Clinical meaning
Congenital diaphragmatic hernia (CDH) is a life-threatening congenital anomaly characterized by a defect in the diaphragm that allows abdominal organs (stomach, intestines, liver, spleen) to herniate into the thoracic cavity during fetal development, severely impeding lung growth and resulting in pulmonary hypoplasia, pulmonary hypertension, and cardiorespiratory compromise at birth. CDH occurs in approximately 1 in 2,500-3,000 live births and carries a mortality rate of 20-50% depending on the severity of pulmonary hypoplasia and associated anomalies. The diaphragm is the primary muscle of respiration, a dome-shaped musculotendinous structure that separates the thoracic and abdominal cavities. Normal diaphragmatic development occurs between 4 and 12 weeks of gestation through a complex process involving the fusion of four embryological structures: the septum transversum (anterior central portion), the pleuroperitoneal membranes (posterior lateral portions), the dorsal mesentery of the esophagus (posterior medial portion), and muscular ingrowth from the lateral body wall. The pleuroperitoneal canals (the communication between the pleural and peritoneal cavities) normally close by the 8th to 10th week of gestation through fusion of the pleuroperitoneal membranes with the septum transversum and dorsal mesentery. Failure of this closure results in a persistent defect through which abdominal viscera can herniate into the thorax. The most common type of CDH is the Bochdalek hernia (posterolateral defect), accounting for approximately 80-85% of cases. It occurs on the left side in approximately 80% of cases (because the right pleuroperitoneal canal closes earlier than the left, and the liver provides a partial barrier on the right side), on the right side in approximately 15% of cases, and bilaterally in approximately 5% of cases. The less common Morgagni hernia (anterior retrosternal or parasternal defect) accounts for approximately 2-5% of CDH cases and typically presents later in life with milder symptoms. The critical pathophysiological consequence of CDH is pulmonary hypoplasia -- underdevelopment of the lungs resulting from compression by herniated abdominal organs during the critical period of fetal lung development. Fetal lung development progresses through five stages: embryonic (3-7 weeks), pseudoglandular (7-17 weeks), canalicular (17-27 weeks), saccular (28-36 weeks), and alveolar (36 weeks through postnatal life). CDH disrupts lung development primarily during the pseudoglandular and canalicular stages, which are the critical periods for airway branching morphogenesis and formation of the gas-exchanging parenchyma. Herniated abdominal organs physically compress the developing ipsilateral lung, reducing airway branching, alveolar number, and cross-sectional area of the pulmonary vascular bed. The ipsilateral lung may be reduced to 10-25% of normal volume in severe cases. The contralateral lung also shows some degree of hypoplasia (typically 50-75% of normal volume) due to mediastinal shift compressing the opposite hemithorax and possibly due to circulating factors that impair bilateral lung development. The pulmonary vascular changes in CDH are equally critical. Normal fetal pulmonary vascular development involves progressive arborization of pulmonary arteries in parallel with airway branching. In CDH, the reduced number of airway generations results in a proportionally reduced pulmonary vascular bed with fewer pulmonary arterioles. Additionally, the existing pulmonary arteries demonstrate abnormal muscularization: the medial smooth muscle layer is thickened (medial hypertrophy), and muscularization extends distally into normally non-muscularized intra-acinar arterioles. This pathological vascular remodeling results in a pulmonary vascular bed with abnormally high resistance that is hyperreactive to vasoconstrictive stimuli. These vascular changes produce persistent pulmonary hypertension of the newborn (PPHN), which is the primary cause of mortality in CDH. In normal fetal-to-neonatal transition, the pulmonary vascular resistance drops dramatically at birth due to lung expansion (mechanical stretching of pulmonary vessels), increased alveolar oxygen tension (oxygen is a potent pulmonary vasodilator through nitric oxide and prostacyclin pathways), and loss of placental circulation (which removes the low-resistance placental vascular bed, increasing systemic vascular resistance). In CDH, the hypoplastic, hyperreactive pulmonary vasculature fails to undergo this normal transition. Pulmonary vascular resistance remains elevated, maintaining fetal circulatory patterns: blood continues to shunt right-to-left through the foramen ovale and ductus arteriosus, bypassing the lungs and producing severe systemic hypoxemia. The right-to-left shunting creates a vicious cycle: hypoxemia and acidosis further increase pulmonary vascular resistance through pulmonary vasoconstriction, worsening the shunt and deepening hypoxemia. This cycle of PPHN is the primary mechanism of death in severe CDH. Additionally, the hypoplastic lungs have reduced surfactant production from the decreased number of type II alveolar pneumocytes. Surfactant deficiency increases alveolar surface tension, promotes atelectasis, and reduces lung compliance, further impairing gas exchange. The combination of reduced alveolar surface area (from hypoplasia), surfactant deficiency, and right-to-left shunting (from PPHN) produces the profound hypoxemia and respiratory failure characteristic of severe CDH. Associated congenital anomalies are present in approximately 30-40% of CDH cases and significantly affect prognosis. Cardiac defects are the most common associated anomalies (present in 10-25% of isolated CDH cases), including ventricular septal defect, atrial septal defect, tetralogy of Fallot, and coarctation of the aorta. Chromosomal abnormalities (trisomy 18, trisomy 13, trisomy 21) are found in approximately 10-15% of CDH cases and carry a particularly poor prognosis. Other associated anomalies include neural tube defects, renal anomalies, and musculoskeletal malformations. CDH can also occur as part of recognized genetic syndromes including Fryns syndrome, Cornelia de Lange syndrome, and Pallister-Killian syndrome.