Define Tetralogy of Fallot. Describe its clinical features, investigations, management and complications.

Definition

• Tetralogy of Fallot (TOF) is the commonest cyanotic congenital heart disease beyond infancy.
• It is a right-to-left shunt lesion resulting from a single developmental abnormality — anterior deviation of the infundibular septum — that produces four components.

The Four Components

• Ventricular Septal Defect (VSD) — large.
• Overriding of the aorta — the aorta sits over the VSD.
• Pulmonary stenosis / RV outflow obstruction — this determines the severity.
• Right Ventricular Hypertrophy (RVH) — a consequence of the outflow obstruction.

Haemodynamics

RV outflow obstruction raises RV pressure → Deoxygenated blood shunts R → L across the VSD into the aorta → Reduced pulmonary blood flow + systemic desaturation → Cyanosis, clubbing and polycythaemia

Clinical Features

• Central cyanosis — onset and severity depend on the degree of pulmonary stenosis (often after the neonatal period).
• Clubbing of the fingers and toes; dyspnoea on exertion; failure to thrive.
• Squatting after exertion (increases systemic vascular resistance → reduces the right-to-left shunt → improves pulmonary flow).
• Hypercyanotic ('Tet') spells — paroxysmal deep cyanosis with hyperpnoea.
• Signs — an ejection systolic murmur of pulmonary stenosis at the left upper sternal border, and a single second heart sound.

Investigations

• Chest X-ray — a 'boot-shaped' heart (coeur en sabot) with an uplifted apex and a concave pulmonary bay; oligaemic (dark) lung fields; a right-sided aortic arch in ~25%.
• ECG — right ventricular hypertrophy and right axis deviation.
• Echocardiography — diagnostic; defines all four components and the RV outflow anatomy.
• Investigations for polycythaemia (raised haematocrit) and its effects.

Management — Hypercyanotic Spell (acute)

• Knee-chest position (or hold the infant with knees to chest) — increases systemic resistance and venous return.
• Oxygen; morphine (calms hyperpnoea); IV fluids (volume).
• Beta-blocker (propranolol/esmolol) to relax the infundibular spasm; phenylephrine to raise systemic resistance; sodium bicarbonate for acidosis.

Management — Definitive & Preventive

• Oral propranolol prophylaxis to reduce the frequency of spells while awaiting surgery.
• Maintain hydration; treat anaemia (iron) as relative anaemia worsens spells.
• Surgery: a palliative Blalock-Taussig shunt in very small/symptomatic infants, and total intracardiac correction (VSD closure + relief of RV outflow obstruction) as the definitive treatment.

Complications

• Hypercyanotic spells; cerebral thrombosis (from polycythaemia) and brain abscess.
• Infective endocarditis; growth failure; and complications of chronic hypoxia.

Spectrum of Severity

• 'Pink' TOF — mild pulmonary stenosis, minimal right-to-left shunt, little/no cyanosis (behaves like a VSD).
• Classic cyanotic TOF — moderate–severe stenosis with cyanosis.
• TOF with pulmonary atresia — the most severe, duct-dependent form.

Differential Diagnosis of a Cyanotic Newborn/child (the '5 TS')

• Tetralogy of Fallot, Transposition of great arteries, Truncus arteriosus.
• Tricuspid atresia, Total anomalous pulmonary venous connection.
• TOF is distinguished by the boot-shaped heart, oligaemic lungs and the ejection systolic murmur.

WHY Squatting Helps

• Squatting kinks the femoral arteries and raises systemic vascular resistance, which reduces the right-to-left shunt and forces more blood through the lungs — relieving cyanosis.
• It is a characteristic self-protective manoeuvre in older children with TOF.

Prognosis

• Untreated TOF carries significant mortality from spells, hypoxia and complications.
• With modern surgery (total correction, usually in infancy) the outlook is excellent, though lifelong cardiology follow-up is needed for residual lesions and arrhythmias.

Associated Anomalies

• A right-sided aortic arch (~25%); atrial septal defect ('pentalogy of Fallot').
• Association with 22q11 deletion (DiGeorge) syndrome — screen for it.

Principles of Chronic Care

• Maintain hydration and treat anaemia (iron) — a relatively low haemoglobin worsens tissue hypoxia and spells.
• Prevent dehydration (fever, diarrhoea) which increases the risk of spells and cerebral thrombosis; ensure good dental hygiene.