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Meconium Aspiration Syndrome (MAS)
Meconium Aspiration Syndrome (MAS) describes the spectrum of disorders and pathophysiology of newborns born in meconium-stained amniotic fluid. 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.

What is Meconium?
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, bile acids, blood, mucus, cholesterol, pancreatic secretions, lanugo, vernix caseosa and cellular debris. Meconium accumulates in the foetal gastrointestinal tract throughout the third trimester of pregnancy and it is the first intestinal discharge released within the first 48 hours after birth. 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.

Prevalence
1 in every 7 pregnancies have meconium-stained amniotic fluid (MSAF) and of these cases approximately 5% of these infants develop meconium aspiration syndrome (MAS). The frequency of MAS increases as the length of gestation increases such that the prevalence is greatest in post-term pregnancies. MSAF is observed 23-52% in pregnancies at 42 weeks and this may lead to MAS in these newborn infants. Conversely, pre-term births are not frequently associated with MSAF (only approximately 5% contain MSAF). Interestingly, the rate of MAS declines in populations where labour is induced in women that have post-term pregnancies that have exceeded 41 weeks. Many pre-disposing factors remain to be elucidated even though much effort has been dome to identify risk factors for MAS development. For example, the risk of MSAF is higher in African American mothers compared to mothers from other ethnic groups.

Signs, Symptoms and Diagnosis
Respiratory distress in an infant who was born through the darkly coloured MSAF is usually sufficient enough to diagnose MAS as there is obvious meconium observed to be obstructing the airways. Sometimes it is hard to diagnose MAS as it can be confused with other diseases that also cause respiratory distress, such as pneumonia. Additionally, newborns with MAS can have other types of respiratory distress such as tachypnoea (abnormal rapid breathing) and hypercapnia (abnormal CO2 levels in the blood). In cases of MAS, there is a need for supplemental oxygen in order to maintain oxygen saturation of haemoglobin at 92% or more. This oxygen is required during the first 2 hours after birth and is needed for at least 12 hours afterwards. The severity of respiratory distress can vary significantly between newborns with MAS as some require minimal or no supplemental oxygen requirement however, in severe cases, mechanical ventilation may be needed. Additionally, lung ultrasound can be a quick, easy and cheap imaging technique to diagnose lung diseases like MAS.

Meconium Passage due to Foetal Distress
It has been long observed that there is an important association between foetal distress and hypoxia with MSAF. However, this association is not always present and does not demonstrate cause-effect, as over three-fourths of infants with MSAF are vigorous at birth.

Foetal distress leads to foetal hypoxia at which point the foetus passes meconium resulting in MSAF. Meconium is inhaled and causes hypoxia in three ways (1) airway obstruction, (2) chemical pneumonitis and (3) surfactant dysfunction.

MASF has also been attributed to stress secondary to hypoxia and infection. Passage of meconium occurs more frequently when umbilical vein oxygen saturations are below 30%.

Meconium Passage due to Foetal Maturity
Peristalsis of the foetal intestines is present as early as 8 weeks gestation and the anal sphincter develops at about 20-22 weeks. The control of the anal sphincter is not well known however the foetus does defecate routinely into the amniotic cavity even in the absence of distress. The presence of intestinal enzymes, including the disaccharidases and alkaline phosphatase, have been recovered from amniotic fluid specimens at as early as 14-22 weeks in pregnancy. Thus, suggesting there is free passage of the intestinal contents into the amniotic cavity.

Although meconium appears very early in the gastrointestinal tract, MSAF rarely occurs before 34 weeks gestation and appears increasingly with advancing gestational age with its incidence increasing to 30e40% over 42 weeks. Motilin, an intestinal polypeptide which stimulates contraction of intestinal muscle, is found in higher concentrations in post-term than pre-term fetal gastrointestinal tracts. Furthermore, intestinal parasympathetic innervation and myelination also increase in later gestations, implying that the increasing incidence may reflect the maturation of peristalsis in the fetal intestine. Therefore, at increasing gestations, particularly post-term, MSAF may be a physiological event, reflecting the maturation of fetal intestinal function.

Pathophysiology
As MAS describes a spectrum of disorders of newborns born through MSAF without any congenital respiratory disorders or other underlying pathology there are numerous hypothesised mechanisms and causes for the onset of this syndrome. Long-term consequences may arise from these disorders, for example, infants that develop MAS have higher rates of developing neurodevelopmental defects due to poor respiration.

