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 ASD and/or persistent patent foramen ovale (PFO) is crucial in residual cardiac function ¹. It 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.

Pathology

HLHS results from underdevelopment of left heart structures inclusive of ⁵ :

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

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

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. 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 give added dynamic assessment.

Treatment and prognosis

HLHS 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 post-natally, 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 (can also be an association)
• interrupted aortic arch: consider as a differential on a four chamber fetal echocardiogram for the dilated fetal right ventricle