This is the commonest variety of diaphragmatic hernias (incidence: 1/2000 in newborn infants). The defect is usually unilateral and presents as a large opening in the posterolateral part of diaphragm. The developmental basis of this anomaly is defective formation (or defective fusion) of a pleuro-peritoneal membrane. In about 80% of the cases the hernia is located on the
left side. The developmental reasons for much lesser involvement of the right side are: (i) the opening of the right pericardioperitoneal membrane is smaller, and (ii) the right sided opening is closed before the left one because of the earlier formation of the right pleuro-peritoneal membrane.
The congenital diaphragmmatic hernia allows the developing abdominal viscera to bulge into the pleural cavity. Because the hernia is more' .common on the ieft side, stomach,spleen and loops of large intestine (colon) may enter the thoracic cavity. If the mass of displaced viscera is large enough, the lung remains small arid hypoplastic on the affected side. the diaphragmatic hernias can be corrected surgically after birth of the body. However,if the defect is large the newborn usually dies because of pulmonary deficiency due to pulmonary hypoplasia.