Evaluating a Normal Chest

Familiarity with the normal appearance of the newborn chest radiograph improves recognition of pathologic changes. Normal lungs appear primarily radiolucent and symmetric in volume. The pulmonary vessels are seen as branching, linear shadows that taper in size as they extend from the hilum to the lung periphery. Normal vessels decrease in size and number in the lateral half of the lung and are not visualized in the lung periphery. The pleural space is normally empty and collapsed and is visualized only when it contains fluid, air, or soft tissue such as thickened pleura. The heart borders should be distinct and the diaphragm should be outlined clearly against the lung. The normal cardiac diameter on an AP radiograph should be less than 60% of the thoracic diameter (cardiothoracic ratio). The normal thymus is visible in most newborns. Extremely variable in size and shape, it is composed of two asymmetric lobes and may therefore have an asymmetric appearance in chest radiography (Figure 40-1). It typically demonstrates “wavy” undulations of the lateral borders and often overlies and obscures a portion of the heart, making the heart look large in appearance (Figure 40-2).

Catheters, Tubes, and Lines

The positions of the endotracheal tube, enteric tube, and the various arterial and venous catheters should be evaluated on all neonatal chest and abdominal radiographs. Tube and line malpositioning is frequent and may lead to complications in the infants. The endotracheal tube tip should lie in the trachea between the medial ends of the clavicles and the carina. Intubation of the right mainstem bronchus is the most common complication. The enteric tube should end in the stomach, beyond the gastroesophageal junction.

Umbilical catheters can be easily identified by their unique paths. The umbilical venous catheter (UVC) courses superiorly and gradually posteriorly within the umbilical vein to intersect the left portal vein, continuing through the ductus venosus, left or middle hepatic vein, and then into the IVC. Its tip should be directed superiorly, ending at the right atrial/inferior vena cava (RA/IVC) junction. Malposition of the UVC within the liver is the most common complication and may result in portal vein thrombosis or portal hypertension. The umbilical artery catheter (UAC) first extends inferiorly and posteriorly to the junction of the umbilical artery and internal iliac artery. The catheter then turns superiorly to course through the common iliac artery and aorta. The tip of the UAC should also be directed superiorly, lying below the ductus arteriosus and above or below the visceral arteries. The UAC therefore may be placed in a high position, ending between T6 and T10, or in a low position ending between L3 and L5. Malposition of the UAC may result in thrombosis of the aortic branches and possibly ischemic injury.

Percutaneously placed catheters, including peripheral inserted central venous catheters (PICC lines) and extracorporeal membrane oxygenation (ECMO) catheters, are increasingly used. Peripheral inserted central venous catheter lines are very small and only faintly radiopaque so that sonography may be used to guide placement. The tip of an upper extremity PICC line should lie within the superior vena cava (SVC) or at the RA/SVC junction. A lower extremity PICC line should end between the 9th and 12th ribs. Extracorporeal membrane oxygenation therapy is reserved for neonates with severe, reversible respiratory failure not responding to conventional treatment and may use a venous and an arterial catheter or a dual lumen venous catheter. The venous catheter is inserted through the right internal jugular vein and ends in the RA. The arterial catheter is inserted into the common carotid artery and ends near the origin of the innominate artery. Complications of all vascular catheters include malposition, thrombosis, perforation, or infection.³⁸

Respiratory distress syndrome (RDS), secondary to surfactant deficiency, is the most common cause of respiratory distress and a leading cause of death in the premature infant. The prematurity of type II pneumocytes in these infants results in an inadequate production of surfactant. This triggers a cascade leading to decreased alveolar distensibility, capillary leak edema, noncompliant lungs, and respiratory distress.

The typical radiographic appearance of RDS largely reflects generalized alveolar collapse and includes a finely granular or ground-glass pattern with diminished lung volumes. The severity of radiographic disease is quite variable and usually correlates with the severity of clinical disease. Mild radiographic disease is characterized by a finely granular pattern that allows visualization of normal vessels (Figure 40-3), whereas severe disease results in silhouetting of the heart borders and diaphragm (Figure 40-4). Peripheral air bronchograms may be seen with severe disease because of air in the bronchi being visualized against a background of alveolar collapse. The distribution of disease is usually diffuse and symmetric; however, patchy or asymmetric disease may be seen. The radiographic changes associated with RDS are often seen immediately after birth, but can also develop over the first 6 to 12 hours of life. The radiographic abnormalities related to uncomplicated RDS should resolve by the time the neonate is 3 to 4 days old or sooner if surfactant therapy is given. Sudden diffuse worsening of densities in the lungs may be seen with pulmonary edema or pulmonary hemorrhage.³⁸ Sudden focal increase in opacity usually indicates focal atelectasis.

