Atrioventricular septal defect (AVSD), also called endocardial cushion defect or atrioventricular canal defect, is a congenital heart disease that originates from inappropriate endocardial cushion development and fusion (2,6). Several studies have suggested that several genes contribute to AVSD formation in humans. However, research in veterinary medicine is limited (10). In small animal medicine, approximately 5-10% of cats with congenital heart disease have AVSD; however, AVSD is rare in dogs (13).

AVSD exhibits variable pathological anatomies and is currently classified as partial or complete in veterinary medicine (2). Partial AVSD has the anatomical features of an ostium primum defect, with a cleft at the anterior mitral leaflet. Transitional AVSD, a subtype of partial AVSD known as transitional AVSD is characterized by the anatomic features of an ostium primum defect with a cleft at the anterior mitral leaflet and a restrictive ventricular septal defect (VSD) filled with fibrous attachments connecting the crest of the interventricular septum and atrioventricular valves. Complete AVSD has the anatomic features of an ostium primum defect that continues into the inlet VSD and a common atrioventricular (AV) valve. Complete AVSD can be classified based on the insertion of the chordae and morphology of the superior bridging leaflet (14).

This case report describes a dog diagnosed with transitional AVSD using echocardiography. Furthermore, we discuss transitional AVSD from the perspectives of terminology and embryology.

A 7-year-old, 2.6 kg, spayed female Maltese dog was referred to the Jeonbuk National University Veterinary Medical Teaching Hospital because of suspected congenital heart disease noted at the previous animal hospital. The dog had a recent history of tachypnea and mild exercise intolerance. Cyanosis was observed during the physical examination. Auscultation revealed a grade II/VI heart murmur in both the left and right apical regions during systole. The systolic blood pressure was 150 mmHg, measured using a Doppler machine. No significant abnormalities were observed in the blood analysis.

Thoracic radiography in both lateral and dorsoventral views revealed generalized cardiomegaly and a reverse D-shape of the heart. Echocardiography revealed eccentric and concentric hypertrophy of the right ventricle, large ostium primum defect, and moderate right atrial enlargement (Supplementary Video 1). The left and right AV valves were noted to be located at the same level (Fig. 1A, B). A gap, suspected to be an inlet VSD was observed at the top of the interventricular septum crest. Fibrous attachments fill the VSD, connecting the crest of the interventricular septum to the AV valves. In contrast, bidirectional atrial communication was observed in a study with agitated 0.9% saline (consisting of 2 mL of saline and 0.5 mL of air) (Fig. 1C, D). Determining the presence of flow through the VSD is challenging because of the presence of contrast in the left ventricle resulting from an ostium primum defect. The ratio of the pulmonary blood flow (Qp) to the systemic blood flow (Qs) was 2:1. In the right parasternal short-axis and left apical 5 chamber view, the left AV valve consisted of one posterior leaflet and one anterior leaflet, which consisted of superior and inferior cushion components, giving the left AV valve a trileaflet-like appearance (Fig. 2, Supplementary Video 1). A cleft was observed between the superior and inferior cushion components (Fig. 2A). The direction of VSD flow was from left to right on color Doppler. The velocity of the VSD flow was too trivial to measure. Regurgitation was observed in both AV valves on color Doppler, and the velocities were measured using continuous doppler: 6.19 m/s at the left AV valve and 3.91 m/s at the right AV valve. Considering the regurgitant velocities at both AV valves, the presence of mild systemic and moderate pulmonary artery hypertension was suspected. Based on these findings, we diagnosed the canine patient with transitional AVSD with pulmonary hypertension due to volume overload in the pulmonary vasculature through the ostium primum defect, along with right heart enlargement. Sildenafil (Unigra Tab; Union Korea Pharm, Korea) was prescribed at a dosage of 1.5 mg/kg twice daily to manage pulmonary hypertension and prevent or delay Eisenmenger’s syndrome. Follow-up echocardiography and thoracic radiography every six months by the owner. At the time of writing this report (approximately 2 years post-diagnosis), the dog had not experienced any further cardiac-related symptoms, showed no structural changes in the heart on echocardiography, and exhibited a slight decrease in regurgitation velocity through the right AV valve from 3.91 m/s to 3.79 m/s.

Figure 1. Echocardiogram images from a dog in this case, showing features of a large ostium primum defect, restrictive ventricular septal defect (VSD) with fibrous attachments, and the left and right atrioventricular (AV) valves at the same level. (A) Right parasternal long axis 4-chamber view at the end of the systolic phase. (B) Right parasternal long axis 4-chamber view at the end of the diastolic phase. (C) Contrast study showing that contrast flows from right atrium to left atrium during diastole. Red arrow: contrast flows right atrium to left atrium. (D) Contrast study showing negative contrast in the right atrium during systole. Green arrow: negative contrast in the right atrium. LA, left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle.

