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Tetralogy of Fallot

Tetralogy of Fallot (TOF) is one of the most common cyanotic congenital heart conditions 1,5 and continues to be a major source of morbidity.

Epidemiology

This anomaly accounts for 10% of all congenital heart disease and has an estimated prevalence of 1 in 2000 births 10.

Clinical presentation

Presentation is dictated by the degree of right ventricular outflow tract obstruction (RVOTO). Typically this is significant, resulting in cyanosis evident in the neonatal period, as a result of the right to left shunt across the VSD. In cases where outflow obstruction is minimal, cyanosis may be inapparent (pink tetralogy) resulting in delayed presentation, even into adulthood, although this is rare.

Pathophysiology

Tetralogy of Fallot is classically characterised by four features which are:

  1. ventricular septal defect (VSD)
    • may be multiple in ~5% of cases 6
  2. right ventricular outlfow tract obstruction (RVOTO) due to:
  3. over riding aorta
  4. right ventricular hypertrophy: only develops after birth

The right ventricular hypertrophy is a result of the VSD and right ventricular outlet obstruction, both contributing to elevated resistance to right heart emptying 6.

Genetics

In approximately 15% of cases it is associated with a deletion on chromosome 21q11 6-7.

Associations

Radiographic features

Plain film

Plain films may classically show a "boot shaped" heart with an upturned cardiac apex due to right ventricular hypertrophy and concave pulmonary arterial segment. Most infants with TOF however may not show this finding 2.

Pulmonary oligaemia due to decreased pulmonary arterial flow. Right sided aortic arch is seen in 25%.

MRI

MRI has the great advantage of providing both exquisite anatomical details and functional information without ionising radiation. Detailed assessment of the pulmonary artery is particularly important because repair of the cardiac defects without addressing pulmonary artery hypoplasia/stenosis has a poor outcome 8.

The main pulmonary artery or right pulmonary artery diameter should be compared to that of the ascending aorta. A ratio of <0.3 usually signifies that primary repair would be unsuccessful and a bridging shunt operation may be of benefit 8.

Assessment of coronary artery origin is also essential to surgical planning.

Treatment and prognosis

Appoximately 90% of untreated tetralogy patients succumb by the age of 10 years 6. Over the years many surgical approaches were performed, until current primary repair was developed. Shunts are now days only performed as a palliative procedure in inoperable cases or to bridge patients until repair can be carried out, typically in the setting of pulmonary arterial hypoplasia 8.

Shunt operations included 6:

Primary repair is now the preferred treatment and is usually performed at the time of diagnosis.

Common post-surgical complications include 6:

  • conduction abnormalities
    • right bundle branch block (RBBB): 80-90% of cases
    • bifasicular block: 15% of cases
    • premature ventricular contractions: ~50% of cases
    • sustained ventricular tachycardias: ~5% of cases
    • atrial arrhythmias: common
  • valvular dysfunction
    • tricuspid regurgitation
    • pulmonary regurgitation

Prognosis is largely dependent on how soon the defect is diagnosed and corrected, with the best outcome seen in patients repaired before the age of 5 6. Overall there is a 90-95% survival rate at 10 years of age, however residual right ventricular dysfunction is common. Up to 10% of patients require re-operation within 20 years 6.

Historical context

It is named after Etienne-Louis Arthur Fallot - French physician (1850-1911)

Differential diagnosis

Findings on a chest x-ray are non-specific and other cyanotic congenital heart diseases should be considered.


Related articles

Congenital heart disease

There is more than one way to present the variety of congenital heart diseases. Whichever way they are categorised, it is helpful to have a working understanding of normal and fetal circulation, as well as an understanding of the segmental approach to imaging in congenital heart disease.

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