Hypoplastic left heart syndrome (HLHS) is a birth defect that affects normal blood flow through the heart. As the baby develops during pregnancy, the left side of the heart does not form correctly. Hypoplastic left heart syndrome is one type of congenital heart defect. Congenital means present at birth.
Hypoplastic left heart syndrome affects a number of structures on the left side of the heart that do not fully develop, for example:
- The left ventricle is underdeveloped and too small.
- The mitral valves is not formed or is very small.
- The aortic valve is not formed or is very small.
- The ascending portion of the aorta is underdeveloped or is too small.
- Often, babies with hypoplastic left heart syndrome also have an atrial septal defect, which is a hole between the left and right upper chambers (atria) of the heart.