Chest radiography can be used to help assess the effectiveness of surfactant replacement therapy in infants with RDS. Typically, improvement in the appearance of the lungs is rapid and uniform after surfactant administration. In 80% to 90% of neonates treated with surfactant, improvement occurs in one or both lungs. When there is a partial response, the improvement may be asymmetric or even restricted to one lung. Explanations for asymmetric radiographic improvement following surfactant therapy include (1) maldistribution of surfactant, (2) insufficient surfactant, and (3) regional differences in lung aeration before surfactant treatment. The absence of radiographic improvement after surfactant administration is a poor prognostic sign and suggests a diagnosis other than surfactant deficiency. In rare circumstances, pulmonary hemorrhage following surfactant therapy may occur.1,38

Complications can result from the high distending pressures of mechanical ventilation that may be required in the treatment of RDS. Alveolar rupture from overdistention results in pulmonary interstitial emphysema (PIE). The radiographic appearance of PIE includes small, rounded or linear lucencies representing interstitial air coursing along the bronchovascular sheaths (Figure 40-5). The abnormalities may be diffuse or localized. Larger focal air collections (pseudocysts) may also form in the interstitium of the lung. Pulmonary interstitial emphysema can dissect into the mediastinum or the pleural space, resulting in a pneumomediastinum or pneumothorax. Radiographic signs of pneumomediastinum include (1) lateral displacement of the mediastinal pleura; (2) continuous diaphragm sign; and (3) superior elevation of the thymus, which is referred to as the spinnaker sail sign or angel wings (Figure 40-6).¹ Radiographic findings of pneumothorax include (1) increased thoracic lucency, (2) identification of the visceral pleural line (Figure 40-7), (3) increased sharpness of the adjacent mediastinal border or hemidiaphragm, and (4) deep sulcus sign. Pneumothoraces may be seen bilaterally (Figure 40-8). Lateral decubitus or cross-table lateral radiographs can be useful in the detection of small pneumothoraces. Large pneumothoraces can produce tension, resulting in contralateral shift of mediastinal structures and depression or eversion of the ipsilateral hemidiaphragm.

Meconium Aspiration Syndrome

The expulsion of meconium before birth is often related to fetal distress leading to a hypoxia-induced vagal response. It typically occurs in full-term or postmature infants. Fetal aspiration of the meconium then causes obstruction of small airways with associated atelectasis and air trapping. Radiographic findings of meconium aspiration syndrome are seen within the first few hours of birth and include coarse, patchy, or nodular opacities and segmental hyperinflation (Figure 40-9). The distribution of disease is bilateral and often asymmetric. Complications include chemical pneumonitis, surfactant inactivation, pulmonary hypertension, and air-leak phenomena, including pneumothorax and pneumomediastinum.³⁸ Pleural effusion may be present. The radiographic appearance of the chest and rate of improvement vary with the amounts of meconium and amniotic fluid aspirated and the complications described in the preceding.

Bronchopulmonary Dysplasia

Chronic lung disease in the premature infant, known as bronchopulmonary dysplasia (BPD), is most often seen in very low birth weight infants. Bronchopulmonary dysplasia is also seen in higher birth weight infants following prolonged mechanical ventilation for conditions including neonatal pneumonia, meconium aspiration, and congenital cardiac disorders. The definition of BPD has evolved, and it is now clinically defined by physiologic criteria.31,60 Pathologically, BPD is considered to be part of the group of alveolar growth abnormalities and can be recognized on imaging. Because BPD is the result of injury and repair of the immature developing lung, the pathologic and radiologic findings are affected by changes in therapy and the degree of prematurity of the infant. Bronchopulmonary dysplasia was originally described as airway injury, obstruction, inflammation, and parenchymal fibrosis. Chest radiograph findings of the later stages of BPD included hyperinflated lungs, asymmetric, coarse, patchy opacities, and cystic emphysematous changes.^1,26,44

In recent years, the widespread adoption of antenatal glucocorticoid administration, postnatal surfactant therapy and refinement of assisted ventilation have decreased lung injury from oxygen toxicity and barotrauma.20,34 Bronchopulmonary dysplasia is now rarely seen in infants of greater than 30 weeks’ gestation or over 1200 grams’ birth weight.¹ Modern therapies have also allowed an increased survival of very low birth weight infants. Despite the absence of prior severe RDS or barotrauma, these infants often have a more insidious development of BPD, known as the “new” BPD. Arrested lung development leading to decreased alveolar and microvascular growth is thought to contribute to this condition. Minimal pathologic changes in the airways, less fibrosis, and more uniform inflation have also been noted. Radiographic and CT findings range from near normal to disordered lung architecture with hyperlucent areas, linear and subpleural opacities, and bullae typical of chronic lung disease (Figure 40-10)^4,20,40 (see Chapter 77).

