time.
Larger fistulae progressively enlarge over time, and complications, such as congestive heart failure, myocardial infarction, arrhythmias, infectious endocarditis, aneurysm formation, rupture, and death, are more likely to arise in older patients. Spontaneous closure has been rarely reported in the setting of large fistulas.
The mortality rate related to surgical repair of coronary artery fistula typically ranges from 0-4%. Variations that may increase surgical risk include the presence of giant aneurysms and a right coronary artery–to–left ventricle fistula. Complications of surgery include myocardial ischemia and/or infarction (reported in 3% of patients) and coronary artery fistula recurrence (4% of patients).
No race predilection is noted.
No sex predilection is noted.
Coronary artery fistula may present in patients at any age but is usually suspected early in childhood when a murmur is detected in an asymptomatic child or with symptoms of congestive heart failure. Older children with murmurs may present with symptoms of coronary insufficiency. In a multicenter review, appreciably more problems related to operative risks and postoperative complications occurred after age 20 years.