What is the most likely diagnosis for a post-term newborn with a history of fetal distress, meconium-stained amniotic fluid, tachypnea, and respiratory distress, with chest X-ray showing hyperinflated lungs and patchy infiltrates?

Meconium Aspiration Syndrome

The most likely diagnosis is C. Meconium aspiration syndrome (MAS), based on the classic triad of post-term delivery, meconium-stained amniotic fluid with fetal distress, and the pathognomonic chest X-ray findings of hyperinflated lungs with patchy infiltrates and atelectasis.

Clinical Reasoning

This case presents the textbook presentation of MAS with all key diagnostic features:

Defining Clinical Features

• MAS is defined as respiratory distress in a neonate born through meconium-stained amniotic fluid whose symptoms cannot be otherwise explained 1, 2

• The characteristic early presentation includes respiratory distress, poor lung compliance, hypoxemia, and radiographic findings of hyperinflation and patchy opacifications—exactly matching this patient's chest X-ray 1

• Post-term status (≥42 weeks gestation) significantly increases the risk of meconium-stained amniotic fluid, occurring in approximately 5-15% of all deliveries 3

• Approximately 3-5% of neonates born through meconium-stained amniotic fluid develop MAS 3

Why Not the Other Diagnoses?

• Transient tachypnea of the newborn (TTN) typically presents with perihilar streaking and fluid in the fissures on chest X-ray, not hyperinflation with patchy infiltrates 4

• Respiratory distress syndrome (RDS) occurs primarily in premature infants (<1,500 g birth weight), not post-term infants, and shows a ground-glass appearance with air bronchograms on imaging 4

• Neonatal pneumonia lacks the specific history of meconium-stained amniotic fluid and fetal distress, and would not typically show the characteristic hyperinflation pattern seen with meconium aspiration 1

Pathophysiology Supporting the Diagnosis

• Meconium aspiration causes airway obstruction, interference with alveolar gas exchange, chemical pneumonitis, and surfactant dysfunction, leading to the hyperinflation (ball-valve effect) and patchy infiltrates (chemical pneumonitis and atelectasis) seen on this patient's chest X-ray 5

• The fetal distress history suggests chronic in utero stress, which triggers meconium passage and increases risk of aspiration during the antepartum or intrapartum period 5

• These pulmonary effects cause gross ventilation-perfusion mismatching and may be complicated by persistent pulmonary hypertension of the newborn 5

Critical Management Considerations

• Do not perform routine tracheal intubation and suctioning in nonvigorous infants born through meconium-stained amniotic fluid, as this delays ventilation without improving outcomes 3, 6

• Proceed immediately with appropriate resuscitation measures, including positive pressure ventilation if needed 6

• Consider intubation and suctioning only if there is evidence of airway obstruction 6

• Use pulse oximetry to guide oxygen therapy and initiate resuscitation with room air for term infants 6

• Consider positive end-expiratory pressure (PEEP) for ventilated infants with MAS to establish functional residual capacity 6

Common Pitfalls to Avoid

• Delaying positive pressure ventilation to perform suctioning can lead to prolonged hypoxia and worse outcomes 3

• Routine suctioning procedures can cause vagal-induced bradycardia, increased infection risk, and lower oxygen saturation 3

• Focusing solely on meconium presence without assessing overall clinical presentation may lead to inappropriate interventions 3