You are in: eMedicine Specialties > Radiology > PEDIATRICS Meconium AspirationArticle Last Updated: Nov 8, 2006AUTHOR AND EDITOR INFORMATIONSection 1 of 9
  Author: Michael Leu, MD, Staff Physician, Department of Pediatrics, Harbor-University of California at Los Angeles Medical Center Coauthor(s): Michael J Diament, MD, Associate Professor, Department of Radiology, University of California at Los Angeles School of Medicine; Virender Rehan, MD, Director, Neonatal Intensive Care Unit, Assistant Professor, Department of Pediatrics, Division of Neonatology, Harbor-University of California at Los Angeles Medical Center; Lynne M Smith, MD, Associate Professor, Department of Pediatrics, UCLA Geffen School of Medicine; Director of Education, Neonatal-Perinatal Medicine Fellowship, Director, Level ll Nursery, Harbor-University of California at Los Angeles Medical Center; Daryoush Bassiri, MD, Neonatology Fellow, Department of Pediatrics, Harbor-University of California at Los Angeles Medical Center, Children's Hospital of Orange County Editors: Henrique M Lederman, MD, PhD, Consulting Staff, Department of Radiology, The Children's Hospital of Philadelphia; Professor of Radiology and Pediatric Radiology, Chief, Division of Diagnostic Imaging in Pediatrics, Federal University of Sao Paulo, Brazil; Bernard D Coombs, MB, ChB, PhD, Consulting Staff, Department of Specialist Rehabilitation Services, Hutt Valley District Health Board, New Zealand; David A Stringer, BSc, MBBS, FRCR, FRCPC, Professor, National University of Singapore; Clinical Director, Diagnostic Imaging, National University Hospital; Head, Diagnostic Imaging, KK Women's and Children's Hospital, Singapore; Robert M Krasny, MD, Consulting Staff, Department of Radiology, The Angeles Clinic and Research Institute; Eugene C Lin, MD, Consulting Staff, Department of Radiology, Virginia Mason Medical Center Author and Editor Disclosure Synonyms and related keywords: MAS, meconium aspiration syndrome, transient tachypnea of the newborn, neonatal pneumonia, Meconium-stained amniotic fluid, MSAF pulmonary hypertension of the newborn, PPHN, extracorporeal membrane oxygenation, ECMO, pulmonary interstitial emphysema, PIE, air-block syndromes INTRODUCTIONSection 2 of 9
  BackgroundThe term meconium is derived from ancient Greek word meconium-arion, or opium-like, from the Greek word mekoni meaning poppy juice. In the time of Aristotle, the term was used because it was believed that the substance induced fetal sleep. Meconium is the first substance discharged from the gastrointestinal tract in the perinatal period. It is a sterile mixture of water (75-95%), mucopolysaccharides (80% dry weight), gastrointestinal secretions (bile salts and pancreatic and liver enzymes), solids (vernix caseosa, lanugo, and squamous cells), blood, minerals, and lipids (free fatty acids). For excellent patient education resources, visit eMedicine's Lung and Airway Center. Also, see eMedicine's patient education article Acute Respiratory Distress Syndrome. PathophysiologyPassage of meconium into amniotic fluid most often represents a normal maturational event. However, in many instances, it may occur in response to fetal hypoxia or acidosis. Meconium passage likely requires neural stimulation of a mature gastrointestinal tract, without which there is no peristalsis and relaxation of the rectal sphincter. This may explain why meconium is rarely found in the amniotic fluid before 34 weeks' gestation. Meconium aspiration syndrome chiefly affects infants at term and afterward. The amniotic fluid and meconium mix to form a greenish-black fluid of variable thickness, or viscosity. Meconium aspiration syndrome occurs when the newborn aspirates the meconium-containing amniotic fluid. In addition to obstruction of the airway, the aspiration leads to an inflammatory response in the lung parenchyma (chemical pneumonitis). It is this inflammation, not the meconium itself, that results in the patchy infiltrates seen on chest radiography. It is not clear which component(or components) of meconium triggers the inflammatory response. However, bile and liver enzymes have been suggested as the causative agents. The viscosity of the meconium and the response to aspiration are highly variable. Also, pulmonary hypertension is a significant comorbidity that is not obvious on chest radiography. Therefore, the clinical picture may be poorly correlated with the radiographic findings. FrequencyUnited StatesMeconium aspiration is a frequent problem in the newborn. Meconium-stained amniotic fluid is noted in 10-15% of deliveries. Overall, 5-15% of infants born with meconium-stained amniotic fluid develop symptoms of respiratory distress. In fact, infants born through meconium-stained amniotic fluid are at 100-fold greater risk of developing respiratory distress than those born with clear amniotic fluid. Mortality/Morbidity
RaceNo racial predilection exists. SexMeconium aspiration syndrome affects the sexes equally. AgeMeconium aspiration syndrome is present only in neonates. It is typically found in infants delivered at term or post-term. AnatomyMeconium aspiration syndrome involves aspiration of meconium through the trachea into the bronchial tree, sometimes down to the alveoli. Three classes of alveoli are described: (1) those not ventilated due to complete proximal obstruction (atelectatic), (2) those with partial meconium obstruction, and (3) those with no proximal obstruction. Alveoli with partial meconium obstruction are thought to trap air because of a ball-valve mechanism by which gas reaches the alveoli on inflation but is trapped secondary to reduced airway diameter during expiration. It is challenging to oxygenate babies with meconium aspiration syndrome because the alveoli where gas trapping occurs may rupture, leading to air leak. Also, meconium in the alveoli may deactivate surfactant. Clinical DetailsDiagnosis Meconium aspiration syndrome is a clinical diagnosis. It is suspected with meconium in the amniotic fluid at the time of birth and with respiratory distress in the newborn. Classically, babies with this disease are post-term, they show signs of weight loss, and they have yellow-stained nails and umbilical cords. Differential diagnosis