A 6-month-old infant attending paediatrics OPD is found to have central cyanosis, clubbing and no hepatomegaly.

Differential Diagnosis

The clinical picture of central cyanosis, clubbing, and absent hepatomegaly in a 6-month-old infant points towards congenital heart diseases (CHDs) with decreased pulmonary blood flow. The most likely diagnoses include:

• Tetralogy of Fallot (TOF): This is the most common cyanotic CHD. It consists of four defects: ventricular septal defect (VSD), pulmonary stenosis, overriding aorta, and right ventricular hypertrophy.
• Transposition of the Great Arteries (TGA): Although often presenting earlier, some milder forms can be diagnosed later. It involves switching of the aorta and pulmonary artery.
• Tricuspid Atresia: A complete absence of the tricuspid valve, leading to no flow from the right atrium to the right ventricle.
• Pulmonary Atresia: Complete blockage of the pulmonary valve.
• Hypoplastic Left Heart Syndrome (HLHS): Though often diagnosed earlier, some cases may present with delayed cyanosis.

Distinguishing Features & Investigations

Differentiating between these conditions requires a detailed clinical examination and appropriate investigations.

Tetralogy of Fallot (TOF)

TOF typically presents with a systolic ejection murmur due to pulmonary stenosis. Cyanotic spells ("tet spells") are common, especially during feeding or crying. Chest X-ray shows a "boot-shaped" heart. ECG reveals right ventricular hypertrophy. Echocardiography is the gold standard for diagnosis, visualizing the four components of TOF.

Transposition of the Great Arteries (TGA)

TGA usually presents with severe cyanosis shortly after birth. A loud S2 is often heard. Chest X-ray shows cardiomegaly and increased pulmonary vascular markings. Echocardiography confirms the diagnosis by demonstrating the switched great arteries. Cardiac catheterization may be needed to assess pulmonary artery pressure.

Tricuspid Atresia & Pulmonary Atresia

These conditions present with severe cyanosis and often require prostaglandin E1 infusion to maintain ductal patency. Echocardiography is crucial for diagnosis, demonstrating the absence of the respective valve. These often require staged surgical palliation.

Hypoplastic Left Heart Syndrome (HLHS)

HLHS typically presents with severe cyanosis and signs of heart failure shortly after birth. A weak femoral pulse is a characteristic finding. Echocardiography is diagnostic, showing underdevelopment of the left ventricle and aorta.

Why No Hepatomegaly?

The absence of hepatomegaly is significant. Hepatomegaly typically indicates right-sided heart failure, which is less common in conditions with decreased pulmonary blood flow. In TOF, for example, the right ventricle isn't overloaded with volume, but with pressure, due to pulmonary stenosis. In TGA, the systemic circulation is supplied by the right ventricle, which may not initially fail. HLHS presents with low cardiac output, but the liver congestion is not prominent early on.

Diagnostic Algorithm

A logical diagnostic approach would involve:

1. Pulse oximetry to confirm cyanosis.
2. Chest X-ray to assess cardiac size and pulmonary vascular markings.
3. ECG to look for signs of ventricular hypertrophy.
4. Echocardiography: The primary diagnostic tool to visualize the heart anatomy and identify the specific CHD.
5. Cardiac catheterization (if echocardiography is inconclusive or to assess pulmonary artery pressure).

Condition	Cyanosis	Murmur	Hepatomegaly	ECG	Echocardiography
TOF	Present	Systolic ejection murmur	Absent	RVH	Confirms all 4 defects
TGA	Severe, early	Loud S2	Absent	Normal or RVH	Switched great arteries
Tricuspid Atresia	Severe	None	Absent	RVH	Absent tricuspid valve