Congenital diaphragmatic hernia (CDH) is a complex life-threatening lesion caused by defective fusion of the pleuroperitoneal membranes during embryologic development. Patent pleuroperitoneal canals located posterolaterally are known as the foramina of Bochdalek. Bowel and solid organs may herniate through the foramen into the affected hemithorax, most commonly on the left side. At birth, the bowel loops may be fluid-filled, making radiographic diagnosis difficult. Eventually, gas-filled bowel loops are seen in the thorax with a paucity of bowel loops in the abdomen (Figure 40-11). The ipsilateral lung is almost universally hypoplastic, and there is usually contralateral shift of the mediastinum, resulting in contralateral lung hypoplasia. Pulmonary hypertension is often present in these infants at birth. Prenatal sonography and fetal MRI (Figure 40-12) allow early diagnosis and may predict neonatal survival by evaluating the degree of pulmonary hypoplasia and defining associated anomalies.⁵⁵ Herniation of the liver predicts a poor prognosis.³⁸

Congenital Pulmonary Airway Malformations

Congenital pulmonary airway malformations (CPAMs), previously known as congenital cystic adenomatoid malformations (CCAMs), are a group of congenital, hamartomatous cystic and noncystic lung masses characterized by overgrowth of the primary bronchioles and a proximal communication with a defective bronchial tree.7,8,32,33,53,62 The new terminology has been recommended because not all lesions are cystic and only one is adenomatoid. Stocker updated his old classification to include five types (0-4) based on cyst size and similarity to segments of the developing bronchial tree and air spaces: Type 0 is acinar dysplasia of tracheal or bronchial origin and is incompatible with life. Type 1, the most common, has a single or multiple large cysts (>2 cm) of bronchial or bronchiolar origin; type 2 has a single or multiple small cysts (≤2 cm) of bronchiolar origin; type 3 is predominantly solid with microcysts (<0.5 cm) of bronchiolar-alveolar duct origin; and type 4, characterized by large air-filled cysts, has a distal acinar origin and is indistinguishable from pleuropulmonary blastoma on imaging.^8,32,33,53 This classification remains controversial and can be difficult to apply. Congenital pulmonary airway malformation can also be seen in association with other foregut anomalies, most commonly pulmonary sequestration.^8,18,62,43

Prenatal sonography and fetal MRI classifies CPAMs mainly based on the presence of macrocysts or microcysts.¹⁸ At birth, chest radiography and CT may show a range of findings from large single or multiple air-filled cystic structures to solid lesions that resemble consolidation (Figure 40-13, A, B). Because of the association of CPAMs with sequestration, precise vascular mapping is essential. Ultrasound may be useful in identifying an abnormal vascular systemic supply (see Figure 40-13, C). However, CT angiography with 2D and 3D reconstructions provides the most accurate assessment of the lung parenchymal and vascular anatomy of these lesions and is, therefore, the modality of choice.

Congenital Lobar Overinflation or Congenital Lobar Emphysema

Congenital lobar overinflation (CLO) or emphysema is a condition characterized by progressive overinflation of one or more pulmonary lobes. This may be caused by intrinsic bronchial narrowing from weak or absent bronchial cartilage or may be caused by extrinsic bronchial narrowing from mass effect of adjacent structures. The collapsed bronchus can result in one-way valve obstruction causing air trapping and progressive distention of the distal airways in the affected lobe.8,33 Congenital lobar overinflation most commonly affects the left upper lobe, followed by the right middle lobe and the right upper lobe. At birth, the involved lobe may be radiographically opaque because of retained lung fluid. The fluid is then cleared and imaging demonstrates progressively increased volume, hyperlucency, and attenuated vascular markings of the involved lobe with compression of the remaining ipsilateral lung and mediastinal structures (Figure 40-14).^8,32,33 Computed tomography may more precisely characterize these findings and identify multilobar involvement. Computed tomography can also exclude causes of extrinsic bronchial compression such as vascular anomalies or mediastinal masses^32,33 (see Chapter 74).

