Meconium aspiration syndrome

# Overview
Meconium aspiration syndrome (MAS, alternatively "Neonatal aspiration of meconium") occurs when infants take meconium into their lungs during or before delivery. Meconium is the first stool of an infant, composed of materials ingested during the time the infant spends in the uterus: intestinal epithelial cells, lanugo, mucus, amniotic fluid, bile, and water.  Meconium is sterile, unlike later feces, and has no odor.
Meconium is normally stored in the infant's intestines until after birth, but sometimes (often in response to fetal distress) it is expelled into the amniotic fluid prior to birth, or during labor.  If the baby then inhales the contaminated fluid, respiratory problems may occur.

# Causes and risk factors
Meconium passage into the amniotic fluid occurs in about five to twenty percent of all births. This is more common in postdate births. Meconium aspiration syndrome develops in only 5-10 percent of these cases.  Frequently, fetal distress during labor causes intestinal contractions, as well as a relaxation of the anal sphincter, which allows meconium to contaminate the amniotic fluid.  Amniotic fluid is normally clear, but becomes greenish if it is tinted with meconium.  If the infant inhales this mixture before, during, or after birth, it may be sucked deep into the lungs. Three main problems occur if this happens:
- the material may block the airways
- efficiency of gas exchange in the lungs is lowered
- the meconium-tainted fluid is irritating, inflaming airways and possibly leading to chemical pneumonia
About a third of those infants who experience MAS require breathing assistance.

# Symptoms and signs
The most obvious sign that meconium may have been aspirated is the greenish appearance of the amniotic fluid.  The infant's skin may be stained green if the meconium was passed a considerable amount of time before birth.  Rapid or labored breathing, slow heartbeat, or low Apgar score are all signs of the syndrome. Inhalation can be confirmed by one or more tests such as using a stethoscope to listen for abnormal lung sounds, performing blood gas tests, and using chest X-rays to look for patchy or streaked areas on the lungs. Infants who have inhaled meconium develop respiratory distress--often requiring ventilatory support. Complications of MAS include pneumothorax and persistent pulmonary hypertension of the newborn.

# Imaging Findings
- High lung volumes (secondary to small airway obstruction).
- Asymmetric, patchy pulmonary opacities.
- Pleural effusions can be seen
- Pneumothorax in 20 - 40% of cases (secondary to small airway obstruction).
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# Prevention and Treatment
MAS is difficult to prevent.  Ensuring that the infant is born before 42 weeks of gestation may lessen the risk. Amnioinfusion is a method of thinning thick meconium that has passed into the amniotic fluid.  In this procedure, a tube is inserted into the uterus through the vagina, and sterile fluid is pumped in to dilute thick meconium. Recent studies have not shown a benefit from amnioinfusion. Until recently it had been recommended that the throat and nose of the baby be suctioned by the  obstetrician as soon as the head is delivered. However, new studies have shown that this is not useful and the revised Neonatal Resuscitation Guidelines published by the American Academy of Pediatrics no longer recommend it. When meconium staining of the amniotic fluid is present and the baby is born depressed, it is recommended that the  pediatrician suction the mouth and nose and use a laryngoscope and suction catheter to suction meconium from below the vocal cords.
If the condition worsens to a point where treatments are not affecting the newborn as they should, extracorporeal membrane oxygenation (ECMO) is recommended. This is essentially a heart-lung machine used for longer than normal, i.e. during surgery.