Updated: Oct 3, 2022
Meconium aspiration syndrome (MAS) occurs when the baby passes meconium in utero and subsequently aspirates it. The stimulus for aspiration is a hypoxia-induced gasping reflex that frequently occurs in utero. Following aspiration, meconium can migrate to the peripheral airway, leading to airway obstruction and subsequent lung inflammation and pulmonary hypertension. The presence of meconium in the endotracheal aspirate automatically establishes the diagnosis of meconium aspiration. Due to the toxic biophysical components of meconium, MAS is one of the most severe aspiration pneumonitis presentation seen in infants.
History & Clinical Presentation
The typical clinical picture is history of meconium-stained amniotic fluid (MSAF) at birth and respiratory distress in the first 24 hours of life. It also typically happens in post term infants, who are delivered after 41 weeks’ gestation.
Clinically, the baby presents with signs of respiratory distress like tachypnea (rapid breathing), grunting, chest retractions, nasal flaring, and cyanosis.
When the baby presents with MAS, there should also be high suspicion for the presence of Persistent Pulmonary Hypertension of Newborn (PPHN).
During pregnancy, the baby gets its oxygen from its mother and the placenta. Very little blood goes to the lungs because the blood vessels in the baby’s lungs are mostly closed. The blood vessels only open after birth when the baby takes his or her first breaths. The vessels then allow blood to travel to the lungs to get oxygen. PPHN happens when the blood vessels do not open enough, which means that there is a limit on how much oxygen is sent to the brain and organs.
Standard of Care for managing MAS infants:
1. Antenatally, if the mother does not go into labour spontaneously, it is advisable to electively induce at 41 weeks of gestation. The incidence of MAS increases with post term deliveries, which is a significant risk factor for early passage of meconium.
2. Meticulous management during labour and delivery is very crucial. Any mother presenting with MSAF and decelerations on the fetal strip should be closely monitored and if necessary, labour should not be prolonged, and steps should be taken to deliver the baby in the safest way possible.
3. According to American Academy of Pediatrics (AAP), if the infant born through meconium-stained amniotic fluid presents with poor muscle tone and inadequate breathing efforts, the initial steps of resuscitation should be completed under the radiant warmer. Positive pressure ventilation (PPV) should be initiated if the infant is not breathing, or the heart rate is <100/min after the initial steps are completed. Routine intubation for tracheal suction in this setting is not suggested because there is insufficient evidence to continue recommending this. Emphasis should be made on initiating ventilation within the first minute of life in non-breathing or ineffectively breathing infants.
4. MAS is managed by effective ventilation and adequate oxygenation, sedation and correction of hypotension, acidosis and other metabolic derangements
5. Inhaled nitric oxide (iNO) therapy, is commonly used in MAS babies. iNO decreases the right-to-left shunting of blood seen in PPHN, by selectively inducing vasodilation in the pulmonary vasculature (lungs) while having no effect on the systemic circulation.
6. Extracorporeal Membrane Oxygenation (ECMO). The ECMO machine, using a pump that works like the heart, pumps blood from the body through an artificial lung. Like a normal lung, it adds oxygen to the blood and removes carbon dioxide. Then the machine sends the blood back to the child.
7. The Canadian Pediatric Society (CPS) recommends the use of surfactant for babies with MAS who are intubated and require oxygen > 50%. While there is evidence to suggest that surfactant incurs some benefit in meconium aspiration syndrome, it has been further demonstrated that surfactant can increase the risk of air leaks, PPHN exacerbations and other detrimental complications. Due to this risk, in tertiary centers with access to iNO and high-frequency ventilation, as well as backup ECMO services, surfactant therapy is sometimes, although rarely, used in MAS.
Surfactant should NOT be ROUTINELY used in a community hospital setting for this indication and consultation with a tertiary center is advised beforehand.
Meconium aspiration syndrome is a complex condition that needs immediate and effective intervention to prevent any long-term damage from the in-utero insult. Most of the MAS babies will eventually be healed in a few days and are sent home. If the standard of care mentioned above is not followed and the baby is not treated promptly, then complications like lung injury, pneumonia, hypoxic ischemic encephalopathy, seizures and cerebral palsy can occur.
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Source: Toronto centre for neonatal health
American Academy of Pediatrics
Canadian Pediatric Society