Meconium aspiration syndrome (MAS) occurs when a neonate inhales thick, particulate meconium. This is usually secondary to fetal hypoxia which causes increased peristalsis, relaxation of anal sphincters and reflex gasping. Most meconium deliveries involve some meconium staining of the liquor but the babies are vigorous, needing no further intervention.
Meconium-stained amniotic fluid may be aspirated before or during labour and delivery. As meconium is rarely found in the amniotic fluid prior to 34 weeks of gestation, meconium aspiration usually affects babies born at term and post-term.
There are several pathomechanisms participating in MAS, particularly airway obstruction, surfactant dysfunction, inflammation, lung oedema, pulmonary vasoconstriction and bronchoconstriction.
Studies of tracheal aspirate confirm an inflammatory response, with increase in inflammatory cell count and the level of pro-inflammatory cytokines, with a corresponding decrease in lung function. In the majority of cases, these changes begin to resolve after the first six hours of life, with consequent improvement in lung function.
One study suggests that fetal pancreatic digestive enzymes may play a part in causing the lung damage seen in MAS.
The figure quoted for infants born with meconium-stained liquor in the industrialised world is 8-25% of births after 34 weeks of gestation. MAS occurs in around 1-3% of live births.
The decrease in the incidence of MAS in the period of a decade has been attributed to the reduction in post-term delivery, aggressive management of abnormal heart rate monitoring, and decreased number of infants with a low Apgar score.Elective induction of labour for pregnancies at or beyond 41 weeks has been shown to be associated with significant reduction in the incidence of MAS and fewer perinatal deaths compared to expectant management.
In developing countries the incidence of MAS is higher and it is associated with a higher mortality rate.
Factors such as placental insufficiency, maternal hypertension, pre-eclampsia, oligohydramnios or maternal drug abuse (tobacco, cocaine) increase the in utero passage of meconium.
Aspiration of meconium can occur in utero with fetal gasping, or after birth, with the first breaths of life. MAS is defined as a respiratory distress that develops shortly after birth, with radiographic evidence of aspiration pneumonitis and presence of meconium-stained amniotic fluid.
Fetal distress and asphyxia are associated with severe MAS.
- Obvious presence of meconium or dark green staining of the amniotic fluid.
- Green or blue staining of the skin at birth.
- Baby appears limp, with a low Apgar score.
- Breathing is rapid, laboured, or absent.
- Signs of postmaturity (eg, peeling skin) are present.
- Fetal monitor may show bradycardia.
- Blood gas analysis showing low blood pH, increased pCO2, decreased pO2.
- Serum electrolytes should be measured in babies with MAS because perinatal stress can lead to inappropriate antidiuretic hormone (ADH) secretion syndrome and acute kidney injury.
- FBC may be useful in excluding infection or features identifying a cause of perinatal stress (eg, thrombophilia suggestive of haemorrhage or polycythaemia associated with decreased pulmonary blood flow).
- CXR shows patchy infiltrates, coarse streaking of both lungs, increased AP diameter and flattening of the diaphragm (due to hyperinflation).
- Lung ultrasound is a quick, easy and cheap imaging technique that is increasingly being used in critical care settings, also for newborns.
- Brain imaging may be indicated if neurological abnormalities are present.
- ECG should be performed to evaluate cardiac structure and assess the severity of any pulmonary hypertension and left-to-right shunting.
All infants at risk for MAS who show signs of respiratory distress should be admitted into the neonatal intensive care units. Close monitoring is important since they can deteriorate very quickly. Maintenance of adequate oxygenation, optimal blood pressure, correction of acidosis, hypoglycaemia and other metabolic disorders is the mainstay of treatment.
- Therapeutic interventions in severe MAS include airway suctioning, oxygen delivery, or ventilatory support.
- Suction - the National Institute for Health and Care Excellence (NICE) does not recommend routinely suctioning the nasopharynx and oropharynx prior to birth of the shoulder and trunk. However, it advises that the upper airways may be suctioned after the shoulders are delivered, if thick or tenacious meconium is present in the oropharynx. If the baby has depressed vital signs after delivery, laryngoscopy and suction under direct vision should be carried out by a healthcare professional trained in advanced neonatal life support.
- Oxygen should be given to keep oxygen saturations at 95-98%. Ventilation may be necessary. Pneumothoraces will need chest drain insertion.
- High-frequency oscillation ventilation may be given in some cases.
- Giving prophylactic antibiotics to neonates born through meconium-stained amniotic fluid has not been shown to reduce the incidence of MAS (or other complications).
- Surfactant - meconium flowing into the lung deactivates the activity of surfactant, causes a rise in surface tension and presages the onset of respiratory distress. Surfactant replacement can be beneficial for babies with MAS, as it can rapidly improve oxygenation.
- Surfactant replacement by bolus or slow infusion in infants with severe MAS has also been shown to reduce the need for extracorporeal membrane oxygenation.
- The development of active synthetic surfactants is very complicated.
- Anti-inflammatory drugs may be given to diminish the adverse action of products of meconium-induced inflammation on both endogenous and exogenously delivered surfactant.
- Inhaled nitric oxide can be useful in the management of pulmonary hypertension associated with MAS.It is thought to act by relaxing smooth muscles in the pulmonary vessels, causing vasodilatation, as well as promoting bronchodilation.
- Enteral sildenafil may be used for the treatment of persistent pulmonary hypertension resulting from MAS.
- Extracorporeal membrane oxygenation (ECMO) may be needed in those babies who deteriorate.
- Steroids - inhaled or systemic - have been used to good effect in some studies. Budesonide has been shown to improve the effects of exogenous surfactant in experimental MAS.
Complications and prognosis
In mild cases, respiratory distress usually subsides in 2-4 days, although tachypnoea can persist for longer. Rarely, more prolonged respiratory damage can occur which can persist for many years. This is more likely if ventilation has been required.
Cerebral hypoxia may lead to long-term neurological damage.
Studies have shown a wide range in the mortality among infants with MAS ranging from 5% to 40%.Myocardial dysfunction, birth weight, and initial oxygen requirement are independent predictors of mortality. MAS is associated with neonatal seizures and chronic seizure disorders.
Babies with high aspartate aminotransferase levels have been shown to be associated with poorer neurodevelopmental outcomes.
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