Figure 2. Echocardiogram images illustrating the left AV valve in this case. (A) Right parasternal short axis view during diastole, showing the cleft (asterisk) at the anterior leaflet. (B) Right parasternal short axis view during systole. The left AV valve’s commissure shows the “Mercedes-Benz sign” (red dotted line). (C) Left apical long axis 5-chamber view during diastole, modified for the left AV valve. (D) Left apical long axis 5-chamber view during systole, modified for the left AV valve. The left AV valve is composed of the posterior leaflet and the anterior leaflet, which in turn comprises the inferior cushion component (ICC) and the superior cushion component (SCC). Ao, aorta; LA, left atrium; LV, left ventricle; PL, posterior leaflet.

AVSD is caused by the inappropriate development and fusion of an endocardial cushion in the fetus, which can lead to various AVSD morphologies. AVSD manifests in diverse forms and various terms denote this spectrum. However, some researchers have perceived these terms as lacking specificity (1). In human medicine, consistent efforts to standardize the nomenclature of pediatric and congenital heart diseases have resulted in the International Pediatric and Congenital Cardiac Code (IPCCC) (4). According to the IPCCC, AVSD can be classified based on communication between chambers as follows: AVSD with communication at the atrial level only, AVSD with communication at the ventricular level only, AVSD with communication at the atrial level and restrictive communication at the ventricular level, and AVSD with communication at the atrial level and unrestrictive communication at the ventricular level. In this case, based on echocardiography, the AVSD in this case can be differentiated as “AVSD with communication at atrial level and restrictive communication at ventricular level.” The terms as partial AVSD, intermediate AVSD, transitional AVSD, and complete AVSD are not preferred by the IPCCC as they detract from consistency in terminology (4,9). An accurate and consistent nomenclature for congenital heart disease is crucial for accurate communication among veterinarians, surgical planning, research endeavors, and educational purposes (1).

In dogs with AVSD, four types of blood flow disturbances can occur: shunting through the ASD, shunting through the VSD, regurgitation at the left AV valve, and regurgitation at the right AV valve (12). In ASD, the shunting volume across the defect is associated with the size of the defect and the relative compliance of the two ventricles during diastole (5). In a large ASD, the shunting volume can be significant owing to the size of the defect, potentially leading to right ventricular volume overload and pulmonary overcirculation, which can result in pulmonary hypertension (11). In a VSD, the shunting volume across the defect is associated with the size of the defect and the pressure gradient across the shunt (5). In the present case, the VSD was too small to be hemodynamically significant. However, the ASD was large enough to cause volume overload in the right ventricle and pulmonary vasculature, leading to pulmonary hypertension. Considering the pathway of blood flow passing through the ASD, left AV valve regurgitation might exacerbate volume overload in the right heart and pulmonary vasculature (8). However, in this case, the effect of the left AV valve regurgitation was not significant in inducing volume overload. Considering the dog’s Qp/Qs ratio, surgical treatment was considered, and there have been reports of successful surgery in dogs with AVSD (3,9). However, the owner declined the surgical intervention. Sildenafil was administered to manage the bidirectional shunt, alleviate symptoms, and prevent the progression of pulmonary hypertension and the transition to a right-to-left shunt (11). After prescription of sildenafil, the dog has no clinical signs and no worsening signs in echocardiography recheck.

During echocardiography, the left AV valve comprised three leaflets. Thus, it can be referred to as a tri-leaflet, that is, it has three leaflets and three commissures. However, solely determining the pathological anatomy without considering the embryology can lead to inaccuracies (15). The embryological processes of common AV canal septation and mitral valve cleft closure are well established in humans and mammals. As fusion occurs between the superior and inferior cushions, the cleft of the anterior mitral valve disappears as fusion progresses from the medial to the lateral aspect (7). Therefore, in AVSD, the cleft is simply a result of incomplete or partial fusion between the superior and inferior cushions. Consequently, the space between the superior and inferior cushion components is not a commissure but rather a cleft of the left anterior valve, separating the anterior valve into the superior and inferior cushion components. The pathological anatomical classification of AVSD can also be elucidated using embryological morphogenetic sequences. Broadly, the AV canal undergoes three distinct processes during fetal development. Initially, the inferior VSD, located beneath the inferior cushion, closes, followed by the closure of the superior VSD, located beneath the superior cushion, resulting in complete VSD closure. Finally, the closure of the mitral cleft and ostium primum defects occurs simultaneously (15). The pathological anatomy and classification of AVSD vary depending on the occurrence of developmental abnormalities. We speculate that the congenital defects in this case arose when the processes outlined above failed to occur in the second step (closure of the superior VSD).

This case report discusses a canine patient diagnosed with transitional AVSD, which is defined by the IPCCC as AVSD characterized by communication at the atrial level and restricted communication at the ventricular level accompanied by pulmonary hypertension. By referring to the IPCCC, consistency of terms for the AVSD subtypes can be achieved. From an embryological perspective, it can be understood that the left AV valve is not a trileaflet but rather consists of a posterior leaflet and an anterior leaflet, comprising the superior and inferior cushion components. This case report describes management with sildenafil alone, which may be effective in cases of transitional AVSD with pulmonary hypertension.

The authors have no conflicting interests.