Transient tachypnoea of the newborn — short-answer questions

This stem concerns a typical transient tachypnoea of the newborn presentation in an infant delivered by elective caesarean before 39 weeks. ^[3]

Question 1 (10 marks)

a) Give the most likely diagnosis and outline the pathophysiology, relating it to the mode of delivery. (5 marks) ^[3]

The most likely diagnosis is transient tachypnoea of the newborn (wet lung syndrome). The fetal lung actively secretes fluid in utero, and at birth this fluid must be cleared by amiloride-sensitive epithelial sodium channels (ENaC) on the alveolar epithelium, which absorb sodium and water into the interstitium for lymphatic removal. ^[2]

Labour is the switch: the catecholamine surge upregulates ENaC and converts secretion to absorption, while vaginal delivery mechanically compresses the thorax to clear fluid. An elective caesarean without labour removes both signals, so ENaC activation is delayed, residual alveolar fluid persists, and compliance falls — producing tachypnoea and grunting that resolve as postnatal ENaC activity rises over 24–72 hours. ^[3]

b) State your initial investigations and the expected chest radiograph findings, and list four differential diagnoses you must exclude. (5 marks) ^[1]

Investigations are a chest radiograph, full blood count with CRP and blood culture to exclude sepsis, and a capillary blood gas to assess hypoxaemia. The expected chest radiograph shows perihilar streaking, hyperinflation, and fluid in the horizontal (minor) fissure; lung ultrasound showing a double lung point is highly specific. ^[1]

The four key differential diagnoses are respiratory distress syndrome from surfactant deficiency, neonatal sepsis or pneumonia, meconium aspiration syndrome, and congenital heart disease with a pre-post-ductal saturation gap. ^[1]