Hypoplastic left heart syndrome

Hypoplastic left heart syndrome (HLHS) is a cyanotic congenital cardiac anomaly where affected individuals can have profound cyanosis and cardiac failure.

It is one of the commonest causes for a neonate to present with congestive cardiac failure and the 4^th most frequent cardiac anomaly to manifest within the 1^st year of life ¹. The presence of an atrial septal defect and/or persistent patent foramen ovale (PFO) is crucial in residual cardiac function ¹. Hypoplastic left heart syndrome is fatal if untreated.

Epidemiology

This anomaly is thought to represent 2-4% congenital cardiac anomalies ³. There is a recognized male predilection. The estimated incidence is ~ 1 in 10 000 births ⁶.

Risk factors

• male ⁸
• genetic disorders ⁸
  • Turner syndrome
  • Holt-Oram syndrome
  • Smith-Lemli-Opitz syndrome
  • Noonan syndrome
  • trisomy 13
  • trisomy 18
  • trisomy 21

Associations

• concurrent aortic coarctation (67 to 80% ⁸)
• endocardial fibroelastosis

Pathology

Hypoplastic left heart syndrome results from the underdevelopment of left heart structures including ⁵:

• left ventricle
• mitral valve: stenosis/atresia
• aortic valve: atresia/hypoplasia
• ascending aortic root/arch

Radiographic features

Plain radiograph

• the overall cardiac silhouette may be small, normal or enlarged ¹
• may also show evidence of pulmonary venous congestion
• the right atrial border may be prominent

Antenatal ultrasound 

The four chamber view is particularly helpful in initial in utero assessment. It may show a small ascending aorta, and a small but thick-walled left ventricle while the right heart chambers may appear enlarged. The movement of the mitral valve may also appear significantly impaired.

CT/CT angiography 

CT allows direct visualization of anomaly and vessel anatomy ². Right-sided cardiac structures inclusive of the right ventricle, right atrium, and pulmonary trunk are often enlarged as a result of compensatory effect. 

Cardiac MRI

Allows direct visualization of anatomy, while SSFP sequences may be used to provide additional dynamic assessment.

Treatment and prognosis

Hypoplastic left heart syndrome can be well-tolerated in utero due to the fetal right ventricle being the dominant chamber and the ductus arteriosus being patent.  

While previously uniformly fatal postnatally, the outlook has somewhat improved with new surgical strategies which include:

• Norwood procedure: most commonly performed initial palliative procedure in the neonatal period ⁵ 
• bidirectional cavopulmonary anastomosis (BDCPA) or hemi-Fontan procedure 
• cardiac transplantation

Prostaglandin E₁ may be given as an initial management option to keep the ductus open.

Differential diagnosis

General considerations include:

• infantile aortic coarctation (may also be an association)
• interrupted aortic arch: consider as a differential on a four-chamber fetal echocardiogram for the dilated fetal right ventricle