Etiology and Pathogenesis

Pleural effusions are the result of an imbalance of hydrostatic and oncotic pressures between the blood in the pulmonary capillary bed and fluid in the pleural space, an alteration in permeability of the pleural membranes, or inadequate uptake by the lymphatic system. Pleural effusions can be divided into transudates and exudates. Exudative effusions occur from pleural inflammation or lymphatic flow obstruction. Transudative effusions occur when there is an imbalance between the formation and reabsorption of pleural fluid. Pleural fluid analysis determines whether the effusion is transudative or exudative. Because exudative effusions result from inflammation of the pleural membranes and leaky capillaries, large molecules such as cholesterol, lactate dehydrogenase (LDH), and proteins enter the pleural space. Conversely, the protein, LDH, and cholesterol levels in transudates are low because the filtration properties of the pleural membranes are not altered.

Typically, small amounts of protein are filtered into the pleural space and are readily absorbed by the parietal pleura via the lymphatic system. If increased amounts of protein enter the pleural space, especially when accompanied by increased capillary permeability (e.g., in pneumonia), the lymphatic system cannot absorb the excessive protein, and an exudative pleural effusion forms. The most common cause of exudative pleural effusions in children is bacterial pneumonia. Additional causes include connective tissue diseases, metastatic intrathoracic malignancy, subdiaphragmatic abscess, and aspiration pneumonitis. Transudative effusions in children are typically associated with overhydration, atelectasis, nephrotic syndrome, and congestive left heart failure. Correcting the oncotic and hydrostatic pressures usually results in resolution of a transudate; drainage of the fluid is only needed for immediate symptomatic relief.

An exudative pleural effusion that is associated with pneumonia is referred to as a parapneumonic effusion. Parapneumonic effusions result from the spread of inflammatory cells and infecting organisms into the pleural space. Initially, the pleurae become inflamed, and the leakage of proteins and leukocytes into the pleural space forms the effusion. Initially, the fluid is sterile with a low leukocyte count. As a parapneumonic effusion progresses and bacteria leak into the pleural space, the pleural fluid becomes purulent, and the effusion is referred to as an empyema, occurring in approximately 0.6% of childhood pneumonia. Loculations (parietal–visceral pleural adhesions) and septations (fibrous strands) form within parapneumonic effusions as the pleural fluid exudate thickens and deposition of fibrin occurs within the pleural space.

The risk of a child developing an empyema increases in certain underlying diseases, such as immunodeficiencies, malignancy, Down syndrome, congenital heart disease, tuberculosis, and cystic fibrosis (CF). Streptococcus pneumoniae remains the most common pathogen causing parapneumonic effusions. Community-acquired methicillin-resistant Staphylococcus aureus is an increasingly common cause of both parapneumonic effusions and empyemas. Empyemas can also be caused by the rupture of lung abscesses into the pleural space; by bacteria entering the pleural space from trauma, thoracic surgery, mediastinitis; or through the spread of intraabdominal abscesses. Complications associated with parapneumonic effusions and empyemas are infrequent in children but include bronchopleural fistula, lung abscess, and empyema necessitatis (perforation through the chest wall). Boys and girls are affected equally by empyemas, and the morbidity and mortality are highest in children younger than 2 years of age.

Clinical Presentation

The size of the effusion, the underlying cause, and when in the course a child presents all determine the clinical presentation. Children with small pleural effusions may be asymptomatic. As the effusion enlarges, it limits lung inflation, causing a decrease in vital capacity. Furthermore, if present, pleural inflammation is associated with dyspnea, chest tightness, and chest pain that is exaggerated by deep breathing, coughing, and straining; all of these further limit full lung expansion. The pain, resulting from stretching of parietal pleura nerve fibers, is often described as a dull ache that worsens with inspiration. The pain is often localized over the chest wall and is referred to the shoulder or the back. Often the child will attempt to decrease the pain by lying on the affected side in an attempt to splint it during breathing.

