Meconium Aspiration Syndrome

Introduction

Meconium aspiration syndrome (MAS) refers to a varying spectrum of respiratory distress in neonates born through meconium-stained liquor.

The clinical presentation can vary from mild respiratory distress through to severe respiratory failure, which cannot be explained by an alternative diagnosis.¹

Aetiology

Meconium is the first material to pass through the fetal intestinal tract. It is composed of cells from intestine, skin, hair, vernix and amniotic fluid.

Although meconium is sterile, if it enters the respiratory tract can stimulate an inflammatory response through cytokine release.

The release of meconium into the amniotic fluid is felt to be related to increased vagal outflow (e.g. umbilical cord compression or hypoxia) which may accompany the clinical picture of MAS.¹

Risk factors

Approximately 2-10% of neonates born through meconium-stained liquor will develop MAS.

Meconium-stained liquor (and therefore MAS risk) increases with postdates gestation and small for gestational age.¹

Clinical features

The diagnosis of MAS is made based on the following clinical features:^1,5

• Meconium-stained liquor
• Respiratory distress at or shortly following birth
• Typical radiographic features on chest X-ray: hyperinflation, patchy opacification and consolidation
• Increased oxygen requirements (mechanical ventilation may be required for severe cases)

Meconium can affect the respiratory system in several ways:¹

• Respiratory distress: meconium has been shown to have a damaging effect on surfactant and its metabolism. This can result in severe effects related to reduced surfactant including increased surface tension, reduced lung volume, reduced compliance and reduced oxygenation.
• Pneumonitis: meconium can lead to irritation and local inflammation resulting in exudative and inflammatory pneumonitis.
• Bacterial pneumonia: meconium-stained liquor is a known risk factor for bacterial infection in utero (Escherichia coli in particular), leading to potentially increased morbidity.
• Pneumothorax: the meconium, if thick, can cause airway obstruction in distal small airways. Meconium plugging and distal gas trapping can lead to distention of distal lung and pneumothorax.

Ultimately, all of the above mechanisms lead to hypoxia and respiratory distress.

Differential diagnoses

Differential diagnoses include other causes of respiratory distress in the newborn.

These include transient tachypnoea of the newborn (TTN), delayed transition from fetal circulation, sepsis, congenital pneumonia, persistent pulmonary hypertension of the newborn (PPHN), pneumothorax and hypovolaemia.¹

These differential diagnoses can be clinically distinguished from MAS:¹

• Infants with TTN may initially present in a similar way to MAS, however, they are quick to recover
• Pneumonia can be difficult to differentiate initially and infants will be treated with antibiotics until blood culture results return
• Pneumothorax is differentiated with transillumination of the chest wall and a chest X-ray

Investigations

Bedside investigations

Relevant bedside investigations include:

• Pre- and post-ductal saturations: to assess respiratory involvement and detect congenital cardiac lesions
• Capillary gas or venous gas: to assess the degree of respiratory compromise and assist in decisions regarding respiratory support and systemic involvement

Laboratory investigations

Relevant laboratory investigations include:

• Full blood count: to look for raised white cell count suggestive of an infective process
• CRP: to look for an infective process
• Blood cultures: to look for bacteraemia suggestive of sepsis and/or pneumonia

Imaging

A chest X-ray should be performed according to local guidelines. Some neonatal units would consider waiting four hours to perform a chest X-ray. This time would be sufficient for TTN to resolve, eliminating the need for an X-ray.¹

However, if the infant is acutely unwell or mechanically ventilated, delaying a chest X-ray is not an option.

Chest X-ray findings in MAS may show hyperinflated lungs due to distal air trapping, patchy pulmonary changes and may show pneumothorax or pneumomediastinum due to raised alveolar tension.⁵

The X-ray also helps differentiate between some of the differential diagnoses including pneumonia and pneumothorax.

Management

Preventative management

There are several preventatives measures to take at various stages of delivery to prevent meconium aspiration syndrome.³

Intrapartum measures include prevention of fetal hypoxia and prevention of postdates gestation.

For infants who are born through meconium-stained liquor, management at time of delivery depends on the clinical status of the infant.

A vigorous infant requires no oropharyngeal suctioning despite the meconium-stained liquor as this does not reduce the risk of meconium aspiration syndrome.²

A non-vigorous infant should not have routine endotracheal suction for meconium. However, these infants may require oropharyngeal suctioning if there is meconium obstructing the airway. The priority should be to rapidly initiate ventilation.²

Post-delivery

Asymptomatic infants with an APGAR score of >9 do not require additional monitoring, other than that of risk of sepsis monitoring given the increased risk of infection with meconium aspiration.³

Infants with respiratory distress after birth should be admitted to a neonatal unit for 4-6 hours to ensure a successful transition.³

Management is supportive to avoid morbidity and mortality associated with MAS.

Supportive management includes oxygen therapy as needed, assisted ventilation if required. Continuous positive airway pressure (CPAP) is used with caution as this may exacerbate air trapping.

Some infants may require sedation (if ventilated) and surfactant therapy may reduce the need for extracorporeal membrane oxygenation (ECMO) in ventilated infants. However, ECMO may be required for severe cases.³

Antibiotics are usually started whilst awaiting blood cultures results. The risk of not treating an infection is significant and it can often be difficult to distinguish MAS from congenital pneumonia in the preliminary stages.³

Complications

Most infants with MAS have a good outcome and are discharged home. Short-term complications include ongoing oxygen requirements, seizures, and necrotising enterocolitis.

Some studies have suggested an increased incidence of reactive airways disease in infants who had MAS. There is limited evidence for altered neurodevelopment in infants who had MAS.³

Key points

• Meconium aspiration syndrome is a spectrum of respiratory distress in a neonate born through meconium-stained liquor.
• When aspirated, meconium can cause obstruction, gas trapping, irritation and inflammation leading to damaged lung surfactant, pneumonitis and hypoxia. It can also create a medium for bacteria to grow in the lung.
• Meconium aspiration syndrome is diagnosed based on the presence of meconium at delivery, respiratory distress, and oxygen requirements.
• Investigations include pre- and post-ductal saturations, blood gas, CRP, blood culture and chest X-ray.
• Key management includes intrapartum and delivery strategies to prevent MAS.
• Once diagnosed, treatment of MAS is primarily supportive and includes oxygen therapy and assisted ventilation. Antibiotic therapy is routinely used in MAS, as pneumonia and sepsis are difficult to differentiate in the initial stages.
• Most infants with MAS have a good outcome and are discharged home. Short-term complications include ongoing oxygen requirements, seizures, and necrotising enterocolitis.

Reviewer

Dr Thuy-Tien Vo

Paediatric Registrar

Editor

Dr Chris Jefferies

References

1. UpToDate. Clinical Features and Diagnosis of Meconium Aspiration Syndrome. Published Aug 12, 2019. Available from: [LINK]
2. ANZCOR (Australia New Zealand Council of Resuscitation). ANZCOR Guideline 13.4 – Airway Management and Mask Ventilation of the Newborn Infant. Available from: [LINK]
3. UpToDate. Prevention and Management of Meconium Aspiration Syndrome. Published Jan 7, 2020. Available from: [LINK]
4. Pandita, A., Murki, S., Oleti, T., Tandur, B., Kiran, S., Narkhede, S. and Prajapati, A., 2018. Effect of Nasal Continuous Positive Airway Pressure on Infants With Meconium Aspiration Syndrome. JAMA Pediatrics, 172(2), p.161.
5. Bickle, I., Radswiki et al. Meconium Aspiration. Available from: [LINK]