Pulmonary Hypoplasia

Failure of development of the lungs in utero which is most often unilateral and can range from hypoplasia to aplasia. The end result is lungs that do not have enough tissue and blood flow for adequate gas exchange.

Seen in nearly 15% of post mortems on peri-natal deaths.¹

Normal lung growth requires the following:²

1. Normal thoracic cavity
2. Normal fetal breathing movements
3. Normal amniotic fluid volume
4. Fetal lung liquid at the right pressure

Pulmonary hypoplasia can therefore be caused by

• Abnormal thoracic cavity: congenital diaphragmatic hernia (in this situation bowel and stomach are sitting in the chest cavity) or malformations of the chest cavity.
• Abnormal fetal breathing movements: neuromuscular disease in utero can result in decreased fetal breathing movement e.g. CNS lesions or space occupying lesions.
• Abnormal amniotic fluid volume: oligohydramnios - due to renal agenesis or urinary outflow obstruction or prolonged rupture of membranes.
• Abnormalities of fetal lung fluid and lung fluid pressure: the underlying pathophysiology which results in abnormal lung fluid pressure and subsequent pulmonary hypoplasia is unclear.

There is probably an overlap of aetiologies.² However, pulmonary hypoplasia may also be idiopathic or related to other syndromes and congenital anomalies e.g. Multiple pterygium syndrome/Fetal akinesia-hypokinesia sequence (autosomal recessive)/Scimitar syndrome³ and Trisomy 21.

• Immediate difficulty in breathing with respiratory distress e.g. cyanosis, intercostal recession with tachypnoea, acid base disturbance (acidosis, hypoxia and hypercarbia).
• There may features of other fetal disorders e.g. skeletal dysplasia or "Potter's facies" in oligohydramnios.
• Other congenital anomalies e.g. cardiac malformations.

• Suspect if oligohydramnios is present.
• Fetal ultrasonography - can be used to measure lung area to head circumference ratio; a ratio of <1 is associated with high rate of neonatal death. Doppler velocimetry has been used experimentally and can detect severe hypoplasia.⁴
• Increased nuchal translucency is seen in almost 50% of fetuses who have a congenital diaphragmatic hernia.

Definitive diagnosis - at post mortem.

• Suspect pulmonary hypoplasia in any neonate with immediate respiratory distress.
• Resuscitation: airway, breathing and circulation. Neonates should be resuscitated for up to several hours.
• Neonates will need to be intubated and ventilated (increased risk of pneumothorax).
• If a congenital diaphragmatic hernia is suspected then ventilation can worsen respiratory distress (decompression of the stomach and bowel is required).
• Measurement of blood gases - usually via umbilical artery catheterisation.
• CXR.
• Correction of underlying cause e.g. surgical repair of a diaphragmatic hernia.

• Extra corporeal membrane oxygenation (ECMO) - this is an invasive method used to enhance oxygenation. It uses a machine to take over the work of the heart and/or lungs when they are failing. However, it is given with anticoagulants and thus there is a high risk of bleeding e.g. intracerebral bleed.⁵
• Inhaled nitric oxide therapy - this is used in many cases of neonatal respiratory distress causing hypoxia and pulmonary hypertension. The nitric oxide is inhaled which makes it non-invasive and attractive. It can abolish the need for ECMO in some neonates. However, some results suggest it is not as useful in congenital diaphragmatic hernia as in other causes of respiratory distress e.g. meconium aspiration.^6,5

Tracheal ligation in fetal rabbits was discovered in the 1960's to result in an increased lung size. This has been tested in fetal sheep with good lung growth. However, the resulting lung tissue has poor compliance and less surfactant. Despite these concerns fetoscopic tracheal occlusion has been used in humans to treat congenital diaphragmatic hernia and pulmonary hypoplasia with some success.⁷ This procedure is flawed and complications such as death in-utero and preterm labour can occur. The most serious complication is hydrops fetalis.

Depends on size of the lung and the underlying cause.

Survivors will have chronic lung problems e.g. reduced lung capacity and recurrent chest infections and impaired growth. However, with ECMO the mortality may decrease although undoubtedly the long-term morbidity will increase as patients are surviving.