Meconium is the earliest stool of a newborn. Occasionally, newborns pass meconium during labor or delivery, resulting in a meconium-stained amniotic fluid (MSAF). Meconium aspiration syndrome (MAS) is the neonatal respiratory distress that occurs in a newborn in the context of MASF when respiratory symptoms cannot be attributed to another etiology. The spectrum of manifestations associated with meconium aspiration is broad, ranging from mild distress to more severe respiratory failure. More life-threatening conditions have been recognized to also be associated with MAS, notably persistent pulmonary hypertension of the newborn (PPHN), and air leak syndromes.
MAS is due to the aspiration of meconium-stained amniotic fluid. MASF is not an uncommon finding and is not always associated with MAS. Uterine stress due to hypoxia or infection can cause early fetal meconium passage. Unlike infant stool, meconium is darker and thicker. It is formed through the accumulation of fetal cellular debris (skin, gastrointestinal, hair) and secretions. Aspiration of these materials causes airway obstruction, triggers inflammatory changes, and inactivates surfactant. Through these mechanisms, the neonate develops respiratory distress.
MSAF is more common in post-term newborns. Its incidence varies with gestational age. One study reported MASF in 5.1%, 16.5%, and 27.1% of preterm, term, and post-term newborns, respectively. Although MASF is necessary for the diagnosis of MAS, only 2 to 10% of babies born through amniotic fluid stained with meconium develop MAS.
The incidence of MAS is also affected by access to care and is higher in areas where post-term deliveries are frequent. It was also lower in areas with a high frequency of early cesarean sections, despite other complications associated with cesarean sections. One study reported a higher rate of MASF in African American patients.
The pathophysiology of MAS is not completely understood, however, 5 important processes have been described: Meconium passage, aspiration, airway obstruction, inflammation, and surfactant inactivation.
All these processes lead to a decrease in alveolar ventilation, causing increased ventilation-perfusion mismatch. This is the main cause of hypoxemia in infants with MAS. Prolonged hypoxemia will trigger pulmonary vascular constriction, which in turn increases the pulmonary vascular resistance (PVR). This is often accompanied by a right to left shunting. These mechanisms can trigger PPHN.
Relevant history for MAS diagnosis includes:
Important findings to note on physical exam that can be present with MAS:
History and clinical presentation/context are key in suspecting a diagnosis of MAS. This is crucial, as early interventions and management can be necessary for respiratory and cardiovascular support.
Evaluation of MAS includes:
Chest radiograph (CXR): Early CXR findings are nonspecific. These include streaky densities bilaterally. Later findings on CXR include hyperinflation, flattening of the diaphragms, and atelectasis. Pneumothorax can also be seen.
Arterial blood gas (ABG): ABG is a tool to assess the degree of respiratory failure and help guide management (intubation, mechanical ventilation). In severe cases, ABG will show hypoxemic, hypercapnic, respiratory acidosis.
Pulse oximetry: To assess oxygenation but also the degree of shunting (pre-ductal and postductal differential).
Echocardiography: (ECG) is an important tool to assess the heart function and help screen for signs PPHN and right ventricular dysfunction. It also helps identify the cardiac anatomy and evaluate for any cardiac level right to left shunting.
Blood and tracheal cultures: Evaluation for sepsis and pneumonia is crucial in the context of neonatal distress. Often empiric antibiotics are started.
Management of infant born with MSAF:
Infants born with MSAF should have routine neonatal care while monitoring for signs of distress according to the general neonatal resuscitation guidelines. According to the 2015 guidelines, the American Heart Association, the International Liaison Committee on Resuscitation, and the American Academy of Pediatrics no longer recommend routine endotracheal suctioning for non-vigorous infants with MSAF.
MASF infants will be observed for signs of MAS.
MAS management is mainly supportive, but early identification and support can improve outcomes and decrease morbidity and mortality. This requires an interprofessional team approach, including the obstetrician, midwife, neonatologist, respiratory therapist, nurse, pediatric pulmonologist, and pediatric cardiologist.
Oxygen therapy: Supplemental oxygen is often needed in MAS with goal oxygen saturation > 90% to prevent tissue hypoxia and improve oxygenation. Hypoxemia is an important trigger of pulmonary vasoconstriction, which can increase PVR and worsen PPHN.
Ventilatory support: This is indicated with refractory hypoxemia despite oxygen therapy, carbon dioxide retention, and increased respiratory distress. It also has a role for respiratory support in PPHN and air leak syndrome. There are no specific ventilation strategies. Oxygenation monitoring and serial ABG to help optimize oxygenation and ventilation are key. In severe cases with refractory hypoxemia, the patient might require extracorporeal membrane oxygenation (ECMO) for cardiorespiratory support.
Surfactant: The use of surfactant in MAS is not standard of care, however, as discussed above, surfactant inactivation has a role in the pathogenesis of MAS. Therefore surfactant may be helpful in some cases.
Nitric oxide: Inhaled nitric oxide is a pulmonary vasodilator that has a role in pulmonary hypertension and PPHN.
The differential diagnosis for MAS includes other causes of newborn distress:
Mortality in MAS is close to 1.2 percent based on a large retrospective study in the United States, this is lower than the mortality reported in developing countries.
The majority of infants recover with a good prognosis.
Short term complications have been discussed above, the main ones include:
Long term complication:
Meconium aspiration usually affects term and post-term newborn babies that are born with amniotic fluid stained with meconium (fetal stool). It occurs in less than 10% of infants born with meconium-stained amniotic fluid. It is an important cause of respiratory distress in a newborn and should be identified and managed early. The medical provider should rule out other conditions, such as neonatal infections.
Management requires admission to the neonatal intensive care unit and is mainly supportive. Most infants recover well with early diagnosis and management.
Early identification of risk factors associated with MAS is crucial. This allows for early preparation through communication and interprofessional partnerships. The obstetrician can help identify patients with infants at risk for MAS. Delivery should happen in a specialized center with access to a neonatal intensive care unit. This will allow trained personnel, including a midwife, nurse, pediatrician/neonatologist, and respiratory therapist to be present for the delivery and ready in the eventual need for respiratory support. Pediatric pulmonologists and pediatric cardiologists should be included in case complications like PPHN or air leak syndrome arise. In extreme cases, a pediatric surgeon is needed for ECMO cannulation or severe air leak syndrome requiring surgical attention.
This highlights the importance of interprofessional communication. Creating an interprofessional working group or engaging in interprofessional rounds to discuss these patients as a team will impact care positively and improve outcomes.
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