What is Meconium Aspiration Syndrome?
Meconium Aspiration Syndrome (MAS) is a clinical condition characterized by respiratory distress in newborns born through meconium-stained amniotic fluid, where the respiratory symptoms cannot be explained by other causes. 1, 2
Definition and Pathophysiology
MAS occurs when a neonate aspirates meconium (the first stool of a newborn) mixed with amniotic fluid into the lungs before, during, or immediately after birth. 3 This aspiration can happen during the antepartum or intrapartum periods and results in multiple pulmonary complications. 4
Mechanisms of Lung Injury
The aspiration of meconium causes respiratory distress through several distinct pathways:
- Airway obstruction from thick meconium particles blocking the tracheobronchial airways 4
- Chemical pneumonitis from the irritant properties of meconium components 4
- Surfactant dysfunction leading to alveolar collapse and impaired gas exchange 4
- Ventilation-perfusion mismatching causing severe hypoxemia 4
- Persistent pulmonary hypertension often develops as a complication, particularly in infants who experienced chronic in utero stress with resultant thickening of pulmonary vessels 4
Epidemiology
- Meconium-stained amniotic fluid occurs in approximately 5-15% of all deliveries 5
- Only 3-5% of neonates born through meconium-stained amniotic fluid actually develop MAS 5
- The condition is more common in post-term pregnancies (≥42 weeks gestation) 5
- MAS remains an important cause of morbidity and mortality in term and post-term newborns despite advances in perinatal care 1, 6
Clinical Presentation
MAS should be suspected in any infant with respiratory distress born through meconium-stained amniotic fluid. 4 The clinical features include:
- Early onset respiratory distress in term and near-term infants 2
- Poor lung compliance requiring increased ventilatory support 2
- Hypoxemia that may be severe and refractory 2
- Radiographic findings showing hyperinflation and patchy opacifications on chest X-ray 2
Risk Factors and Timing
The aspiration can occur at different time points:
- Before delivery during periods of fetal distress 3
- During birth as the infant passes through the birth canal 3
- During resuscitation if meconium is present in the airway 3
Complications
Beyond the immediate respiratory distress, MAS can lead to life-threatening complications:
- Respiratory failure requiring mechanical ventilation 7
- Pulmonary air leaks including pneumothorax 7
- Persistent pulmonary hypertension of the newborn which significantly worsens prognosis 7, 4
- Secondary pulmonary infection, as meconium in the airways predisposes to bacterial colonization despite being sterile itself 4
Current Understanding of Management
Historical approaches to MAS prevention have evolved significantly. 7 Routine suctioning of the oropharynx before delivery of the shoulders was abandoned after randomized controlled trials showed no benefit. 3 Similarly, routine endotracheal intubation and tracheal suctioning for all meconium-stained newborns was discontinued after evidence showed no value in vigorous infants. 3
For nonvigorous infants born through meconium-stained amniotic fluid, current guidelines no longer recommend routine immediate direct laryngoscopy with or without tracheal suctioning, as this delays ventilation without improving outcomes. 5, 8 Tracheal suctioning has not been associated with reduction in the incidence of MAS or mortality in depressed infants. 3
Long-term Considerations
Despite improved survival rates over recent decades due to advances in supportive care, surfactant therapy, inhaled nitric oxide, and high-frequency ventilation, long-term morbidity among survivors remains a significant concern. 1, 6