Anatomic Alterations of the Lungs

During normal fetal development, the diaphragm first appears anteriorly between the heart and liver and then progressively grows posteriorly. Between the eighth and tenth week of gestation, the diaphragm normally completely closes at the left Bochdalek foramen, which is located posteriorly and laterally on the left diaphragm. At about the tenth week of gestation (close to the same time the Bochdalek foramen is closing), the intestines and stomach normally migrate from the yolk sac. If, however, the bowels reach this area before the Bochdalek foramen closes, a hernia results—a congenital diaphragmatic hernia (CDH) (also called a Bochdalek hernia or posterior-lateral diaphragmatic hernia). In other words, a Bochdalek hernia is an abnormal hole in the posterolateral corner of the left diaphragm that allows the intestines—and in some cases the stomach—to move directly into the chest cavity and compress the developing lungs.*

As shown in Figure 38-1, the effects of a diaphragmatic hernia are similar to the effects of a pneumothorax or hemothorax—the lungs are compressed. As the condition becomes more severe, atelectasis and complete lung collapse may occur. When this happens, the heart and mediastinum are pushed to the right side of the chest—called dextrocardia. In addition, long-term lung compression in utero causes pulmonary hypoplasia, which is most severe on the affected (ipsilateral) side but also occurs on the unaffected (contralateral) side.

This pathologic process causes a marked reduction in the number of bronchial generations and alveoli per acinus. The concomitant increased muscularity of the small pulmonary arteries may contribute to the increased pulmonary vascular resistance and pulmonary hypertension commonly seen in these patients. Respiratory distress usually develops soon after birth. As the infant struggles to inhale, the increased negative intrathoracic pressure generated during each inspiration causes more bowel to be sucked into the thorax. Further compression of the heart occurs as the infant cries and swallows air, causing the intestine and stomach to distend further.

Finally, as a consequence of the hypoxemia associated with a diaphragmatic hernia, these babies often develop hypoxia-induced pulmonary arterial vasoconstriction and vasospasm, which produces a state of pulmonary hypertension. As a general rule, however, these babies only have a transient state of pulmonary hypertension until the diaphragmatic hernia is repaired. This is different from persistent pulmonary hypertension of the newborn (PPHN) (see Chapter 31).

The major pathologic or structural changes associated with diaphragmatic hernia may include the following:

Etiology and Epidemiology

A CDH occurs in an overall incidence ranging from 1 in 2000 to 4000 live births. The baby is usually mature, and two thirds of affected infants are male. About 95% of CDHs occur on the left side through the Bochdalek foramen. The mortality rate is about 40%. The prognosis depends on (1) the size of the defect, (2) the degree of hypoplasia, (3) the condition of the lung on the unaffected side, and (4) the success of the surgical diaphragmatic closure.