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.