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 hypoxaemia, hypercapnia, atelectasis (partial collapse or incomplete inflation of the lungs) and respiratory acidosis. After 60 minutes of exposure the meconium travels further down into the smaller airways. Within the terminal bronchioles and alveoli, the meconium triggers inflammation, pulmonary oedema, vasoconstriction, bronchoconstriction, collapse of airways and inactivation of surfactant.

Partial obstruction will lead to air trapping and hyperinflation of certain lung fields and pneumothorax may occur.

Foetal Hypoxia
The lung areas which do not or only partially participate in ventilation (because of obstruction and/or destruction) will become hypoxic and subsequently, an inflammatory response may follow. Chronic hypoxia will lead to an increase in pulmonary vascular smooth muscle tone and persistent pulmonary hypertension causing respiratory and circulatory failure.

Infection
Romero et al. (2014) found that microorganisms, most commonly Gram-negative rods, were found in their sample of patients with MSAF at a higher rate (19.6%) than in those with clear amniotic fluid (4.7%). They also observed that endotoxins were more frequently found in MSAF than in clear amniotic fluid such that 46.9% of patients with MSAF also had endotoxins present. A microbial invasion of the amniotic cavity (MIAC) is more common in patients with MSAF and this could ultimately lead to an intra-amniotic inflammatory response. MIAC is associated with high concentrations of cytokines (such as IL-6), chemokines (such as IL-8 and monocyte chemoattractant protein-1), complement-split products, phospholipase A2 and matrix-degrading enzymes. Therefore, these aforementioned mediators within the amniotic fluid during MIAC and intra-amniotic infection could, when aspirated in utero, induce lung inflammation within the foetus.

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 (inflammation of the lung caused by aspirating or inhaling irritants) as it is a potent activator of inflammatory mediators which include cytokines, complement, prostaglandins and reactive oxygen species.

Meconium is a source of pro-inflammatory cytokines, including tumor necrosis factor (TNF) and interleukins (IL-1, IL-6, IL-8), and mediators produced by neutrophils, macrophages and epithelial cells that may injure the lung tissue directly or indirectly. For example, neutrophils are activated by macrophages and cytokines and adhere to the endothelium. Within several hours, meconium causes an accumulation of neutrophils within the lungs. Proteolytic enzymes are released from neutrophilic granules and these proteases may damage the membranes and surfactant proteins. Meconium contains high amounts of pancreatic phospholipase A2 (PLA2) which may directly, or through the stimulation of arachidonic acid metabolites, injure the lung epithelium, endothelium, surfactant and intensify apoptosis. Activated leukocytes and cytokines generate reactive nitrogen and oxygen species which have cytotoxic effects. Oxidation stress results in vasoconstriction, bronchoconstriction, platelet aggregation and accelerated cellular apoptosis.

Phospholipase A2 is a potent proinflammatory enzyme, triggering proinflammatory cells to produce cytokines and possibly leading to surfactant dysfunction and cellular destruction with tissue necrosis and, presumably, apoptosis. PLA2 activity has been detected in human meconium and in meconium-contaminated lungs, indicating that meconium itself is a source of this enzyme. Possibly, bile acids present in meconium raise PLA2.

Activated neutrophils and macrophages may damage the lungs by different pathways. Meconium is a potent activator of inflammatory mediators, including cytokines, complement, prostaglandins and reactive oxygen species. Meconium can also activate the coagulation cascade, production of platelet activating factor (PAF) and other vasoactive substances that may lead to destruction of capillary endothelium and basement membranes. Injury to the alveolcapillary membrane results in leakage of liquid, plasma proteins, and cells into the interstitium and alveolar spaces.

Recently, it has been hypothesised that meconium is a potent activator of complement, a key mediator of inflammation, and may thus contribute to the inflammatory response in MAS. TLRs and complement are the two main danger recognition systems. Activation of these systems initiates the secondary inflammatory response, that is the activation of leukocytes and the release of cytokines, ROS, expression of adhesion molecules etc..

Surfactant Inactivation
Surfactant is synthesised by type II alveolar cells and is made of a complex of phospholipids, neutral lipids, proteins and saccharides. It functions to lower surface tension, stabilise alveoli and terminal airways at the end of expiration, decreases suction forces into alveoli and thereby prevents lung oedema. Surfactant also contributes to lung protection and defence as it is also an anti-inflammatory agent. Surfactant enhances the removal of inhaled particles and senescent cells away from the alveolar structure. Surfactant reduces alveolar surface tension to facilitate lung expansion, preventing alveolar collapse after (the onset of) breathing

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 following meconium exposure. The major features were detachment of airway epithelium from stroma and shedding of epithelial cells into the airway, indicating a direct deleterious effect on meconium on lung alveolar cells.

