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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. One study found that babies with an Apgar score above 8 at five minutes rarely developed MAS and could be discharged.
Significant aspiration of thick meconium, however, can induce 4 major pulmonary effects, viz: airway obstruction, surfactant dysfunction, chemical pneumonitis and pulmonary hypertension.
Studies of tracheal aspirate confirm an inflammatory response, with increase in inflammatory cell count and the level of proinflammatory 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.
Midtrimester intrauterine meconium aspiration is occasionally seen. One study of 21 fetuses (18 miscarriages and 3 terminations on medical grounds) reported an association with abnormalities of the umbilical cord or placenta.
The figure quoted for infants born with meconium-stained liquor in the industrialised world is 8-25% of births after 34 weeks of gestation. However, improvements in obstetric practice have resulted in a reduction in the incidence of meconium aspiration syndrome (MAS) over recent years. One unit reported a four-fold decrease from 1990-1992 to 1997-1998 (5.8% to 1.5% of meconium-stained infants more than 37 weeks).
Such a dramatic decrease is not seen in countries with less well developed standards of antenatal care.
- 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 meconium aspiration syndrome (MAS) because perinatal stress can lead to inappropriate antidiuretic hormone (ADH) secretion syndrome and acute renal failure.
- A full blood count 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).
- Chest X-ray shows patchy infiltrates, coarse streaking of both lungs, increased AP diameter and flattening of the diaphragm (due to hyperinflation).
- 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.
- Suction - the National Institute for Health and Clinical Excellence (NICE) does not recommend routinely suctioning of the nasopharynx and oropharynx prior to birth of the shoulder and trunk. However, they advise that the upper airways may be suctioned if thick or tenacious meconium is present in the oropharynx. If the baby has depressed vital signs, laryngoscopy and suction under direct vision should be carried out by a healthcare professional trained in advanced neonatal life support.
- Oxygen - depending on the degree of respiratory distress, respiratory support should be provided with oxygen via a nasal cannula, continuous positive pressure ventilation or conventional mechanical ventilation.
- Antibiotics (eg gentamicin) - these may be useful in ventilated cases but in non-ventilated babies there is little evidence to support their use.
- Surfactant - meconium flowing into the lung deactivates the activity of surfactant, causes a rise in surface tension and presaging the onset of respiratory distress. Surfactant lavage combats this effect and significantly improves infant mortality and morbidity. Modern synthetic surfactants are to be preferred to animal extracts due to their more consistent content of surfactant protein. Recently, therapeutic surfactant lung lavage has been used with good result in neonates affected by severe MAS . Combination of surfactant with continuous positive pressure airways' administration has also proved successful.
- Inhaled nitric oxide - this is useful in the management of pulmonary hypertension associated with meconium aspiration syndrome (MAS). It is thought to act by relaxing smooth muscles in the pulmonary vessels causing vasodilatation, as well as promoting bronchodilation. It is often more effective when combined with high-frequency oscillatory ventilation.
- Extracorporeal membrane oxygenation (ECMO) - this uses a heart-lung machine to take over the work of the lungs. Venovenous ECMO seems as effective as veno-arterial ECMO.
- Steroids - inhaled or systemic - have been used to good effect in some studies.
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. The mortality rate is approximately 5%. Risk factors for mortality include resuscitation outside hospital, first-born babies, shock, pneumothorax, pulmonary hypertension and renal failure. If severe parenchymal lung disease or pulmonary hypertension develops, the mortality rate can be as high as 20%.
In one study of 2,603 deliveries 11.6% had meconium-stained amniotic fluid, Of these infants, 21.1% developed meconium aspiration syndrome (MAS). Of pregnancies in which meconium-stained amniotic fluid was a feature, severity of meconium, low Apgar score at 5 minutes and non-reassuring fetal heart rate tracing was associated with MAS.
Another study of term infants with MAS, admitted to a neonatal intensive care unit, concluded that, despite modern advances in perinatal care, such patients remained a high-risk population who developed significant morbidity and often required intensive therapies.
More frequent diagnosis of abnormal fetal heart rate patterns and the avoidance of post-mature delivery by elective Caesarean section have both been shown to reduce the incidence of meconium aspiration syndrome (MAS).
The use of uterine stimulants such as misoprostol is associated with meconium staining of amniotic fluid and amniotomy during labour may be a risk factor for MAS.
Further research on the exact mechanisms involved in the distribution of aspirated meconium and its effect on pulmonary and extrapulmonary tissues may hopefully lead to the development of further options for the prevention and management of this condition.
Further reading & references
- Whitfield JM, Charsha DS, Chiruvolu A; Prevention of meconium aspiration syndrome: an update and the Baylor experience. Proc (Bayl Univ Med Cent). 2009 Apr;22(2):128-31.
- van Ierland Y, de Boer M, de Beaufort AJ; Meconium-stained amniotic fluid: discharge vigorous newborns. Arch Dis Child Fetal Neonatal Ed. 2010 Jan;95(1):F69-71. Epub 2009 Apr 23.
