Meconium aspiration syndrome (MAS)
Introduction
Meconium is the first intestinal discharge of newborn, viscous
and dark green composed of intestinal epithelial cells,
lanugo, mucus, intestinal secretions and swallowed amniotic
fluid elements. Meconiumstained amniotic fluid may be than
aspirated before or during birth causing neonatal respiratory
distress syndrome. The causes of premature meconium passage in
utero are not clear, hypoxic stress may play a role. Meconium
aspiration syndrome is seen in term and postterm infants,
it is rare before 34 weeks.
Etiology
- Risk factors (not always found)
- placental insufficiency
- maternal hypertension and preeclampsia
- oligohydramnion, maternal drug abuse
- Aspiration of meconium induces hypoxia via
- airway obstruction
- surfactant dysfunction
- chemical pneumonitis
- pulmonary hypertension due to hypoxic
vasoconstriction of pulmonary arteries
- the presence of meconium in the airways
also predispose the infant to bacterial pneumonia
Clinical signs
- severe respiratory distress
- pneumothorax is frequent
Macroscopic appearance
- the lungs are greenish brown in massive meconium inhalation but this is rare.
- mostly the lungs are congested and airless with petechial hemorrhages.
- meconium staning of skin and fingernails may be seen concurrently
Histology
- meconium in the lungs is present as eosinophilic granular
material with small yellowish meconium bodies and mucus
- acute inflammation appears in survivors after several hours