Persistent  Pulmonary Hypertension
Persistent pulmonary hypertension (PPHN) is the failure of the foetal circulation to adapt to extra-uterine conditions after birth. PPHN is associated with various respiratory diseases, including MAS, but also pneumonia and sepsis among others. A combination of hypoxia due to MAS, pulmonary vasoconstriction and ventilation/perfusion mismatch can trigger PPHN. Meconium contamination induces a concentration-dependent pulmonary hypertensive response with 15-20% of infants with MAS developing PPHN (Fanaroff, 2008). In clinical MAS, persistent pulmonary hypertension of the newborn is the leading cause of death in MAS. Pulmonary hypertension plays a significant role in the severity and outcome.

Apoptosis
Meconium induces apoptosis and DNA cleavage of lung airway epithelial cells, this is detected by the presence of fragmented DNA within the airways and in alveolar epithelial nuclei. Zagariya et al. (2005) found that the total amount of apoptotic cells was at 54% after 8 hours of meconium exposure in rabbit foetuses however after 24 hours of exposure there was no difference in apoptosis. Meconium induces an inflammatory reaction within the lungs which includes the presence of autophagocytic cells and an increased level of caspase-3 after exposure. Therefore, the majority of meconium-induced lung damage may be due to damage of airway epithelium cell barrier from apoptosis.

Apoptosis, programmed cell death, is an important mechanism in the clearance of injured cells and in tissue repair, however too much apoptosis may cause harm. Increased apoptosis may also play a role in acute lung injury, leading to damage and detachment of lung airway or alveolar cells.

Treatment
Most infants born through MSAF do not require any treatments as they show no signs of respiratory distress as only approximately 5% of infants born through MSAF develop MAS. In this case, infants should receive routine postnatal care. However, infants which do develop MAS need to be administered to a neonatal unit where they will be closely observed and provided any treatments needed. Observations include monitoring heart rate, respiratory rate, oxygen saturation and blood glucose (to detect worsening respiratory acidosis or the development of hypoglycaemia). In general, treatment of MAS is more supportive in nature.

Assisted Ventilation
Acute intrapulmonary meconium contamination induces a concentration-dependent pulmonary hypertensive response, with 15 to 20% of infants with the MAS showing persistent pulmonary hypertension.

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. However, supplemental oxygen can be given to the infant to help maintain ideal oxygen levels. The desired oxygen saturation is between 90-95% and PaO2 may be as high as 90mmHg. 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. While on ECMO, the body can have time to absorb the meconium and for all the associated disorders to resolve. There has been an excellent response to this treatment, as the survival rate while on ECMO is more than 94%.

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 can have homogenous lung changes and others can have inconsistent and patchy changes to their lungs in response to MAS. Infants with severe MAS often require a high airway pressure and high FiO2. It is also common for sedation and muscle relaxants to be used to optimise ventilation and minimise the risk of pneumothorax associated with dyssynchronous breathing.

Inhaled nitric oxide (iNO) acts on vascular smooth muscle causing selective pulmonary vasodilation. This is ideal in the treatment of PPHN as it causes vasodilation within ventilated areas of the lung thus, decreasing the ventilation-perfusion mismatch and improves oxygenation. Treatment utilising iNO decreases the need for ECMO and mortality in newborns with hypoxic respiratory failure and PPHN as a result of MAS. However, approximately 30-50% of infants with PPHN do not respond to iNO therapy.

Anti-Inflammatories
As inflammation is such a huge issue in MAS, treatment has consisted of anti-inflammatories.

Glucocorticoids
Glucocorticoids (GCs) have a strong anti-inflammatory activity and works to reduce the migration and activation of neutrophils, eosinophils, mononuclears and other cells. GCs reduce the penetration of neutrophils into the lungs ergo, decreasing their adherence to the endothelium. Thus, there is a reduction in the action of mediators released from these cells and therefore, a reduced inflammatory response.

GCs also possess a genomic mechanism of action in which, once bound to a glucocorticoid receptor, the activated complex moves into the nucleus and inhibits transcription of mRNA. Ultimately, effecting whether various proteins get produced or not. Inhibiting the transcription of nuclear factor (NF)-κB and protein activator (AP)-1 attenuates the expression of pro-inflammatory cytokines (IL-1, IL-6, IL-8 and TNF etc.), enzymes (PLA2, COX-2, iNOS etc.) and other biologically active substances. The anti-inflammatory effect of GCs is also demonstrated by enhancing the activity of lipocortines which inhibit the activity of PLA2 and therefore, decrease the production of arachidonic acid and mediators of lipooxygenase and cyclooxygenase pathways.