and other problems to consider Transient tachypnea of the newborn: usually has patchy opacities caused by pulmonary fluid in the process of resorption. Follow-up radiographs show rapid clearing of infiltrates, in contrast with meconium aspiration syndrome or pneumonia. Neonatal pneumonia usually has patchy opacities representing consolidation, with pleural effusion present in up to two thirds of cases. Lung volumes are usually normal, but the lung fields may be hyperinflated. Respiratory distress syndrome usually has a uniform distribution of opacities, classically with a ground-glass appearance and decreased lung volumes due to alveolar collapse. Air bronchograms may be seen. Pleural effusions are rare. This is most often seen in preterm infants (in contrast to those with meconium aspiration syndrome). In addition to persistent pulmonary hypertension of the newborn (PPHN), other issues of neonates should be considered, including sepsis, pulmonary hypoplasia, congenital anatomic pulmonary anomalies, congenital diaphragmatic hernia, and congenital heart disease. Treatment The interventions for meconium aspiration syndrome depend primarily on the clinical course. Infants are typically suctioned at the perineum. In those with weak or no respiratory effort, an endotracheal tube is placed immediately so that meconium can be suctioned from beneath the vocal cords prior to positive pressure ventilation. Respiratory support is provided initially by using oxygen via nasal cannula, continuous positive airway pressure, conventional mechanical ventilation, or high-frequency oscillatory ventilation. Antibiotics may be started for the empirical treatment of pneumonia, and surfactant may be administered to overcome the surfactant-deactivating properties of meconium. In severe cases, pulmonary resistance is high because of a number of factors, which include vasoconstriction from hypoxia/hypercarbia, and mechanical compression on the pulmonary vasculature from atelectatic lung. This continued high resistance or PPHN leads to the continuation of fetal circulation, with blood bypassing the lungs instead of going through the foramen ovale and the ductus arteriosus. Clinically, the infant may appear ill and cyanotic, with tachycardia and tachypnea. PPHN can be confirmed with echocardiography, and can be managed with oxygen and vasodilatory support (hyperventilation, nitric oxide therapy). If these fail, extracorporeal membrane oxygenation may be used to provide oxygenation and systemic arterial pressure support. Complications and outcomes Other neonatal complications (sepsis/infection, causes of fetal distress that lead to meconium passage, and damage from hypoxia) should be evaluated and managed concurrently. The clinical course of meconium aspiration syndrome is highly variable. In mild cases, the respiratory distress generally subsides in 2-4 days, though rapid breathing may persist for days. Meconium aspiration rarely leads to permanent lung damage. Most infants have complete recovery of pulmonary function, though infants with severe aspiration that require mechanical ventilation, nitric oxide therapy, or extracorporeal membrane oxygenation have a more guarded outcome. A few infants have an increased incidence of pneumonia as their lungs recover. Some children may develop chronic lung disease secondary to meconium aspiration syndrome and intense pulmonary intervention. The events that trigger meconium aspiration syndrome (eg, hypoxia) may lead to long-term neurologic deficits or even death. Preferred ExaminationChest radiography is the preferred examination. Limitations of TechniquesThe inflammatory process of meconium aspiration may have various presentations on chest radiography, and it is initially indistinguishable from transient tachypnea of the newborn or neonatal pneumonia. Without visualization of meconium below the vocal cords during resuscitation, the diagnosis is made on the basis of the clinical course and the results of follow-up imaging studies. DIFFERENTIALSSection 3 of 9
Hyaline Membrane Disease Pneumonia, Neonatal Transient Tachypnea of the Newborn
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| Media file 1: Meconium aspiration. Serial radiographs in a newborn with uncomplicated meconium aspiration. Radiograph obtained shortly after birth shows ill-defined, predominantly perihilar opacities in the lungs; these are more severe on the right than on the left. The lungs are hyperexpanded. The neonate's heart size is within normal limits. The abnormalities on the initial chest radiograph, aside from the presence of an endotracheal tube and an umbilical artery catheter, are identical to those seen in severe cases of transient tachypnea of the newborn. | |
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| Media file 2: Meconium aspiration. Radiograph obtained 2 days after Image 1 shows almost complete resolution of the pulmonary opacities. | |
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| Media file 3: Meconium aspiration. Radiographic findings in a more severe case of meconium aspiration. This initial radiograph obtained shortly after birth shows patchy, coarse parenchymal opacities and severe hyperexpansion. In addition, pneumomediastinum is present on the right (arrows), outlining the right lobe of the thymus (T). | |
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| Media file 4: Meconium aspiration. Follow-up radiograph in the patient in Image 3 obtained after placement of bilateral thoracostomy tubes for pneumothoraces shows pneumopericardium (arrows) and extensive imaging lucencies in the lungs. These findings indicate pulmonary interstitial emphysema. | |
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| Media file 5: Meconium aspiration. Radiographic abnormalities in a patient with meconium aspiration who was treated with extracorporeal membrane oxygenation (ECMO). The lungs are airless because of pulmonary bypass. Cannula (arrows) enters from the right neck and extends to the right atrium, providing venous-venous ECMO. An endotracheal tube, a nasogastric tube, and an umbilical artery catheter are also in place. | |
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Article Last Updated: Nov 8, 2006
