Hypoplastic left heart syndrome (HLHS) is a serious congenital heart disease that occurs because the left side of the heart does not form properly during fetal life.
Both the heart chambers and the valvular structures on the left side of the heart are affected. This includes the left atrium, the left ventricle, the mitral valve that separates these two chambers, the aortic valve that guards the opening of the left ventricle into the aorta and the first or ascending part of the aorta. All these structures are drastically underdeveloped and very small in size, and therefore unable to function as expected.
In many babies with HLHS, an atrial septal defect is also present. This refers to the persistence of an opening in the septum separating the two upper heart chambers or atria. This type of foramen is present in fetal life but becomes pathologic when it persists into post-natal life, leading to an abnormal communication between the atria.
A normal heart has four chambers, the upper two atria (right and left) to receive deoxygenated venous blood and oxygenated blood from the lungs, respectively, and the lower two ventricles or pumping chambers, pumping blood received from the atria to the pulmonary and systemic circulation respectively.
In the baby with HLHS, however, the undeveloped left ventricle cannot pump oxygenated blood to the body. This blood is shunted to the right side of the heart through the two openings between the left and right sides of the heart that are normally present in the newborn. These are called the patent foramen ovale and the patent ductus arteriosus. This allows a mixing of oxygenated blood (from the left heart) and deoxygenated blood (from the right side of the heart) in the right atrium and right ventricle. This mixture is pumped to both the lungs and to the body by the right ventricle.
The closure of these openings, which occurs within a few days of birth, cuts off this escape route and prevents oxygenated blood from reaching the systemic circulation in babies with HLHS. This is because their left ventricle is incapable of taking up the job of pumping this blood out through the aorta to the whole body. In babies without HLHS, the left ventricle adjusts to the closure of the right-left communications with ease.
Babies with HLHS present with symptoms within the first 3-4 days of life, such as:
- Cyanosis, or the presence of a bluish color around the mouth and lips
- A rapid pounding heartbeat
- A weak pulse
Diagnosis and management
This syndrome is diagnosed shortly before birth or in the newborn period. Prenatal ultrasound imaging or fetal echocardiograms may allow the diagnosis to be made in the intrauterine period. After birth, the presence of the symptoms of heart disease, coupled with heart murmurs, may lead to the suspicion of congenital cyanotic heart disease, confirmed by an echocardiogram.
Management relies on diuretics, digoxin and anti-hypertensive drugs initially. Special formulas may be prescribed to help the baby gain weight with small feedings, as such infants tire quickly while feeding.
Definitive management to enable the infant to survive is surgical, and consists of staged surgeries. The commonly used protocol includes:
- The Norwood procedure: starting with the creation of a new aorta connected to the functioning right ventricle, so that blood may be pumped to the whole body. A connection between either the right subclavian or the right ventricle and the pulmonary artery is also fashioned to allow the right ventricle to pump blood both to the lungs and to the systemic blood vessels.
- The bidirectional Glenn procedure: connecting the superior vena cava to the right pulmonary artery, and therefore letting venous blood from the upper half of the body flow directly to the lungs, reducing the pumping work of the right ventricle
- The Fontan procedure: connecting the inferior vena cava to the right pulmonary artery, which completely separates venous inflow from the oxygenated inflow. This corrects the cyanosis.
Lifelong care is required after corrective surgery, both in the form of medications such as anticoagulants and digoxin, and regular monitoring of heart function by a cardiologist. Complications may occur and persist throughout the patient’s life. This condition is not cured by surgery, therefore, but improved to the point that the patient can survive.
Reviewed by Yolanda Smith, BPharm