Coronary Artery Fistula

Etiology

There are three major causes of coronary artery fistulas. Most commonly, coronary artery fistulas arise congenitally as a result of abnormal embryological development. The most common form of acquired coronary artery fistulas result from trauma, such as gunshot wounds or stab injuries. Iatrogenic causes of coronary artery fistulas include interventional cardiac procedures such as coronary artery bypass grafting, cardiac angiography, valve replacements, device implantations, or biopsies.[2]

Epidemiology

Although there are more and more studies being done regarding the prevalence of coronary artery fistulas in the general population, one study found its prevalence to be 0.9%[3] Previously, the prevalence of coronary artery fistulas was thought to be somewhere around 0.05% to 0.25% of the population, as imaged through standard coronary angiography. However, with the increasing use of coronary CT angiography, this prevalence was found to be 0.9%, and the most common malformation was between a coronary artery and either of the pulmonary arteries.

Pathophysiology

Coronary artery fistulas can lead to several cardiopulmonary functional abnormalities. The severity of the presenting symptoms depends on the origin, site of insertion, and the length of the fistula, as well as the volume of blood that is shunted. The most common site of origin is the right coronary artery, followed by the left coronary artery. The most common sites of insertion (from most common to least common) are the right ventricle, right atrium, and pulmonary arteries. However, newer studies have shown that the overall most common fistula is one between the left main artery or the left anterior descending artery and the pulmonary arteries.[3][4]

Histopathology

Studies have shown that these aberrant coronary arteries, which form the malformation, have thicker tunica intima. They also have thicker tunica media with tightly packed smooth muscle cells. The anomalous arteries stained positive for alpha-smooth muscle actin (SMA), calponin, and desmin. The endothelium was found to be CD34 positive. Oftentimes, the anomalies would be accompanied by aneurysmal dilations at the neck of the aberrant coronary artery.[5]

History and Physical

As stated previously, there are a variety of ways that coronary artery fistulas can present. Some of the determinants of severity are the site of origin, place of termination, length, and overall size of the fistula. However, with that being said, the majority of patients with coronary artery fistulas are asymptomatic. Generally speaking, fistulas remain asymptomatic for twenty or so years. However, once the fistulas start to become hemodynamically significant, signs and symptoms can develop. Below are some of the more common presentations of coronary artery fistulas.

Congenital coronary artery fistulas - In infants who are around 3-4 months of age, hemodynamically significant coronary artery fistula may manifest itself as congestive heart failure. Infants experience tachycardia, tachypnea, and are usually irritable, restless, and diaphoretic. A continuous murmur during auscultation of the heart can be heard. 

Asymptomatic adult patients - The majority of patients with coronary artery fistula remain asymptomatic at least for a decade or two. A physical examination can occasionally reveal a continuous murmur normally heard best over the precordium. Most fistulas are found incidentally on coronary angiography.[6] 

Myocardial infarction/chronic myocardial ischemia - One way that coronary artery fistulas can affect the myocardium is to shunt blood away, through a coronary "steal" syndrome. In these situations, the vessels distal to the fistula do not receive blood, and therefore the areas they supply become ischemic. Another mechanism by which fistulas can cause myocardial ischemia is thrombosis at the junction of the normal and aberrant coronary arteries. Patients may present with typical/atypical signs of chest pain. However, in some cases, patients can present with unrelenting, chronic pain that might not get better after rest.[7]

Congestive heart failure - Coronary artery fistulas that increase "left to right shunting" can lead to volume overload of the pulmonary circuitry, and could, therefore, lead to signs of right-sided heart failure. Patients may present with shortness of breath, dyspnea on exertion, orthopnea, or leg swelling. In patients with congenital fistulas, this is the most common presenting finding that usually occurs during infancy/childhood.

Atrial/ventricular arrhythmias - The mechanism by which coronary artery fistulas can bring about arrhythmia is by inducing low output congestive heart failure, leading to a backup of blood flow and subsequent dilation of the atria and/or ventricles.[8]

Cardiac tamponade - Although very rare, in certain cases, the rupture of coronary artery fistulas could lead to hemopericardium. This would then result in cardiac tamponade with patients presenting with shortness of breath, hypotension, jugular venous distension, and a paradoxical pulse.[9]

Evaluation

The evaluation of coronary artery fistulas begins with the auscultation of a continuous murmur over the lower precordium. Initial diagnostic exams could include chest radiography and electrocardiography (EKG). Although these modalities do not yield a sufficient diagnosis, they are helpful in uncovering any ensuing complications. For example, ST-segment changes on an EKG could be indicative of acute myocardial infarction or chronic myocardial ischemia. Chest radiography would be able to assess volume overload.[10]

