Hypoplastic left heart syndrome

Hypo means “beneath” or “under,” and plasia means “molding” or “formation,” so hypoplastic means “under form” or in this case, “underdeveloped.” Therefore, hypoplastic left heart syndrome means that the left side of the heart doesn’t fully develop.

Hypoplastic left heart syndrome, or HLHS, is a congenital heart defect that affects the left side of the heart; it specifically affects the left ventricle and ascending aorta. Also, the aortic valve and mitral valve might be either too small to allow enough blood to flow through, or they might be absent altogether, which is called atresia. The exact mechanism that causes HLHS isn’t known, but one popular theory is that there might be some other primary congenital heart defect that reduces the blood flow through the LV and outflow tract during fetal development, so that part of the heart does not grow and develop normally. This said, HLHS is also often associated with other congenital heart defects, and in particular, with an atrial septal defect, which is an opening between the left and right atria. In fact, not only do these babies usually have this defect, they essentially need it to survive after birth. This is in addition to a patent ductus arteriosus, or PDA, a blood vessel that connects the aorta to the pulmonary artery, which usually closes within a few days after birth.

Let’s switch to a more simplified version of the heart to see what happens with blood flow. So, right here you have the atrial septal defect connecting the left and right atria, and the ductus arteriosus, which connects the aorta to the pulmonary artery. This is a really underdeveloped left ventricle and a smallish aortic and mitral valve.