Tetralogy of Fallot

Introduction

Tetralogy of Fallot is the commonest cyanotic anomaly in those who survive cyanotic congenital heart disease.

It has up to 4 features (although only the first 2 are technically essential for diagnosis):

1. Large ventricular septal defect (VSD)
2. Pulmonary stenosis/Right ventricular outflow obstruction – mostly subvalvular and degree of obstruction determines severity of condition
3. Overriding aorta
4. Right ventricular hypertrophy

Epidemiology

Tetralogy of Fallot is the most common cause of cyanosis after 1 year. It accounts for 6% of congenital heart disease cases and is the 3^rd most common cyanotic congenital heart malformation.

Causes

There is no clear genetic or environmental cause. However, it is associated with several conditions, including:

1. Down’s syndrome
2. Noonan’s syndrome
3. Thalidomide use during pregnancy

Pathophysiology

The embryological cause is abnormal development of the bulbar septum that separates into the ascending aorta and pulmonary artery.

Right ventricular outflow obstruction in combination with a large VSD leads to right-to-left shunting. This results in:

• ↓Oxygen saturation in the left ventricle (as deoxygenated blood from the right ventricle mixes with oxygenated blood in the left ventricle)
• ↓Pulmonary artery pressure (as some blood from the right ventricle enters the left ventricle instead of the pulmonary circulation)
• ↑Pressure and hypertrophy in right ventricle (because of the increased resistance it is pumping against from the pulmonary stenosis)

Clinical features

Children

• Central cyanosis – develops in children, not neonates
• Dyspnoea
• Fatigue
• Fallot’s spells – hypoxic episodes on exertion (e.g. crying, feeding) that are characteried by deep cyanosis, apnoea and syncope. May be relieved by squatting, which increases peripheral vascular resistance and consequently reduces the degree of right-to-left shunting

Adults

Do not usually suffer from Fallot’s spells but may have:

• Exertional dyspnoea and fatigue
• Erythrocytosis (secondary to hypoaxaemia)
• Infective endocarditis

Signs

• Central cyanosis
• Ejection systolic murmur – loudest in pulmonary area
• Clubbing
• Polycythaemia
• Retarded growth

Investigations

• Chest x-ray: ‘Boot-shaped’ heart, large right ventricle
• ECG: Right ventricular hypertrophy, right axis deviation
• Echocardiography: demonstrates the lesions

Management

Surgical correction of VSD and pulmonary stenosis are the definitive treatment options for tetralogy of Fallot.

It is usually done in 2 stages:

1. Palliative – create shunt between aorta and pulmonary artery (Blalock shunt)
2. Total repair, usually done around school age

Complications

• Polycythaemia
• Cerebral venous sinus thrombosis (from polycythaemia)
• Infective endocarditis
• Heart failure
• Sudden cardiac death

Prognosis

If surgically treated: 95% of patients live beyond the age of 8

If not surgically treated: 30% die in infancy and 75% die by age 10