pulmonary hypoplasia
description
pulmonary hypoplasia is a histopathological diagnosis based on lung weight/body weight ratio less than or equal to 0.012 with a radial/alveolar count more than 1 sd below the mean for gestational age.
a spectrum of disease probably exists, ranging from the lethal form of pulmonary hypoplasia with histopathological correlation, to the milder form which is associated with air leaks in the early neonatal period.
pulmonary hypoplasia is usually secondary to either extrathoracic compression (early chronic amniotic fluid leakage), thoracic cage compression (as seen in some forms of skeletal dysplasia), or intrathoracic compression (large pleural effusions, congenital diaphragmatic hernia).
diagnosis
the ultrasound diagnosis of pulmonary hypoplasia is primarily based on the fetal chest circumference measurement.
several nomograms are available, with measurements below the 5th percentile being highly suspicious for pulmonary hypoplasia.
this parameter is useful when the aetiology is due to extrathoracic and thoracic cage compression.
the fetal chest circumference:abdominal circumference ratio birth values, less than 2 standard deviations below the mean, have proved diagnostic. this ratio is essentially constant throughout gestation with a mean of 0.89 and a lower limit of 0.77 (2 sd).
chest circumferences are not predictive of pulmonary hypoplasia due to intrathoracic causes (pleural effusion, diaphragmatic hernia) and are technically difficult to obtain in the presence of oligohydramnios; the circumference must be measured in a plane at right angles to the fetal spine at the level of the atrioventricular valves.
the lung length measurement has also been used with good predictive values in one report.
second trimester oligohydramnios is the commonest associated sonographic feature and may be secondary to congenital renal problems (renal agenesis, renal dysplasia or obstructive uropathy) or chronic amniotic fluid leakage.
large pleural or pericardial effusions and intrathoracic space occupying masses (such as intrapericardial teratomas, diaphragmatic hernia) are sonographically obvious causes.
long bone measurements may reflect the presence of an associated skeletal dysplasia, and in these cases the chest will appear small relative to the size of the abdomen.
differential diagnosis
in the presence of a small chest circumference, management is dependent on differential diagnosis of the causative lesion.
sonographic features
- fetal chest circumference less than 2 sd for gestational age.
- evidence of primary condition may be apparent (oligohydramnios, skeletal dysplasia, intrathoracic lesions)
** the chest circumference must be measured in a plane at right angles to the fetal spine at the level of the atrioventricular valves. **
associated syndromes