Studies have shown that transthoracic echocardiograms (TTE) are able to visualize hemodynamically significant coronary artery fistulas, but were not helpful in determining sites of origin and termination. TTEs were sufficient to diagnose this disease in children. Doppler echocardiography was also useful in pinpointing dilated or tortuous arteries and helping map out the blood flow. A greater than usual flow is usually seen at the origin and termination points of the fistula. Also, inappropriate blood flow into the right/left chambers of the heart can be seen through Doppler echocardiography. Transesophageal echocardiograms (TEE) were helpful in assessing turbulent blood flow and delineating the origin and insertion points of the fistulas in adults.[11] Cardiac magnetic resonance imaging can also be used to appreciate coronary artery fistulas.[12]

Undoubtedly, coronary catheterization and subsequent coronary angiography are the gold standards in diagnosing coronary artery fistulas. In general, studies have shown that aortic root angiograms should be done initially to help guide the next step in management. This initial step will help in selecting the most appropriate coronary artery to undergo angiography. The next step is for each of the arteries to selectively undergo coronary angiography, with the goal of establishing sites of origination and termination as well as estimating the amount of blood flow.[13] In certain cases, retrograde thoracic angiography can be used as well. Of recent, there has been a shift from coronary angiography to coronary computerized tomography angiography (CTA). Not only are coronary CTAs noninvasive, but they are also able to detect coronary artery fistulas at a higher rate as compared to standard invasive coronary catheterization with angiography.[3]

Treatment / Management

Treatment and further management are only indicated in patients with the following:

• Hemodynamically significant left to right shunt
• Congestive heart failure with either left ventricular volume overload or left ventricular dysfunction
• Myocardial ischemia

Although surgical repair was once considered to be a treatment for coronary artery fistulas, recently catheterized closure has become the treatment of choice. During the process of catheterized closure, the objective is to embolize the artery at the most distal aspect at the fistula, closest to the site of termination. The rationale behind this is to ensure that tissues supplied by the aberrant artery are still perfused, and in addition, to decrease the shunting of blood through the fistula. Some mechanisms by which embolization can be achieved are through the use of detachable balloons, or steel/platinum microcoils. After embolization, the next step is to perform a post-procedure coronary angiography to ensure successful embolization as well as to possibly visualize smaller fistulas or branches from the fistula.[14] 

In the past, surgical repair was used to treat coronary artery fistulas, whereby internal closure was the goal in the receiving artery/heart chamber. However, surgery was noted to be associated with a higher level of fistula recurrence. As a result, catheterized closure is preferred.

Patients are then placed on antiplatelet therapy and, in some cases, anticoagulants, for the first six months after cardiac catheterization.

Differential Diagnosis

The following are differential diagnoses to be considered in patients presenting with signs and symptoms of coronary artery fistula:

• Patent ductus arteriosus
• Arteriovenous malformation 
• Acute myocardial ischemia
• Congestive heart failure
• Arrhythmia
• Cardiac tamponade

Although these are some of the conditions that could also be presenting signs of a hemodynamically stable coronary artery fistula, it is important to exclude other more serious underlying etiologies such as a myocardial infarction, secondary to thrombosis, and primary ventricular arrhythmias.

Pertinent Studies and Ongoing Trials

Perhaps one of the most pertinent studies was the 2014 retrospective study that was done to assess the prevalence of coronary artery fistulas. This study, as stated above, uncovered a prevalence of 0.9% in the general population, using coronary CT angiography. This study was not only able to delineate a more accurate representation of the prevalence but was also able to show that coronary CT angiography was better able to diagnose coronary artery fistulas, as opposed to conventional coronary angiography.[3]

Prognosis

Although the prognosis of coronary artery fistulas is highly variable, the complications they predispose patients to will be the ultimate determining factor. In adults, some patients may remain asymptomatic for their entire lives, if the fistula was not hemodynamically significant. In other patients with ensuing complications such as congestive heart failure, atrial/ventricular arrhythmias, and cardiac tamponade, an immediate reversal of the complication followed by treatment of the coronary artery fistula by catheterization closure is necessary to decrease morbidity. Although recurrence of the coronary artery fistula is rare, surgery is known to increase the rate of recurrence as opposed to catheterized closure.

Complications

Coronary artery fistulas are often asymptomatic. The majority of presentations of patients with coronary artery fistulas are usually secondary to its complications. The following are the list of complications that could arise from coronary artery fistulas:

• Steal syndrome - Whereby blood is shunted away through the fistula, rendering decreased distal blood flow to the distal part of the original artery leading to myocardial ischemia.
• Thrombosis/embolism - Leading to myocardial infarction.
• Volume overload - Leading to cardiac failure.
• Arrhythmias
• Rupture - Leading to hemopericardium and cardiac tamponade.
• Endocarditis/endarteritis