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Describe the embryology and pathophysiology of congenital diaphragmatic hernia.

The incidence of congenital diaphragmatic hernia (CDH) is 1 in 2000 to 1 in 5000 live births. There may be associated congenital anomalies of the central nervous, gastrointestinal, genitourinary, and cardiovascular systems. Chromosomal anomalies also may be present. In the fetus, during the first month of life, there is a single pleuroperitoneal cavity. During the second month, the pleuroperitoneal membrane begins to form, separating the pleural and peritoneal cavities. The last portion of this membrane to form is the posterolateral portion; the right side closes before the left side. The fetal gut is outside the pleuroperitoneal cavity in the yolk sac during the first month of fetal life and returns to the peritoneal cavity during the second month of development. If the gut returns before full closure of the pleuroperitoneal membrane, any or all portions of the gut may migrate up into the pleural cavity. There are three sites where migration of the gut may occur:

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  Posterolateral (foramen of Bochdalek)

  
  
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  Anteromedial (foramen of Morgagni)

  
  
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  Esophageal hiatus

The most common site of migration (80%) is through the posterolateral portion, the left side more commonly than the right. Approximately 1% of diaphragmatic hernias occur through the anteromedial portion, and the remaining cases occur through the esophageal hiatus ( Table 60-1 ).

Lung development is impaired by the presence of abdominal contents in the pleural cavity during fetal growth. The degree of impairment of lung development is determined by both the amount of abdominal contents in the pleural cavity and the time of migration. The greater the amount of abdominal contents in the pleural cavity and the earlier the migration, the greater the degree of pulmonary hypoplasia that is present at birth. Not only is the ipsilateral lung affected, but also there are developmental changes in the contralateral lung. These changes include the following:

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  Decreased number of bronchi and alveoli

  
  
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  Smaller pulmonary artery

  
  
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  Inappropriately muscularized pulmonary arteries

  
  
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  Decreased cross-sectional area of pulmonary artery branches

The physiologic changes that occur secondary to developmental changes in the lung are an increase in pulmonary vascular resistance (PVR) and persistent pulmonary hypertension. Decreased pulmonary blood flow and right-to-left shunting through the foramen ovale and ductus arteriosus contribute to progressive hypoxia and acidosis. These physiologic changes can be divided into two components: (1) irreversible (owing to pulmonary hypoplasia and abnormal vasculature) and (2) reversible (owing to vasoconstriction of abnormal muscularized arteries). The greater the irreversible component, the poorer the prognosis. To date, there is no method available to determine accurately which component is predominant.

Although compression of the lung by abdominal contents in the pleural cavity is detrimental, it does not contribute significantly to hypoxia and acidosis. CDH is no longer considered a surgical emergency. The initial management of these neonates is directed at improving oxygenation and ventilation. Surgery should be considered only after stabilization of the neonate’s condition. In a stable neonate, surgery is usually scheduled in the next 24–48 hours.