Esophageal Atresia and Tracheoesophageal Fistula

The pairing of the terms esophageal atresia and tracheoesophageal fistula (TEF) describes a disorder in formation and separation of the primitive foregut and esophagus.⁷ A spectrum of malformations is noted, ranging from esophageal atresia (with or without a proximal or distal tracheoesophageal fistula) to a tracheoesophageal fistula without esophageal atresia. The most common type, involving proximal esophageal atresia with a distal tracheoesophageal fistula, accounts for more than 80% of cases. At chest radiography, a blind-ending, air-filled proximal esophageal pouch is noted with the most common type (Figure 40-15). The presence of a distal fistula is supported by air in the gastrointestinal tract. An esophagram can be performed to evaluate for a fistulous tract. Tracheoesophageal fistula is associated with multisystem abnormalities in approximately one third of cases, including vertebral, cardiac, renal, and limb anomalies as well as other gastrointestinal tract atresias. All infants with a tracheoesophageal abnormality should, therefore, undergo a more extensive evaluation to assess for associated anomalies.

Heart

Cardiac problems in infants are usually congenital in origin. Although radiography is an important part of the evaluation of children with suspected cardiac disease, it alone rarely leads to a specific diagnosis. Rather, its usefulness is to exclude pulmonary conditions as a cause of the respiratory distress. The clinical examination, cardiac echocardiography, and MRI are the primary means of diagnosing congenital heart conditions.

Great variability in cardiac size is found in congenital heart anomalies. Typically, cardiomegaly is present in infants and neonates with large left-to-right shunts (ventricular septal defect, atrioventricular canal), Ebstein anomaly, hypoplastic left heart syndrome, or cardiomyopathy. In addition, cardiomegaly may be seen transiently in the absence of cardiac disease in association with hypoglycemia, hypocalcemia, or severe anemia, as well as in infants of diabetic mothers. Enlargement of specific cardiac chambers cannot be accurately assessed by chest radiography.

In cyanotic congenital heart disease, the caliber of the pulmonary arteries is reduced, the hila appear small, and the lungs may appear more lucent. However, it is difficult to differentiate normal from small pulmonary vessels by chest radiography, as there is too much overlap in their appearance. Persistent pulmonary hypertension may mimic cyanotic congenital heart disease, both clinically and radiographically, because of cardiomegaly and pulmonary oligemia.

Radiography is useful in diagnosing increased pulmonary arterial vascularity associated with large left-to-right shunts, including patent ductus arteriosus, ventricular septal defect, atrial septal defect, and endocardial cushion defect. Increased pulmonary arterial vascularity is seen at radiography when the ratio of left-to-right shunt is greater than 3 : 1. Increased branching linear shadows will be seen in the perihilar region, and vascular markings will be seen in the periphery of the lung fields. The pulmonary arterial vascularity usually appears normal with shunts of a lesser degree (see Chapter 80).

Midgut malrotation is the most important cause of upper intestinal obstruction. Abnormal in utero rotation of the midgut results in abnormal mesenteric fixation and a short mesenteric base that may allow twisting of the bowel and mesentery around the axis of the superior mesenteric artery. This is known as midgut volvulus, which can lead to vascular compromise, bowel ischemia, and necrosis. Most infants with malrotation present with bilious vomiting. Up to 75% of patients present in the first month of life.⁵² Bilious vomiting in an infant should be considered a potential surgical emergency and, in the absence of another defined cause, evaluation for midgut malrotation should be performed.

The diagnosis of midgut malrotation by imaging is challenging and the findings variable.3,52 The abdominal radiographic findings may be normal in infants with malrotation, so normal results on an abdominal radiograph do not exclude the condition. A high obstruction pattern can be seen, either because of abnormal peritoneal attachments (Ladd bands) or midgut volvulus.^30,59 Dilatation of multiple bowel loops may indicate volvulus-induced ischemia. The diagnostic examination of choice is the UGI. The location of the duodenum seen on this study is predictive of the mesenteric attachment. Normally, the third portion of the duodenum (D3) courses posteriorly in the retroperitoneum, crossing the midline to reach the duodenal-jejunal junction (DJJ), which is normally located to the left of the spine and at the same level as, or more superior to, the duodenal bulb. An abnormal course of the duodenum and abnormal location of the duodenal-jejunal junction is diagnostic of midgut malrotation (Figure 40-18, A). A spiral or corkscrew appearance of the duodenum and jejunum indicates midgut volvulus (see Figure 40-18, B).^3,30

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