Drs. Sumner, Cox, and Auringer are from the Department of Radiology at Wake Forest University Baptist Medical Center in Winston-Salem, NC.

Usually, emergency neonatal chest imaging is required for causes of respiratory distress necessitating surgery in the newborn. Although conventional chest radiography remains the main imaging modality, certain conditions are better evaluated with ultrasonography, computed tomography (CT), and magnetic resonance (MR) imaging. The appearance of the initial chest x-ray determines which additional imaging procedure(s), if any, will be used. For the purpose of this article, the three conditions that predispose to intervention or surgery are: tension pneumothorax, congenital lesions in the neonate, and acquired lesions occurring in the perinatal period.

Tension pneumothorax

Tension pneumothorax may be spontaneous in the newborn or may develop later as a result of barotrauma or positive pressure ventilation, frequently associated with pulmonary interstitial emphysema. Conventional single-view supine radiographs of the chest are often diagnostic, as pleural air outlines the edge of the lung. Additional findings include: inversion of the ipsilateral hemidiaphragm, contralateral mediastinal shift, and herniation of the parietal pleura across the midline due to mass effect of tension pneumothorax (figure 1). Because the lung falls posteriorly, its lateral edge may not be outlined by pleural air in the supine position. If a pneumothorax is suspected, additional views, such as cross-table lateral or lateral decubitus chest films with the side of interest positioned uppermost, are often helpful. A chest tube should be inserted anteriorly for optimal decompression of a tension pneumothorax.

In the newborn, congenital lesions requiring intervention or surgery include three "C" lesions (congenital diaphragmatic hernia [CDH], congenital cystic adenomatoid malfunction [CCAM], and congenital lobar emphysema [CLE]), plus sequestration. These "C" lesions ultimately produce a large lucent or cystic-appearing hemithorax. Sequestration usually presents as a solid lesion because it is airless and remains opaque on follow-up radiographs of the chest.

Congenital diaphragmatic hernia

Bochdalek hernia is a posterolateral developmental defect of the diaphragm. It results from maldevelopment or failure of fusion of the pleuroperitoneal membrane (or both). It occurs in approximately 1 of every 2000 to 5000 births and occurs on the left five times as frequently as on the right. ¹ Initially, CDH may appear as a large intrathoracic mass of soft-tissue density, progressing to its more characteristic cystic-mass appearance after several hours (figure 2A and B). Other classic findings include absence of bowel gas with a scaphoid abdomen; and Morgagni hernia, which usually develops later in life and is an anterior hernia that results from failure of fusion of the septum transversum with the body wall.

Differential diagnosis includes CLE, CCAM, sequestration, and bronchogenic cysts. Air rapidly fills bowel loops after air-swallowing, resuscitative efforts, or a combination of these, thereby eliminating diagnoses other than CDH. If necessary, careful injection of several cubic centimeters of air through a nasogastric tube will distinguish CDH from other considerations. Sonography of the chest is confirmatory in that it reveals peristalsis of fluid-filled bowel loops and mesenteric vessels in the herniated thorax.

Clinically, the infant usually shows symptoms of severe respiratory compromise. Placement of a nasogastric tube helps to decompress the bowel and reduces the mass effect. If fetal circulation persists and pulmonary hypertension ensues, extracorporeal membrane oxygenation (ECMO) may be necessary for survival (figure 2C).

Congenital cystic adenomatoid malformation

A developmental abnormality of the lung, CCAM is characterized
by adenomatoid proliferation and bronchial structures forming cysts at the expense of normal alveoli. Clinical clues include maternal polyhydramnios and fetal hydrops, anasarca, or both. Polyhydramnios is secondary to fetal esophageal compression with impaired fetal swallowing; hydrops may be due to fetal inferior vena cava or heart compression, and anasarca may also be related to intrauterine vena cava obstruction. From 50% to 85% of neonates with CCAM have respiratory compromise that ultimately requires surgery. ² CCAM is usually unilateral, occurring in a single lobe in 95% of cases. ² It may be airless initially, but unlike sequestration, it communicates with the tracheobronchial tree. Anatomic and clinical differences distinguish three types of CCAM. Type 1 (50% of cases of CCAM) contains few large cysts (3 to 10 cm), type 2 (40% of cases) contains numerous small- to medium-size cysts (0.5 to 3 cm), and type 3 (10% of cases) is mostly solid with smaller cysts (<2 cm). The prognosis is best for patients with type 1. With type 2, the prognosis depends upon associated anomalies (often genitourinary). Type 3 is associated with the worst prognosis (80% mortality) because of its large size, with resultant cardiovascular compromise and pulmonary hypoplasia. ³

In the newborn, radiographs of the chest reveal an opaque hemithorax that, in type 1 and type 2 malformations, has fluid-filled cysts becoming air-filled over the course of hours, days, or weeks to produce a cystic-appearing hemithorax (figure 3A). Chest sonography in patients with type 1 or type 2 CCAM may demonstrate fluid-filled cystic spaces (figure 3B). Chest radiographs in type 3 malformation differ from type 1 and type 2 malformations by showing a persistent opacity (figure 3C); chest sonography reveals a solid structure (figure 3D). Computed tomography reveals cystic changes associated with abnormal soft tissue in types 1 and 2 (figure 3E).

