Pulmonary Agenesis, Aplasia, and Hypoplasia
Three different degrees of arrested development of the lungs may occur: (1) agenesis, in which there is a complete absence of one lung or both lungs and no trace of bronchial or vascular supply or parenchymal tissue; (2) aplasia, in which there is a suppression of all but a rudimentary bronchus ending in a blind pouch and there are no pulmonary vessels and no parenchyma; and (3) hypoplasia, in which there is incomplete development of the lung, which is smaller in weight and volume, and there is a reduced number of airways branches, alveoli, arteries, and veins.
The incidence of pulmonary agenesis is low. There is no clear-cut gender predominance; and it does not occur more frequently on one side or the other. Experimental work in rat fetuses has shown that mothers deprived of vitamin A have a greater incidence of pulmonary agenesis in their offspring; however, a similar degree of malnutrition of this type is unlikely to occur in humans. Although absence of the lung is often associated with other congenital defects that terminate life in infancy, many patients with a single lung have lived well into adult life. Sixty percent of patients with agenesis of the lung are found to have other congenital anomalies. The most frequently associated anomalies are patent ductus arteriosus, tetralogy of Fallot, anomalies of the great vessels, and bronchogenic cysts. One normal lung can sustain life because the single lung probably hypertrophies. The condition alone may be asymptomatic, but pulmonary function can more easily be compromised by pneumonia, foreign body, or other insults if there is only one functional lung present. The mortality rate of patients with an absent right lung is 75%, but 25% if the left lung is absent. The difference in mortality rate is caused by the higher frequency of cardiac abnormalities with an absent right lung.
There are many causes of secondary lung hypoplasia, including a reduction in amniotic ﬂuid volume, reduction in intrathoracic space, reduction in fetal breathing movements (neurologic abnormalities or neuromuscular disorders), genetic disorders (trisomy 18 or 21), malnutrition (vitamin A deﬁciency), maternal smoking, and medications such as glucocorticoid administration.
The ﬁnding in cases of agenesis, aplasia, or whole lung hypoplasia is, as might be expected, total or almost total absence of an aerated lung. The marked loss of volume is indicated by approximation of the ribs, elevation of the ipsilateral hemidiaphragm, and shift of the heart and mediastinum into the unoccupied hemithorax. Because of distension and herniation of the remaining functioning lung tissue across the mediastinum, however, breath sounds may be audible bilaterally, and auscultation alone may not be diagnostic. The diagnosis depends on bronchoscopic and bronchographic determination along with tomography and angiography to demonstrate the absence of the main bronchus on the affected side together with the absence of the pulmonary artery. On histologic study of the hypoplastic lung, a pleural surface can be seen under which there is a small, poorly developed bronchus but no bronchial or alveolar tissue.
Congenital absence of a pulmonary lobe presents similar but less dramatic ﬁndings. Physical and radiographic examinations show diminished volume of the affected hemithorax, shift of the heart and mediastinum into the affected side, and ipsilateral elevation of the hemidiaphragm. Bronchography establishes the diagnosis by proving the absence of the bronchus to the missing lobe, and angiography is conﬁrmatory.
Treatment consists in managing intercurrent diseases as they arise. Patients must take precautions to avoid infection, and their prognosis is always guarded because those who survive into adult life have progressively decreased pulmonary function.