Meconium aspiration syndrome

Meconium aspiration syndrome
Classification and external resources
Micrograph of fetal membranes with meconium-laden macrophages, a finding that may accompany meconium aspiration. H&E stain.
ICD-10	P24.0
ICD-9	770.11, 770.12
DiseasesDB	7907
MedlinePlus	001596
eMedicine	ped/768
MeSH	D008471

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 almost 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 5-20 percent of all births. This is more common in postdate births. Of the cases where meconium is found in the amniotic fluid Meconium Aspiration Syndrome develops less than 5 percent of the time^[1]. 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 (pneumonitis) 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 has been passed during or before labor is the greenish or yellowish appearance of the amniotic fluid. The infant's skin, umbilical cord, or nailbeds may be stained green if the meconium was passed a considerable amount of time before birth. These symptoms alone do not necessarily indicate that the baby has inhaled in the fluid by gasping in utero or after birth. After birth, rapid or labored breathing, cyanosis, slow heartbeat, a barrel-shaped chest 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 (diffuse crackles and rhonchi), performing blood gas tests to confirm a severe loss of lung function, and using chest X-rays to look for patchy or streaked areas on the lungs. Infants who have inhaled meconium may develop respiratory distress syndrome often requiring ventilatory support. Complications of MAS include pneumothorax and persistent pulmonary hypertension of the newborn.

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 delivery attendant 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 by the guidelines that an individual trained in neonatal intubation use a laryngoscope and endotracheal tube 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) can be necessary to keep the infant alive. This is essentially a heart-lung machine that can be used for days, rather than only hours or minutes.

See also

• Aspiration pneumonia

Footnotes and references

External links

• eMedicine's article about meconium aspiration syndrome