Meconium aspiration syndrome

Background

• May cause meconium aspiration syndrome (MAS) in a newborn infant that was born through meconium-stained amniotic fluid (MSAF)
• Presentation ranges from mild respiratory distress to life-threatening respiratory failure
• Incidence is 2-10% of infants born through MSAF
• Thought to be associated with fetal hypoxia and post-term delivery
• Causes hypoxemia and acidosis via airway obstruction, chemical irritation/inflammation, infection, and surfactant inactivation
• Associated with persistent pulmonary hypertension of the newborn (PPHN)

Clinical Features

• Tachypnea
• Cyanosis
• Accessory muscle use (intercostal/subxiphoid retractions, paradoxical breathing, grunting, nasal flaring)
• Barrel-shaped chest
• Rales and rhonchi on lung auscultation
• Pneumothorax
• Pneumomediastinum
• Usually develop symptoms immediately after birth, but sometimes are born asymptomatic and develop symptoms as meconium moves into lower tracheobronchial tree

Differential Diagnosis

• Transient tachypnea of the newborn (TTN)
• Sepsis
• Pneumonia
• Delayed transition from fetal circulation
• Pneumothorax
• Pulmonary edema
• Blood aspiration
• Pulmonary hypertension
• Congenital heart disease
• Respiratory distress syndrome

Evaluation

• Workup
  • CXR
  • ABG
  • Echocardiogram
  • Blood and sputum cultures

• Clinical diagnosis based on the following:
  • Evidence of meconium on infant
  • Respiratory distress shortly after birth
  • Characteristic CXR findings
    • Initially, streaky, linear densities
    • Next, development of hyperinflated lungs and flattened diaphragm
    • Finally, diffuse patchy opacities (may appear similar to ARDS if severe)
  • If intubation required, meconium visualized in trachea

• Ways to differentiate between other causes of respiratory distress in a neonate:
  • TTN is more common in late preterm infants (34-37 weeks) and RDS in preterm infants, whereas MAS more common in postmature infants (>41 weeks)
  • Delayed transition from fetal circulation symptoms improve more quickly than those of MAS
  • Congenital cyanotic heart disease is differentiated by physical exam (murmurs, hepatomegaly), CXR (cardiac size/shape), and echocardiogram (cardiac anatomy and function)

Management

• Empiric antibiotics while awaiting culture results (because of difficulty differentiating between pneumonia initially)

• Prevention
  • Fetal heart rate monitoring intrapartum to reduce risk of fetal hypoxia
  • Induction of labor after 41 weeks gestation to reduce postmature delivery
  • Amnioinfusion with isotonic fluid to dilute thick meconium is NOT routinely recommended