- Clark M et al; Meconium Aspiration Syndrome, eMedicine, Mar 2010
- Cayabyab RG, Kwong K, Jones C, et al; Lung inflammation and pulmonary function in infants with meconium aspiration syndrome. Pediatr Pulmonol. 2007 Oct;42(10):898-905.
- Ivanov VA, Gewolb IH, Uhal BD; A New Look at the Pathogenesis of the Meconium Aspiration Syndrome: A role for Pediatr Res. 2010 Jun 14.
- Mortensen E, Kearney MS; Meconium aspiration in the midtrimester fetus: an autopsy study. Pediatr Dev Pathol. 2009 Nov-Dec;12(6):438-42.
- Yoder BA, Kirsch EA, Barth WH, et al; Changing obstetric practices associated with decreasing incidence of meconium aspiration syndrome. Obstet Gynecol. 2002 May;99(5 Pt 1):731-9.
- Meconium aspiration syndrome; Department of Health, Victoria, Australia
- Intrapartum care; NICE Clinical Guideline (2007)
- Lin HC, Su BH, Tsai CH, et al; Role of antibiotics in management of non-ventilated cases of meconium aspiration syndrome without risk factors for infection. Biol Neonate. 2005;87(1):51-5. Epub 2004 Sep 30.
- Taylor P; Surface Tension in the Lungs 2002. Case Western Reserve University, Soft Condensed Matter Theory Group
- Wiswell TE, Knight GR, Finer NN, et al; A multicenter, randomized, controlled trial comparing Surfaxin (Lucinactant) lavage with standard care for treatment of meconium aspiration syndrome. Pediatrics. 2002 Jun;109(6):1081-7.
- Sinha SK, Lacaze-Masmonteil T, Valls i Soler A, et al; A multicenter, randomized, controlled trial of lucinactant versus poractant alfa among very premature infants at high risk for respiratory distress syndrome. Pediatrics. 2005 Apr;115(4):1030-8.
- Lo CW, Jeng MJ, Chang FY, et al; Therapeutic lung lavage with diluted surfactant in neonates with severe meconium aspiration syndrome. J Chin Med Assoc. 2008 Feb;71(2):103-9.
- Kribs A, Vierzig A, Hunseler C, et al; Early surfactant in spontaneously breathing with nCPAP in ELBW infants - a single centre four year experience. Acta Paediatr. 2008 Mar;97(3):293-8.
- Ichinose F, Roberts JD Jr, Zapol WM; Inhaled nitric oxide: a selective pulmonary vasodilator: current uses and therapeutic potential. Circulation. 2004 Jun 29;109(25):3106-11.
- Kinsella JP, Truog WE, Walsh WF, et al; Randomized, multicenter trial of inhaled nitric oxide and high-frequency oscillatory ventilation in severe, persistent pulmonary hypertension of the newborn. J Pediatr. 1997 Jul;131(1 Pt 1):55-62.
- No authors listed; UK collaborative randomised trial of neonatal extracorporeal membrane oxygenation. UK Collaborative ECMO Trail Group. Lancet. 1996 Jul 13;348(9020):75-82.
- Kugelman A, Gangitano E, Taschuk R, et al; Extracorporeal membrane oxygenation in infants with meconium aspiration syndrome: a decade of experience with venovenous ECMO. J Pediatr Surg. 2005 Jul;40(7):1082-9.
- Basu S, Kumar A, Bhatia BD, et al; Role of steroids on the clinical course and outcome of meconium aspiration syndrome-a randomized controlled trial. J Trop Pediatr. 2007 Oct;53(5):331-7. Epub 2007 May 29.
- Swaminathan S, Quinn J, Stabile MW, et al; Long-term pulmonary sequelae of meconium aspiration syndrome. J Pediatr. 1989 Mar;114(3):356-61.
- Hamutcu R, Nield TA, Garg M, et al; Long-term pulmonary sequelae in children who were treated with extracorporeal membrane oxygenation for neonatal respiratory failure. Pediatrics. 2004 Nov;114(5):1292-6.
- Lin HC, Su BH, Lin TW, et al; Risk factors of mortality in meconium aspiration syndrome: review of 314 cases. Acta Paediatr Taiwan. 2004 Jan-Feb;45(1):30-4.
- Khazardoost S, Hantoushzadeh S, Khooshideh M, et al; Risk factors for meconium aspiration in meconium stained amniotic fluid. J Obstet Gynaecol. 2007 Aug;27(6):577-9.
- Singh BS, Clark RH, Powers RJ, et al; Meconium aspiration syndrome remains a significant problem in the NICU: outcomes J Perinatol. 2009 Jul;29(7):497-503. Epub 2009 Jan 22.
- Hofmeyr GJ; What (not) to do before delivery? Prevention of fetal meconium release and its Early Hum Dev. 2009 Oct;85(10):611-5. Epub 2009 Oct 12.
- Kaapa PO; Meconium aspiration syndrome (MAS) - Where do we go? Research perspectives. Early Hum Dev. 2009 Oct;85(10):627-9. Epub 2009 Oct 12.
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Dr Laurence Knott
Dr Laurence Knott