Author: Tanner Marshall, MS
Editor: Rishi Desai, MD, MPH, Tanner Marshall, MS
Hypo means beneath or under, and plasia means molding or formation, so hypoplastic means under form or in this case under develop, hypoplastic left heart syndrome therefore is where 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, specifically affecting 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, 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, which reduces the blood flow through the LV and outflow tract during fetal development, making that part of the heart not grow and develop normally.
With that said, HLHS is also often associated with other congenital heart defects, in particular though, an atrial septal defect, which is an opening between the left and right atria. Actually, not only do these babies usually have this defect, they essentially need it to survive after birth, along with 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 actually 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, and then this is a really underdeveloped left ventricle and a smallish aortic and mitral valve.
Alright so now let’s take out this septal defect and PDA. What happens? Well, deoxygenated blood comes from the body and goes into the right atrium, it then goes to the right ventricle and is pumped to the lungs, then oxygenated blood comes back to the left atrium. At this point, blood has a super hard time getting to the left ventricle, so the pressure in the left atrium is high. Blood that does manage to get into the left ventricle, is really weakly pumped out to the body. This situation, obviously isn’t sustainable, right?
Alright, so now let’s add in the septal defect and PDA. Oxygenated blood comes into the left atrium, and since the pressure on the left side is so high it takes the path of least resistance, and goes through the septal defect and mixes with deoxygenated blood in the right atrium. It then goes to the right ventricle and is pumped out the pulmonary artery, where some of it travels back to the left side through the PDA to the aorta, and out to the body. Clearly this system isn’t the most efficient way to move blood around the heart, right? But it can be enough to keep the baby alive, and sometimes even relatively asymptomatic in the hours immediately after birth. Normally, within the first day after birth the ductus arteriosus starts clamping shut, and within 3 weeks, it’s completely closed off and turned into the ligamentum arteriosum.
As it closes, blood flow gets more and more restricted to the body, causing cyanosis, or bluish purple discoloration of the skin. Eventually this leads to cardiogenic shock, and without intervention of some kind, HLHS is fatal.
HLHS is often diagnosed by prenatal ultrasound, but occasionally doesn’t get diagnosed until after the baby is born using echocardiography. Prostaglandin can often be given to babies to help keep the ductus arteriosus open until a staged surgery can be performed to fix the heart defects. Occasionally, a heart transplant can be done as well, but because of the complexity of HLHS, these treatment decisions are made on a patient-by-patient basis.
Sources edit
http://emedicine.medscape.com/article/890196-overview#a5
https://en.wikipedia.org/wiki/Hypoplastic_left_heart_syndrome