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Pulmonary hypoplasia refers to deficient or incomplete development of parts of the lung. It can develop as a result of a number of other in utero anomalies.
Pulmonary hypoplasia has been reported to affect 1.4% of all births 13 however the true prevalence is not well known. In cases of premature rupture of membranes at 15-28 weeks gestation, the reported prevalence of pulmonary hypoplasia is 9-28% 2.
Pulmonary hypoplasia is characterized by the presence of both bronchi (albeit rudimentary) and alveoli in an underdeveloped lobe. Both the size and the weight of the lung are reduced.
There are normally four stages of fetal lung development where the pathological process can occur at any level
embryonic: conception to ~5th week
pseudoglandular: ~5th to 17th week
canalicular: ~16th to 24th week: this stage may be more affected by oligohydramnios
terminal sac or alveolar period: 24th week to term and beyond
There are several key factors required for the adequate development of the lung. These are:
sufficient amniotic fluid volumes
adequate volume of the thoracic cavity
normal breathing movement
normal fluid within the lung
A deficiency in any of these could lead to pulmonary hypoplasia.
Most cases of pulmonary hypoplasia are secondary to other congenital anomalies or pregnancy complications. Some cases however can occur as a primary event 5.
With secondary causes, it can result from factors directly or indirectly compromising the thoracic space available for lung growth.
Intrathoracic causes include:
congenital diaphragmatic hernia: most common intrathoracic cause
decreased pulmonary vascular (arterial) perfusion
Extrathoracic causes include:
oligohydramnios and its causes
large intra-abdominal mass compressing the thorax
neuromuscular conditions interfering with fetal breathing
Other associations include:
Antenatal ultrasound may show ancillary features such as the presence of oligohydramnios and/or also show any of the causative anomalies.
Several sonographic parameters may give indirect clues as to the presence and extent of pulmonary hypoplasia. Such include:
fetal lung : head ratio: reduced (ratios <1 usually indicate a poor prognosis)
fetal chest circumference (or thoracic circumference) 6: reduced
In the case of intra-thoracic causes, both these parameters can be normal.
Peripheral pulmonary arterial resistance is often increased with pulmonary hypoplasia.
Fetal MRI may be useful to more accurately calculate lung volumes.
T1: normal fetal lung is usually low signal on T1
T2: normal fetal lung is usually high signal on T2; fetuses with pulmonary hypoplasia are thought to have low signal 1
Treatment and prognosis
The presence or development of pulmonary hypoplasia is often a critical determining feature in fetal survival in context of other potential causative anomalies. Treatment will significantly depend on the presence of underlying/associated anomalies.
In adult cases of unilateral pulmonary hypoplasia, consider conditions associated with a small hemithorax such as:
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