Pulmonary interstitial emphysema

Pulmonary interstitial emphysema (PIE) refers to the abnormal location of air within the pulmonary interstitium and lymphatics. It typically results from rupture of overdistended alveoli following barotrauma in infants who have hyaline membrane disease. Interstitial emphysema can also occasionally be incidentally detected in adults⁵.  

Epidemiology

PIE is almost always associated with mechanical ventilation or continuous positive airway pressure in the first weeks of life. Reduced lung compliance, prematurity, low birth weight, meconium aspiration syndrome and pneumonia are other risk factors. The reported incidence of PIE amongst infants admitted to the neonatal ICU is 2-3%⁶. The incidence rises to 20-30% amongst premature infants. The highest frequencies are seen in low-birth weight infants⁷.

Clinical presentation

PIE is typically an incidental finding on routine chest radiographs in the NICU before the onset of symptoms. If it is more extensive it may result in compression of adjacent lung and vasculature. This can impede both ventilation and pulmonary blood flow, which can affect oxygenation, ventilation, and blood pressure. PIE may also present with air-block complications such as pneumomediastinum, pneumothorax, pneumopericardium, pneumoperitoneum, or subcutaneous emphysema.

Pathophysiology

PIE almost always occurs during the first weeks of life in newborns on ventilatory support. Alveolar pressure increases in the setting of mechanical ventilation or continuous positive airway pressure. A combination of increased pressure and poor compliance can result in alveolar rupture with escape of air into the adjacent interstitium and lymphatics. This interstitial air may resolve spontaneously, persist chronically or progress with further air-leak into adjacent spaces, causing pneumomediastinum, pneumothorax, pneumopericardium, pneumoperitoneum, or subcutaneous emphysema.

Overall lung volumes are increased; however, the lungs are less compliant because they are splinted at a large volume by the air within the interstitium. This may further compromise the already critically ill child. Air-block complications of PIE such as pneumomediastinum, pneumothorax and pneumopericardium can be fatal.

Distribution

PIE may be focal affecting one lobe or diffuse and bilateral. It has no predilection for any particular lobe.

Radiographic features

Plain film

Chest radiographs will demonstrate some or all of the following features:

• radiograph shows bubbles (round)cystic or streaky (linear)linear radioculencies in the interstitum radiating from the hilum
• affected segment is often hyperexpanded and static in volume on multiple radiographs
• patients may have pneumothorax, pneumomediastinum, or pneumopericardium
• in supine patients, pneumomediastinum is evident by the sharp mediastinum sign
• the heart tends to get smaller as intrathoracic pressure increases and results in diminished venous return into the chest

CT chest

• not usually part of routine investigation (unnecessary risk of radiation exposure)
• shows cystic radiolucencies in affected segment
• may characteristically show a line and dot pattern with pulmonary arterial branches surrounded by radiolucent air⁴
• may help differentiate persistent PIE from a hyperlucent mass such as congenital lobar emphysema, congenital pulmonary airway malformation (CPAM)
• allows better visualisation of a pneumothorax or pneumomediastinum 
• if incidentally detected in adults, it may appear as perivascular lucent or low-attenuating halos and small cysts ⁵

Treatment and prognosis

Switching from conventional to high frequency ventilation is commonly performed in infants with PIE. Many cases resolve spontaneously. Frequent radiographs may be performed to assess for complications such as pneumothorax or pneumomediastinum. 

Selective bronchial intubation, lobectomy and pneumonectomy are rarely used to treat progressive PIE⁸.

Differential diagnosis

General imaging differential considerations include

•  partially treated hyaline membrane disease (surfactant deficiency)
  • partial clearing of collapsed alveoli may be seen with treatment of hyaline membrane disease with exogenous surfactant; this pattern of alternating distended and collapsed alveoli can mimic PIE
• bronchopulmonary dysplasia
  • similar bubble like lucencies; PIE typically occurs abruptly in the first week of life, BPD is more gradual and manifests later
•   congenital pulmonary airway malformation (CPAM)
  • typically present at birth; cysts are more variable in size and static on serial radiographs
•  congenital lobar emphysema
  • hyperlucent, hyperexpanded lobe; vessels may appear attenuated
• congenital diaphragmatic hernia 
  • larger hyperlucencies corresponding to gas in herniated bowel loops, mass effect

See also

• neonatal respiratory distress (causes)