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.
Meconium aspiration syndrome (MAS, alternatively "Neonatal aspiration of meconium") is a medical condition affecting newborn infants. It occurs when meconium is present in 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.
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.
Signs and symptoms 
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.
Fetal distress during labor causes intestinal contractions, as well as relaxation of the anal sphincter, which allows meconium to pass into the amniotic fluid and contaminate the amniotic fluid. Meconium passage into the amniotic fluid occurs in about 5-20 percent of all births and is more common in overdue births. Of the cases where meconium is found in the amniotic fluid, meconium aspiration syndrome develops less than 5 percent of the time. Amniotic fluid is normally clear, but becomes greenish if it is tinted with meconium.
Pathophysiology and Mechanism 
The pathophysiology of MAS is due to a combination of primary surfactant deficiency and surfactant inactivation as a result of plasma proteins leaking into the airways from areas of epithelial disruption and injury.
The leading three causes of MAS are
- Due to physiologic maturational event,
- A response to acute hypoxic events, and
- A response to chronic intrauterine hypoxia.
If an infant inhales this mixture before, during, or after birth, it may be sucked deep into the lungs. Three main problems occur if this happens:
These can lead to possibly fatal or long term health problems for the infant.
High risk infants may be identified by fetal tachycardia, bradycardia or absence of fetal accelerations upon CTG in utero, at birth the infant may look cachexic and show signs of yellowish meconium staining on skin, nail and the umbillical cord, these infants usually progress onto Infant Respiratory distress syndrome within 4 hours. Investigations which can confirm the diagnosis are fetal chest x-ray, which will show hyperinflation, diaphragmatic flattening, cardiomegaly and pathy atelectasis, and ABG samples, which will show decreased oxygen levels.
Prevention and Screening 
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Amnioinfusion, a method of thinning thick meconium that has passed into the amniotic fluid through pumping of sterile fluid into the amniotic fluid, has not shown a benefit in treating MAS . 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.
Lucinactant (Surfaxin) is used as a treatment of MAS. Albumin-lavage has not demonstrated to benefit outcomes of MAS. Steroid use has not demonstrated to benefit the outcomes of MAS.
The mortality rate of meconium-stained infants is considerably higher than that of non-stained infants; meconium aspiration used to account for a significant proportion of neonatal deaths. Residual lung problems are rare but include symptomatic cough, wheezing, and persistent hyperinflation for up to 5-10 yr. The ultimate prognosis depends on the extent of CNS injury from asphyxia and the presence of associated problems such as pulmonary hypertension.
In a study conducted between 1995 and 2002, MAS occurred in 1061 of 2,490,862 live births, reflecting an incidence of 0.43 of 1000. MAS requiring intubation occurs at higher rates in pregnancies beyond 40 weeks. 34% of all MAS cases born after 40 weeks required intubation compared to 16% prior to 40 weeks.
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Research directions 
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See also 
- Usta, I.; Mercer, B.; Sibai, B. (1995). "Risk factors for meconium aspiration syndrome". Obstetrics and gynecology 86 (2): 230–234. doi:10.1016/0029-7844(95)00124-A. PMID 7617354.
- Wirbelauer, J.; Speer, C. (2009). "The role of surfactant treatment in preterm infants and term newborns with acute respiratory distress syndrome". Journal of perinatology : official journal of the California Perinatal Association. 29. Suppl 2: S18–S22. doi:10.1038/jp.2009.30. PMID 19399004.
- Klingner, MC; Kruse, J (1999). "Meconium aspiration syndrome: pathophysiology and prevention". The Journal of the American Board of Family Practice / American Board of Family Practice 12 (6): 450–66. PMID 10612363.
- Cheng, Y.; Nicholson, J.; Nakagawa, S.; Bruckner, T.; Washington, A.; Caughey, A. (2008). "Perinatal outcomes in low-risk term pregnancies: do they differ by week of gestation?". American journal of obstetrics and gynecology 199 (4): 370.3e1–7. doi:10.1016/j.ajog.2008.08.008. PMID 18928977.
- Hofmeyr, G. (2009). "What (not) to do before delivery? Prevention of fetal meconium release and its consequences". Early human development 85 (10): 611–615. doi:10.1016/j.earlhumdev.2009.09.010. PMID 19822401.
- Xu, H.; Hofmeyr, J.; Roy, C.; Fraser, W. (2007). "Intrapartum amnioinfusion for meconium-stained amniotic fluid: a systematic review of randomised controlled trials". BJOG: an International Journal of Obstetrics & Gynaecology 114 (4): 383. doi:10.1111/j.1471-0528.2007.01262.x. PMID 17378813.
- Short, B. L. (2008). "Extracorporeal membrane oxygenation: use in meconium aspiration syndrome". Journal of Perinatology 28: S79. doi:10.1038/jp.2008.152. PMID 19057615.
- Halliday, H. L. (2006). "Recent Clinical Trials of Surfactant Treatment for Neonates". Biology of the Neonate 89 (4): 323. doi:10.1159/000092869. PMID 16770072.
- Donn, S. (2005). "Lucinactant: a novel synthetic surfactant for the treatment of respiratory distress syndrome". Expert Opinion on Investigational Drugs 14 (3): 329–334. doi:10.1517/135437184.108.40.2069. PMID 15833063.
- Wiswell, T. E.; Knight, G. R.; Finer, N. N.; Donn, S. M.; Desai, H.; Walsh, W. F.; Sekar, K. C.; Bernstein, G. et al. (2002). "A multicenter, randomized, controlled trial comparing Surfaxin (Lucinactant) lavage with standard care for treatment of meconium aspiration syndrome". Pediatrics 109 (6): 1081–1087. doi:10.1542/peds.109.6.1081. PMID 12042546.
- Salvesen, B.; Mollnes, T. E.; Saugstad, O. D. (2008). "Albumin lavage does not improve the outcome of meconium aspiration syndrome". Journal of Maternal-Fetal and Neonatal Medicine 21 (10): 719. doi:10.1080/14767050802255561. PMID 19012188.
- Ward, M. C.; Sinn, J. K. (2003). "Steroid therapy for meconium aspiration syndrome in newborn infants". Cochrane Database of Systematic Reviews (1). doi:10.1002/14651858.CD003485.
- Dargaville, P. A.; Copnell, B.; Australian and New Zealand Neonatal Network (2006). "The Epidemiology of Meconium Aspiration Syndrome: Incidence, Risk Factors, Therapies, and Outcome". Pediatrics 117 (5): 1712. doi:10.1542/peds.2005-2215. PMID 16651329.
- eMedicine's article about meconium aspiration syndrome
- Surfactant treatment of the meconium aspiration syndrome