On physical examination, a child with a significant pleural effusion can appear ill but is rarely toxic appearing. Most children are tachypneic with shallow breathing to minimize the pain. It is important to look for signs and symptoms of underlying conditions that predispose to the development of pleural effusions. If the child has an empyema, he or she is usually febrile with a cough and malaise. If a child is being appropriately treated for pneumonia and is not improving within 48 hours, a parapneumonic effusion must be suspected. A malignant effusion must be suspected in a child with an effusion accompanied by a mediastinal mass or lymphadenopathy. A history of recurrent serious bacterial infections, failure to thrive, or chronic diarrhea is suggestive of a primary immunodeficiency.

Because of splinting toward the affected side, the child may appear to have mild scoliosis. There can be ipsilateral bulging of the intercostal spaces and contralateral displacement of the heart and trachea. A pleural friction rub caused by roughened pleural surfaces may be the only physical examination finding early in the course of disease, heard during inspiration and exhalation. As the pleural effusion increases and separates the pleural membranes, the plural rub disappears. Diminished thoracic wall excursion, decreased breath sounds, dullness to percussion, and decreased tactile and vocal fremitus can be observed over the affected area in an older child with a moderate effusion. If pneumonia is present, crackles and rhonchi can also be audible. In infants, the physical signs of an effusion are less noticeable. Breath sounds can be deceptively loud and clear throughout both lungs because of the small lung volume in an infant.

Evaluation and Management

Children with the clinical history and findings suggestive of a pleural effusion should be evaluated with an upright chest radiograph and a lateral decubitus view (Figure 40-2). Performing radiographic examinations with the child in multiple positions helps to demonstrate a shift in the effusion with position changes. These radiographic images can help in making the diagnosis of pleural effusion and in determining the need for thoracocentesis or chest tube placement.

Figure 40-2 Pleural effusion.

Radiographic signs of pleural effusion include a homogenous density overlying the normal markings of the underlying lung, obliteration of the costophrenic angle, the “meniscus sign” or “pleural stripe” (a rim of fluid ascending the lateral chest wall), and possible scoliosis. Infants or children whose radiograph is taken in a recumbent position will not have a meniscus, but instead demonstrate only a denser hemithorax on the affected side. Air-fluid levels within the pleural space suggest the presence of gas-forming organisms, pneumothorax, perforated viscus, or bronchopleural fistula. If there is no shift in the fluid on chest radiograph with a change in position, the effusion is most likely a loculated empyema.

Ultrasonography is useful in confirming the presence of fluid in the pleural space. Additionally, if a child has findings consistent with a parapneumonic effusion, ultrasonography should be undertaken to determine whether loculations are present.

Chest computed tomography (CT) can also be helpful in determining the presence of pleural fluid; however, CT findings do not typically affect management decisions, and they should not be performed routinely in the evaluation of children with pleural effusions. Chest CT can be useful in the evaluation of complicated cases when more detail about the effusion or the underlying lung parenchyma is desired.

When infection is in the differential diagnosis, pleural fluid aspiration via thoracocentesis should be performed if at least 1 cm of fluid is seen on decubitus radiographs to determine the type of effusion (i.e., infectious exudate, empyema, hydrothorax, hemothorax, or chylothorax) before starting therapy (Figure 40-3). Certain laboratory studies should always be performed on the pleural fluid aspirate. Additional laboratory studies should also be obtained during the evaluation of a child with a parapneumonic effusion or empyema (Table 40-1). Microbiologic studies of the pleural fluid can identify a causative organism, and the analysis of pleural fluid is helpful in guiding therapeutic options. Pleural fluid can be obtained through thoracocentesis, aspiration under ultrasonographic guidance, or through video-assisted thoracoscopic surgery (VATS), depending on the presence of loculations or the availability of pediatric surgeons trained in VATS.

Figure 40-3 Pleural fluid aspiration.