Hypoplastic left heart syndrome (HLHS) is a rare congenital heart defect in which the left side of the heart is severely underdeveloped and incapable of supporting the systemic circulation. It is estimated to account for 2-3% of all congenital heart disease. Early signs and symptoms include poor feeding, cyanosis, and diminished pulse in the extremities. The etiology is believed to be multifactorial resulting from a combination of genetic mutations and defects resulting in altered blood flow in the heart. Several structures can be affected including the left ventricle, aorta, aortic valve, or mitral valve all resulting in decreased systemic blood flow.
Diagnosis can occur prenatally via ultrasound or shortly after birth via echocardiography. Initial management is geared to maintaining patency of the ductus arteriosus - a connection between the pulmonary artery and the aorta that closes shortly after birth. Thereafter, a patient subsequently undergoes a three-stage palliative repair over the next few years of life. The Norwood procedure is typically done within a few days of birth. The Glenn procedure is typically performed at three to six months of age. Finally the Fontan procedure is done sometime between the age of two and five years of age.
If left untreated patients with HLHS die within the first weeks of life while 70% of those that undergo three-staged palliative surgery reach adulthood. After surgery children with HLHS typically experience neurodevelopmental as well as motor delay and are at an increased risk of heart failure as adults.
Epidemiology
HLHS occurs in an estimated 1 out of 3,841 live births in the United States, or an estimated total of 1,025 live births per year in the US. Overall, it is estimated to make up 2-3% of all cases of congenital heart disease, and is the most common single-ventricle defect. It is thought to be more common in male infants, 1.5 times as common as in female infants. A recent systematic review found a slight decrease in prevalence of HLHS from 0.689/1000 in 1995 -1999, to 0.475/1000 in 2010 - 2017. This is thought to be due to improvements in prenatal detection and prenatal counseling and subsequent pregnancy.
Signs and symptoms
Closing of the ductus arteriosus in a heart that is severely underdeveloped on the left results in cyanosis and respiratory distress which can progress to cardiogenic shock and death. Early symptoms might include poor feeding or cyanosis that does not respond to oxygen administration. Peripheral pulses may be weak and extremities cool to the touch.
HLHS often co-occurs with low birth weight and premature birth.
In neonates with a small atrial septal defect, termed "restrictive", there is inadequate mixing of oxygenated and deoxygenated blood. These neonates quickly decompensate and develop acidosis and cyanosis.
On EKG, right axis deviation and right ventricular hypertrophy are common, but not indicative of HLHS. Chest x-ray may show a large heart (cardiomegaly) or increased pulmonary vasculature. Neonates with HLHS do not typically have a heart murmur, but in some cases, a pulmonary flow murmur or tricuspid regurgitation murmur may be audible.
Co-occurring tricuspid regurgitation or right ventricular dysfunction can cause hepatomegaly to develop.
Pathogenesis
The majority of HLHS cases are sporadic meaning they arise in patients with no family history of HLHS. Some cases may have a genetic component, as HLHS has been shown to be heritable and associated with specific gene mutations. Possible contributing factors may include intrauterine infarction, infectious changes, and a selective left ventricular cardiomyopathy.
Genetics
Genetic loci associated with HLHS include GJA1 (connexin 43), HAND1, NKX2.5, 10q22, and 6q23. There is a slight risk of recurrence in future pregnancies, estimated to be 2-4%, which increases to 25% in families with two affected children. This is thought to be mediated by genetic mutations with incomplete penetrance.
HLHS is also associated with several genetic syndromes, including trisomy 13 (Patau syndrome), trisomy 18 (Edwards syndrome), partial trisomy 9, Turner's syndrome (XO), Jacobsen syndrome (11q deletion syndrome), Holt-Oram syndrome, and Smith-Lemli-Opitz syndrome.