Meconium aspiration syndrome (MAS), also known as neonatal aspiration of meconium, is a medical condition affecting newborn infants. It describes the spectrum of disorders and pathophysiology of newborns born in meconium-stained amniotic fluid (MSAF) and have meconium within their lungs. Therefore, MAS has a wide range of severity depending on what conditions and complications develop after parturition. Furthermore, the pathophysiology of MAS is multifactorial and extremely complex which is why it is the leading cause of morbidity and mortality in term infants.
The word meconium is derived from the Greek word mēkōnion meaning juice from the opium poppy as the sedative effects it had on the foetus were observed by Aristotle.
Meconium is a sticky dark-green substance which contains gastrointestinal secretions, amniotic fluid, bile acids, bile, blood, mucus, cholesterol, pancreatic secretions, lanugo, vernix caseosa and cellular debris. Notably, since meconium and the whole content of the gastrointestinal tract is located 'extracorporeally,' its constituents are hidden and normally not recognised by the foetal immune system.
For the meconium within the amniotic fluid to successfully cause MAS, it has to enter the respiratory system during the period when the fluid-filled lungs transition into an air-filled organ capable of gas exchange.
The association between foetal distress and meconium passage is not a definite cause-effect relationship as over of infants with MSAF are vigorous at birth and do not have any distress or hypoxia.
Motilin is found in higher concentrations in post-term than pre-term foetal gastrointestinal tracts. Similarly, intestinal parasympathetic innervation and myelination also increases in later gestations. Therefore, the increased incidence of MAS in post-term pregnancies may reflect the maturation and development of the peristalsis within the gastrointestinal tract in the newborn.
Airway obstruction
In the first 15 minutes of meconium aspiration, there is obstruction of larger airways which causes increased lung resistance, decreased lung compliance, acute hypoxemia, hypercapnia, atelectasis and respiratory acidosis. After 60 minutes of exposure, the meconium travels further down into the smaller airways. Once within the terminal bronchioles and alveoli, the meconium triggers inflammation, pulmonary edema, vasoconstriction, bronchoconstriction, collapse of airways and inactivation of surfactant.
Pulmonary inflammation
Meconium has a complex chemical composition, so it is difficult to identify a single agent responsible for the several diseases that arise. As meconium is stored inside the intestines, and is partly unexposed to the immune system, when it becomes aspirated the innate immune system recognises as a foreign and dangerous substance. The immune system, which is present at birth, responds within minutes with a low specificity and no memory in order to try to eliminate microbes. Meconium perhaps leads to chemical pneumonitis as it is a potent activator of inflammatory mediators which include cytokines, complement, prostaglandins and reactive oxygen species. Recently, it has been hypothesised that meconium is a potent activator of toll-like receptor (TLRs) and complement, key mediators in inflammation, and may thus contribute to the inflammatory response in MAS.
The extent of surfactant inhibition depends on both the concentration of surfactant and meconium. If the surfactant concentration is low, even very highly diluted meconium can inhibit surfactant function whereas, in high surfactant concentrations, the effects of meconium are limited. Meconium may impact surfactant mechanisms by preventing surfactant from spreading over the alveolar surface, decreasing the concentration of surfactant proteins (SP-A and SP-B), and by changing the viscosity and structure of surfactant. Several morphological changes occur after meconium exposure, the most notable being the detachment of airway epithelium from stroma and the shedding of epithelial cells into the airway. These indicate a direct detrimental effect on lung alveolar cells because of the introduction of meconium into the lungs. Therefore, the majority of meconium-induced lung damage may be due to the apoptosis of lung epithelium.
Prevention
In general, the incidence of MAS has been significantly reduced over the past two decades as the number of post-term deliveries has minimized. In general, treatment of MAS is more supportive in nature.
Assisted ventilation techniques
To clear the airways of meconium, tracheal suctioning can be used however, the efficacy of this method is in question and it can cause harm.
In cases of MAS, there is a need for supplemental oxygen for at least 12 hours in order to maintain oxygen saturation of haemoglobin at 92% or more. The severity of respiratory distress can vary significantly between newborns with MAS, as some require minimal or no supplemental oxygen requirement and, in severe cases, mechanical ventilation may be needed. In cases where there is thick meconium deep within the lungs, mechanical ventilation may be required. In extreme cases, extracorporeal membrane oxygenation (ECMO) may be utilised in infants who fail to respond to ventilation therapy.
Ventilation of infants with MAS can be challenging and, as MAS can affect each individual differently, ventilation administration may need to be customised. Some newborns with MAS can have homogenous lung changes and others can have inconsistent and patchy changes to their lungs. It is common for sedation and muscle relaxants to be used to optimise ventilation and minimise the risk of pneumothorax associated with dyssynchronous breathing.
Inhibitors of cyclooxygenase
Arachidonic acid is metabolised, via cyclooxygenase (COX) and lipoxygenase, to various substances including prostaglandins and leukotrienes, which exhibit potent pro-inflammatory and vasoactive effects. By inhibiting COX, and more specifically COX-2, (either through selective or non-selective drugs) inflammation and oedema can be reduced. However, COX inhibitors may induce peptic ulcers and cause hyperkalemia and hypernatremia. Additionally, COX inhibitors have not shown any great response in the treatment of MAS.
Previous treatments
Originally, it was believed that MAS developed as a result of the meconium being a physical blockage of the airways. Thus, to prevent newborns, who were born through MSAF, from developing MAS, suctioning of the oropharyngeal and nasopharyngeal area before delivery of the shoulders followed by tracheal aspiration was utilised for 20 years. This treatment was believed to be effective as it was reported to significantly decrease the incidence of MAS compared to those newborns born through MSAF who were not treated. This claim was later disproved and future studies concluded that oropharyngeal and nasopharyngeal suctioning, before delivery of the shoulders in infants born through MSAF, does not prevent MAS or its complications.
Future research
Research is being focused on developing both a successful method for preventing MAS as well as an effective treatment. For example, investigations are being made in the efficiency of anti-inflammatory agents, surfactant replacement therapy and antibiotic therapy. More research needs to be conducted on the pharmacological properties of, for example, glucocorticoids, including dosages, administration, timing or any drug interactions.