Congenital lobar emphysema

Congenital lobar emphysema consists of progressive lobar overdistension without destruction of alveolar septae. CLE usually produces respiratory compromise in the newborn. It has a male:female incidence of 3:1. The left upper lobe is involved in 43% of cases, right middle lobe in 32%, and the right upper lobe in 20%. ⁴ The etiology of CLE is unknown in more than 50% of cases; often it is believed to be secondary to a congenital bronchial abnormality that causes a high-grade partial obstruction. The involved lobe may be fluid-filled at birth, appearing as a large opaque hemithorax (figure 4A). Subsequent chest radiographs reveal increasing radiolucency as fetal lung fluid is replaced by air (figure 4B). If an entire lung appears overinflated, a contrast swallow may help to exclude a bronchogenic cyst or obstructing vascular abnormality. CT demonstrates the hyperlucent lobe with attenuated or thin vascular structures (figure 4C). ⁵ Definitive treatment of CLE is lobectomy to prevent progression of symptoms and repeated infections.

Sequestration

The term sequestration was introduced by Pryce in 1946 to describe a disconnected bronchopulmonary mass or cyst with an anomalous arterial supply. ⁶ Thus, sequestration has no normal connection with the tracheobronchial tree or with the pulmonary arteries. It may result from an abnormally caudal accessory budding of the foregut, dissociating it from the trachea while retaining its embryonic systemic arterial connection. ⁷ Classically, intralobar sequestration (ILS) and extralobar sequestration (ELS) are defined according to their relationship to lung parenchyma. Shanji et al ⁸ support the theory that ILS occurs earlier and results in incorporation of the sequestered lung within the lung without a separate pleural covering; ELS occurs later and has its own pleural covering. ⁸ ILS is usually drained by pulmonary veins, whereas ELS is usually drained by the azygos venous system; however, this distinction is variable. Arterial supply to ILS derives from the thoracic aorta in 80% to 90% of cases, whereas ELS may be supplied by the thoracic aorta or supraceliac infradiaphragmatic aorta. ILS, the most common form, accounts for 75% of cases. It most often manifests during childhood or early adulthood with recurrent infection; its vascular supply is usually best demonstrated with MR imaging. ELS is encountered frequently in the neonate with respiratory distress, cyanosis, and feeding disorders. Associated congenital anomalies occur in more than 50% of patients with ELS; the most common is diaphragmatic hernia (30%). ⁸ Associated congenital anomalies are rare with ILS. ⁸ Demonstrating the vascular anatomy in ILS may be difficult with CT because the vessels are small; Doppler sonography or MR imaging may be preferable.

Sequestration may appear as solid or hyperlucent areas on chest radiographs. ILS frequently is hyperlucent and may be difficult to distinguish from types 1 and 2 CCAM. ELS is usually solid and, therefore, is distinguishable from types 1 and 2 CCAM; but it may be difficult to distinguish from type 3 CCAM. Because ELS is the most common type of sequestration in neonates and it occurs most commonly on the left side (90%), usually in a subpulmonic location, it should be the primary differential consideration for a solid lower-lung mass in a neonate (figure 5). Surgical resection is usually performed for confirmation and to prevent recurrent infection.

Acquired lesions occurring in the perinatal period are pulmonary interstitial emphysema (PIE) and bronchopulmonary dysplasia (BPD).

Pulmonary interstitial emphysema

PIE results from air dissection into the perivascular and peribronchial spaces. It occurs most often in neonates requiring assisted ventilation for severe respiratory distress syndrome caused by rupture of overdistended alveoli into the pulmonary interstitium. Chest radiographs reveal linear lucencies radiating from the hilar regions; because they are located within the noncompliant lung, these lucencies usually do not empty on expiration (figure 6).

PIE occurs in two forms: acute and chronic (persistent). Acute PIE can resolve or persist. Recognition of PIE itself is important, but its clinical significance is that it may be the precursor of such air-block complications as pneumothorax, pneumomediastinum, and pneumoperitoneum. Alveolar rupture may also permit air to enter the pulmonary venous system, causing fatal systemic air embolism.

As previously mentioned, pneumothorax may produce mass effect requiring decompression with a chest tube. Emergency exploratory surgery is frequently required to determine the cause of pneumoperitoneum. Pneumomediastinum is more often self-limiting and, therefore, less likely to necessitate surgical intervention.

