Foetal hypoxia
The lung areas which do not or only partially participate in ventilation, because of obstruction and/or destruction, will become hypoxic and an inflammatory response may consequently occur. Partial obstruction will lead to air trapping and hyperinflation of certain lung areas and pneumothorax 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
Microorganisms, most commonly Gram-negative rods, and endotoxins are found in samples of MSAF at a higher rate than in clear amniotic fluid, for example 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, 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.
Apoptosis
Apoptosis is an important mechanism in the clearance of injured cells and in tissue repair, however too much apoptosis may cause harm, such as acute lung injury. 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. Meconium induces an inflammatory reaction within the lungs as there is an increase of autophagocytic cells and levels of caspase 3 after exposure. After 8 hours of meconium exposure, in rabbit foetuses, the total amount of apoptotic cells is 54%. Therefore, the majority of meconium-induced lung damage may be due to the apoptosis of lung epithelium. Diagnosis
Respiratory distress in an infant born through the darkly coloured MSAF as well as meconium obstructing the airways is usually sufficient enough to diagnose MAS. Additionally, newborns with MAS can have other types of respiratory distress such as tachypnea and hypercapnia. Sometimes it is hard to diagnose MAS as it can be confused with other diseases that also cause respiratory distress, such as pneumonia. Additionally, X-rays and lung ultrasounds can be quick, easy and cheap imaging techniques to diagnose lung diseases like MAS. 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. Prevention during pregnancy
Prevention during pregnancy may include amnioinfusion and antibiotics but the effectiveness of these treatments are questionable.