Meconium Aspiration Syndrome (MAS) Flashcards
What is meconium aspiration syndrome (MAS)?
Meconium Aspiration Syndrome (MAS) is the term used to describe a spectrum of disorders, marked by various degrees of respiratory distress in the new born infant. This follows the aspiration of meconium stained amniotic fluid (MSAF), which can happen either antenatally (2.8% of all pregnancies) or during birth (up to 23% of all pregnancies).
What is meconium?
Meconium is the dark green, sticky and lumpy faecal material produced during pregnancy. It is usually released from the bowels after birth.
Briefly differentiate between meconium aspiration syndrome (MAS) and aspiration of meconium stained amniotic fluid (MSAF)
Meconium is the dark green, sticky and lumpy faecal material produced during pregnancy. It is usually released from the bowels after birth, but in 8-25% of pregnancies, the baby can pass meconium in utero, leading to MSAF. Of these, 5-12% of babies can aspirate MSAF and develop MAS.
Why is MAS a problem?
MAS can cause the new born to develop respiratory distress which may be life threatening. It is still an important cause of morbidity and mortality amongst neonates. Also, MAS can lead or exacerbate other neonatal complications such as neonatal sepsis and ischaemic insults. MAS is highest amongst infants who are post term, had thick meconium at birth and suffered birth asphyxia.
Briefly descirbe the pathophysiology of MSAF
MSAF is the after-effect of in-utero peristalsis. In term or post term infants, where gastrointestinal maturation is appropriate, this movement leads to meconium passage. The peristalsis usually is the result of foetal hypoxic stress or vagal stimulation due to cord compression. There is also some evidence that chronic hypoxia may lead to it as well. These factors can also lead to fetal gasping which results in MAS.
Briefly describe the pathophysiology of MAS
The pathophysiology of MAS is complex. It depends not only on the meconium’s varied chemical composition (the inflammatory interactions of which are not fully understood) but other factors affecting the neonate’s health as well, such as fetal hypoxia andairway obstruction.
However, once aspirated, it can stimulate the release of many vasoactive and cytokine substances that activate inflammatory pathways, as well as triggering vasculature changes. It also inhibits the effect of surfactant in the lungs.
What are the common features of MAS-related respiratory distress in neonates?
- Partial or total airway obstruction
- Foetal hypoxia
- Pulmonary inflammation
- Infection
- Surfactant inactivation
- Persistent Pulmonary Hypertension (PPHN)
How can MAS lead to partial or total airway obstruction?
Due to its thick and sticky consistency, meconium can cause partial or total mechanical airway obstruction. This leads to a decrease in pulmonary ventilation of the small airways. This may lead to atelectasis and a “ball-valve” effect with air trapping. Currently, there is evidence that there is little correlation between the presence of meconium in the trachea and clinical signs of severe MAS. Once the airways are obstructed, the pulmonary pressure increases, leading to right-to-left shunt (through a patent ductus arteriosus or foramen ovale). This creates a V:Q mismatch, leading to a downward spiral of severe foetal hypoxia.
How can MAS lead to foetal hypoxia?
Foetal hypoxia results from a myriad of reasons in MAS: a V/Q mismatch, increase of pulmonary vascular pressures, mechanical obstruction, airway oedema and/or surfactant inactivation. All these can cause a decrease in gas exchange and a drop in foetal oxygen saturations. Although it is an important factor to consider in MAS (as it prompts the clinicians to react quicker to the patient) it is neither the cause nor the determinant factor in the severity of MAS in ¾ of the cases.
How can MAS lead to pulmonary inflammation?
Meconium contains many pro-inflammatory cytokines such as tumour necrosis factor (TNF) and interleukins that directly and indirectly contribute to lung tissue injury, surfactant inactivation and infection.
How can MAS lead to infection?
The inflammation process predisposes the foetal lung to an increased risk of infection and can cause a chemical pneumonitis. (N.B. even though both are usually considered sterile, meconium has been shown to be a good medium for microorganisms to grow in compared to clear amniotic fluid).
How can MAS lead to surfactant inactivation?
The inflammatory reaction caused by meconium deactivates surfactant which increases the surface tension of the alveoli. This reduces the efficiency of gas exchange and in turn further exacerbates foetal hypoxia.
How can MAS lead to Persistent Pulmonary Hypertension (PPHN)?
Persistent Pulmonary Hypertension (PPHN): PPHN results from remodelling of the pulmonary vascular bed in response to hypoxia, vasoactive mediators in the meconium and ventilation/perfusion mis-match. PPHN is one of the major causes of morbidity and mortality in MAS.
What are the risk factors for MAS?
The risk factors for MAS are not completely understood. It is though that conditions leading to foetal hypoxia and foetal gasping increase the risk of aspirating meconium in a term or post-term baby, they are as follows;
- Gestational Age > 42 weeks
- Foetal distress (tachycardia / bradycardia)
- Intrapartum hypoxia secondary to placental insufficiency
- Thick meconium particles
- Apgar Score <7
- Chorioamnionitis +/- Prolonged pre-rupture
- Oligohydramnios
- In utero growth restriction (IUGR)
- Maternal hypertension, diabetes, pre-eclampsia or eclampsia, smoking and drug abuse
What are the clincal features of MAS?
A diagnosis of MAS is difficult given the non-specific presentation of respiratory distress on examination at birth (which often overlaps with other conditions). Having said this, it is still a clinical diagnosis. A full history of the risk factors, confirmed presence of meconium in the amniotic fluid and aspirated meconium (clinical signs of post-maturity and MSAF staining) are necessary to diagnose MAS in an infant presenting with respiratory distress where no other diagnosis can be confirmed ie. MAS is a diagnosis of exclusion.
On examination the newborn infant will show signs of respiratory distress:
- Tachypnoea- a respiratory rate of >60 breaths per minute
- Tachycardia- a heart rate of >160 beats per minute
- Cyanosis- this requires immediate management
- Grunting
- Nasal flaring
- Recessions- intercostal, supraclavicular, tracheal tug
- Hypotension- systolic blood pressure of <70 mmHg
Briefly describe what is shown on the chest x-ray
Chest x-ray showing bilateral asymmetric opacification of MAS.