Persistent PIE (PPIE) is much less common than acute PIE. It typically occurs in neonates with surfactant deficiency, but can occur in those with no history of lung disease or assisted ventilation. ⁹ Chest radiographs show progressive expansile cystic areas (figure 7A and B). The air cysts are usually isolated to one or two lobes, can be as large as 3 cm, and can produce mediastinal shift and respiratory distress. ⁹ CT may help to differentiate PPIE from other causes of multicystic abnormality seen on chest radiographs, such as CCAM and CLE. ^5,10 In patients with PPIE, abnormal air collections are located in the interstitial space, and CT shows interstitial air surrounding bronchovascular bundles, thereby documenting the interstitial location of the cystic air spaces. The bronchovascular bundles appear as soft-tissue-attenuation nodular or linear densities in the center of the air-filled cyst(s) (figure 7C). ⁹ By contrast, chest CT in patients with CCAM shows cystic changes associated with abnormal soft tissue (figure 3E). Chest CT in those with CLE shows attenuated thinned vascular structures at the periphery of the expanded air spaces, rather than in the center as with PPIE (figure 4C). Initial treatment of PPIE with decubitus positioning or selective intubation may be sufficient; however, in some cases, surgical resection is required to relieve respiratory distress.

Bronchopulmonary dysplasia (BPD)

A distinct pulmonary disease, BPD results from prolonged oxygen or respirator therapy for causes of respiratory distress, including hyaline membrane disease, neonatal pneumonia, and meconium aspiration. ¹¹ The combination of high oxygen concentration and barotrauma of respirator therapy ultimately injures alveolar epithelium and results in overdistended alveoli and acini within scarred lungs. ¹² Corresponding chest radiographs show hyperexpansion with irregular aeration and a "soap-bubble" appearance that resembles multiple cysts (figure 8A). The bubbly lucencies are due to overdistended alveoli, which alternate with curvilinear strands of soft-tissue density representing scarred lung. Associated hyperaeration is often most marked at the bases. CT may be helpful in distinguishing BPD from other causes of cystic-appearing lungs. Changes associated with BPD typically demonstrated on CT scans of the chest include: volume loss (especially in the upper and middle lobes), fibrosis, retraction, and bronchial dilatation (figure 8B). ⁵ Occasionally, development of a focal large interstitial cyst or acquired emphysema produces increasing respiratory distress; CT may again prove helpful by defining the type and severity of involvement with PPIE or lobar emphysema.

Finally, some extrapulmonary conditions may mimic lung masses and produce respiratory distress in the neonate. Two such processes are pleural fluid and mass lesions. Pleural fluid is detected easily on a chest radiograph; a large amount may produce significant respiratory distress, necessitating aspiration, an indwelling chest tube, or both. However, imaging may exclude a solid mass lesion before intervention with aspiration or drainage. Sonography of the chest may suffice in this regard; CT will also differentiate fluid from a solid mass lesion (figure 9). The most common cause of a large pleural effusion in the newborn is chylothorax. The etiology of spontaneous chylothorax is usually unknown; thoracic duct rupture secondary to birth trauma and transient central venous hypertension during delivery are two possible causes. Fifty percent of infants with spontaneous chylothorax have symptoms within the first day of life, and 70% have symptoms by the end of the first week. Right-sided effusion is more common than left-sided effusion. ¹³ Treatment consists of single or multiple thoracentesis; chest-tube drainage may be required. Medium-chain triglycerides are often therapeutic and thoracic duct exploration is rarely necessary; prognosis is excellent. Other etiologies of pleural effusion in the newborn include Turner's syndrome, congestive heart failure, infantile polycystic kidney disease, polycythemia, CCAM, idiopathic hypervolemia, iatrogenic hypervolemia, obstructed pulmonary venous return, esophageal rupture, erosion by gastroenteric cyst, and wet-lung disease.

Rarely, respiratory distress may be secondary to an extrapulmonary mass lesion producing cardiovascular compromise. Figure 10A illustrates respiratory distress in an infant whose chest radiograph suggested the possibility of a cardiac or pericardial mass lesion. Ultrasonography demonstrated an echogenic mass within the pericardium (figure 10B). Subsequent surgical exploration revealed the mass to be a pericardial teratoma.

Summary

The conditions described above may produce respiratory distress requiring intervention or surgery. Diagnosis may be possible with conventional radiography, which is undoubtedly the most rapid imaging technique. However, ultrasonography followed by CT or MR imaging may also be necessary to establish the diagnosis. AR

Acknowledgment

We thank David H. Binstadt, MD, St. John's Hospital, Springfield, IL, for providing the images in figure 4.