Etiology

• During early organogenesis, the atria and ventricles are formed from a common chamber.
• The atria are partitioned, first by the septum primum and secondly by the septum secundum that develops to its right, leaving a slit like opening between them, the foramen ovale.
• The foramen ovale allows blood to pass from right to left atrium in the fetal heart.
• At birth when pulmonary resistance falls dramatically with the first breath, left atrial pressure exceeds that of the right atrium causing functional and anatomical closure of the foramen ovale.
• The atrial and ventricular septae are eventually joined by upward and downward proliferation and differentiation of the endocardial cushions.
• Defects in either of the embryonic atrial septae (primum or secundum) or the endocardial cushions can result in atrial septal defects and they are then classified according to their anatomical positions:
  • Ostium secundum defects are usually smaller and situated more dorsally than the larger primum atrial septal defects.  More rarely in other domestic species, defects can also occur high in the atrium in the region of the sinus venosus close to the entry of the cranial vena cava.
  • In horses, the most common post-natal atrial intercommunication results from persistence of blood flow through the foramen ovale.  In this case, the atrial septae have formed correctly but patency of the foramen results from failure of the two septae to seal; usually as a result of failure to reverse fetal right left atrial pressure gradient at birth generally arising from severe congenital heart disease, causing elevation of right ventricular systolic pressure to systemic levels.

Pathophysiology

• Depends upon the presence or absence of other congenital abnormalities.
• If there is an isolated atrial septal defect, blood shunts from the left to the right atrium resulting in right ventricular volume overload and pulmonary overcirculation, if the defect is very large. Isolated defects not yet reported in horses.

Timecourse

• Depends upon the presence or absence of other congenital abnormalities and the size of the defect.

Epidemiology

• Sporadic and very rare.
• A fossa ovalis defect has been reported as an isolated lesion in one foal with atrial fibrillation and progressive heart failure. More often, ASD is part of more complex congenital defects.