Epidemiology

• Transposition is the most common cyanotic congenital heart lesion presenting in the neonate.
• The overall annual incidence is 20-30 per 100,000 live births.^[1]
• It is more common in males than females, with a ratio of about 3:1.
• Maternal factors associated with an increased risk include rubella or other viral illness during pregnancy, alcoholism, maternal age over 40 and diabetes.
• Transposition is rarely associated with syndromes or extracardiac malformations.

Presentation

• Most affected infants present with cyanosis, most often within the first 2 or 3 days, and many require resuscitation.^[2] Some cases are more mild in presentation and not diagnosed until the end of the first week of life or beyond.
• Those with a large ventricular septal defect or patent ductus arteriosus may not be diagnosed until several weeks of age.
• Infants may develop breathlessness and heart failure over the first 3-6 weeks as pulmonary blood flow increases.
• If there is a ventricular septal defect and left ventricular outflow tract obstruction, the presentation may be similar to that of an infant with Fallot's tetralogy.

Signs

• Cyanosis and may also be breathless with tachycardia and tachypnoea.
• Heart sounds are often normal with no audible murmurs.
• In patients with a ventricular septal defect there may be a systolic murmur, which increases in intensity as the pulmonary vascular resistance falls.
• An ejection systolic murmur is usually present in those with left ventricular outflow tract obstruction.

Differential diagnosis

• Non-cardiac causes of a severely ill neonate, eg infection, respiratory problems (such as respiratory distress syndrome, meconium aspiration, pneumothorax, pneumonia, congenital diaphragmatic hernia)
• Other causes of congenital heart disease, especially:
  • Pulmonary atresia
  • Fallot's tetralogy
  • Total anomalous pulmonary venous connection
  • Tricuspid atresia
  • Truncus arteriosus

Investigations

Antenatal diagnosis by ultrasound scanning results in better clinical status before surgery and improved postoperative outcome.^[2]

• Chest X-ray may appear normal in newborns with transposition of the great arteries (TGA) and intact ventricular septum but may demonstrate the classic 'egg on a string' appearance (heart is slightly enlarged and appears like an egg lying on its side, narrow vascular pedicle because aorta and pulmonary artery lie one in front of the other, and increased vascular lung markings).
• With an associated ventricular septal defect, the chest X-ray usually shows cardiomegaly with increased pulmonary arterial vascular markings.
• Initial investigations also include pulse oximetry, ECG and assessment of an unwell baby, eg infection screen, full blood count, renal function, electrolytes.
• Echocardiogram images are diagnostic of transposition and associated anatomic lesions.
• Cardiac catheterisation may be needed if echocardiogram does not adequately demonstrate the anatomical abnormality.

Management

• Palliative treatment with prostaglandin E1 infusion to maintain ductal patency and balloon atrial septostomy are usually required soon after birth.^[3]
• All patients require antibiotic prophylaxis prior to dental and indicated surgical procedures in order to reduce the risk of subacute bacterial endocarditis.

Surgical

• Cardiac catheterisation and balloon atrial septostomy are used to increase the atrial level shunt and to improve mixing.
• The definitive corrective procedure is the arterial switch operation, which has been found to have similar mortality and less morbidity than atrial (Mustard or Senning) repairs.^[4]
• Most fullterm neonates with uncomplicated transposition of the great arteries (TGA) can undergo an arterial switch procedure in one operation, with minimal mortality.

Complications

• Congestive heart failure
• Arrhythmias
• Right ventricular dysfunction in long-term survivors
• Eisenmenger's syndrome
• Polycythaemia and hyperviscosity syndrome
• Seizures may occur in about 5% of patients before surgery^[2]
• Thrombocytopenia

Prognosis

• The mortality rate in untreated patients is approximately 30% in the first week, 50% in the first month, and 90% by the end of the first year.
• Death is usually due to anoxia, acidosis, heart failure and complications associated with polycythaemia, including thromboembolic events.
• The overall survival rate following arterial switch operation is approximately 90% at 15 years of age. However, exercise performance, cognitive function and quality of life may be impaired.^[3]
• Low gestational age and a high preoperative lactate are the most important predictors of poor developmental outcome.^[2]