Right Coronary Artery to Right Ventricular Outflow Tract Fistula in an Aged Dog - Imaging Diagnosis

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Background: Coronary artery fistulas (CAFs) are uncommon congenital or acquired vascular anomalies that cause abnormal communication between the coronary artery and cardiac chamber or great vessels. If the drainage site is the cardiac chamber, the lesion is classified as a coronary-cameral fistula. Right coronary artery (RCA) to right ventricular outflow tract (RVOT) fistulas have not been clinically documented in dogs; however, other CAFs have been reported occasionally in small animal practice. This case report describes the echocardiography and computed tomography (CT) features of an RCA to RVOT fistula in an aged dog.

Case: An 18-year-old, castrated male dog mixed-breed, weighing 3.6 kg, was presented for evaluation of a left intrathoracic wall mass with no overt clinical signs related to cardiopulmonary function. A grade 2/6 systolic murmur was auscultated, and systolic blood pressure was 150 mmHg. Echocardiography revealed an abnormal jet originating from the aortic root region and directed toward the RVOT with diastolic predominance and a peak velocity of 1.8 m/s. Mild pulmonary regurgitation with a peak velocity of 2.1 m/s was also detected, although no echocardiographic indices suggested secondary pulmonary hypertension. Based on the echocardiographic characteristics and anatomic location, a CAF was considered the most likely diagnosis, although alternative differentials could not be excluded based on echocardiography alone. Subsequent CT angiography demonstrated a small tubular communication between the proximal RCA and RVOT, without RCA dilation. These findings excluded alternative diagnoses, namely, supracristal ventricular septal defect, atypical patent ductus arteriosus, and aorto-right ventricular fistula or tunnel, thereby confirming a clinically silent RCA-to-RVOT fistula.

Discussion: This report describes the 1^st documented case of an RCA-to-RVOT fistula in a geriatric dog along with diagnostic imaging findings. The clinical relevance of CAFs depends primarily on the fistula size, associated hemodynamic changes, and the presence of related clinical signs. In this patient, the lesion was considered to have low clinical significance because it was small, demonstrated predominantly diastolic flow with relatively low velocity, and was not associated with any clinical signs. Long-term follow-up to rule out progressive changes in the fistulous tract and delayed onset of clinical signs was not available; however, based on the overall imaging findings and the patient’s advanced age, clinically meaningful progression was considered unlikely. This case also highlights the complementary roles of echocardiography and CT angiography in diagnosing congenital cardiovascular malformations. Echocardiography enables detection of abnormal flow that is not expected in a normal heart, and provides functional information regarding hemodynamic relevance. Additionally, CT angiography using appropriate intravascular enhancement, optimized window settings, and multiplanar reformation provides three-dimensional anatomic information, allowing detailed definition of lesion morphology and its relationship to adjacent structures. When echocardiography identifies a predominantly diastolic jet coursing toward the RVOT, CAF should be included in the differential diagnoses. Combining echocardiography with CT is valuable for establishing a definitive diagnosis.

Keywords: computed tomography, coronary artery, diagnostic imaging, dog, echocardiography, fistula.