Anti-inflammatories need to be administered as quickly as possible as the effect of these drugs can diminish even just an hour after meconium aspiration. For example, early administration of dexamethasone significantly enhanced gas exchange, reduced ventilatory pressures, decreased the number of neutrophils in the bronchoalveolar area, reduced oedema formation and oxidative lung injury.

However, GCs may increase the risk of infection and this risk increases with the dose and duration of glucocorticoid treatment. Other issues can arise, such as aggravation of diabetes mellitus, osteoporosis, skin atrophy and growth retardation in children.

Inhibitors of Phosphodiesterase
Phosphodiesterases (PDE) degrades cAMP and cGMP and, within the respiratory system of a newborn with MAS, various isoforms of PDE may be involved due to their pro-inflammatory and smooth muscle contractile activity. Therefore, non-selective and selective inhibitors of PDE could potentially be used in MAS therapy. However, the use of PDE inhibitors can cause cardiovascular side effects. Non-selective PDE inhibitors, such as methylxanthines, increase concentrations of cAMP and cGMP in the cells leading to bronchodilation and vasodilation. Additionally, methylxanthines decreases the concentrations of calcium, acetylcholine and monoamines, this controls the release of various mediators of inflammation and bronchoconstriction, including prostaglandins. Selective PDE inhibitors target one subtype of phosphodiesterase and in MAS the activities of PDE3, PDE4, PDE5 and PDE7 may become enhanced. For example, Milrinone (a selective PDE3 inhibitor) improved oxygenation and survival of neonates with MAS.

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 hyperkalaemia and hypernatremia. Additionally, COX inhibitors have no shown any great response in the treatment of MAS.

Antibiotics
Meconium is typically sterile however, it can contain various cultures of bacteria so appropriate antibiotics may need to be prescribed.

Surfactant Treatment
Lung lavage with diluted surfactant is a new treatment with potentially beneficial results depending on how early it is administered in newborns with MAS. This treatment shows promise as it had a significant effect on air leaks, pneumothorax, the need for ECMO and death. Early intervention and using it on newborns with mild MAS is more effective. However, there are risks as a large volume of fluid instillation to the lung of a newborn can be dangerous (particularly in cases of severe MAS with pulmonary hypertension) as it can exacerbate hypoxia and lead to mortality.

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 prevention treatment was embraced as a study by Carson et al. (1976) claimed a significant decrease in the incidence of MAS compared to no preventative treatment in newborns born through MSAF. However, the results between the two groups were later found to not be significant. Later studies, concluded that oropharyngeal and nasopharyngeal suctioning before delivery of the shoulders in infants born through MSAF does not prevent MAS or its complications. In fact, it can cause more issues and damage (e.g. mucosal damage), thus it is not a recommended preventative treatment. Suctioning may not significantly reduce the incidence of MAS as meconium passage and aspiration may occur in-utero. Thereby making the suctioning redundant and useless as the meconium may already deep within the lungs at the time of birth.

Historically, amnioinfusion has been used when MSAF was present, which involves a transcervical infusion of fluid during labour. The idea was to dilute the thick meconium to reduce its potential pathophysiology and reduce cases of MAS, since MAS is more prevalent in cases of thick meconium. However, there are associated risks such as ambilical cord prolapse and prolongation of labour. The UK National Institute of Health and Clinical Excellence (NICE) Guidelines recommend against the use of amnioinfusion in women with MSAF.

Prevention
In generally, the incidence of MAS has been significantly reduced over the past two decades as the number of post-term deliveries has minimised. Currently, labour is induced in women who have been pregnant for longer than 41 weeks gestation.

Prevention During Pregnancy
Prevention during pregnancy may include amnioinfusion and antibiotics but the effectiveness of these treatments are questionable.

Prevention During Partituition
As previously mentioned, oropharyngeal and nasopharyngeal suctioning is not an ideal preventative treatment for both vigorous and depressed (not breathing) infants. In the presence of MSAF, it was recommended to suction the mouth, nose and nasopharynx of the foetus before delivery of the shoulders.

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. Additionally, there is still research being conducted on whether intubation and suctioning of meconium in newborns with MAS is beneficial, harmful or is simply a redundant and outdated treatment. In generally, there is still no generally accepted therapeutic protocol and effective treatment plan for MAS.