Diseases of the Pleura
James S. Kemp
Terrence W. Carver Jr.
STRUCTURE AND PHYSIOLOGY OF THE NORMAL AND INFLAMED PLEURA
The embryonic coelomic cavity is lined by mesothelial cells and fibroelastic tissue. This embryonic mesothelial lining gives rise to the pleura and peritoneum. The parietal and visceral pleural mesothelia each is a single cell layer thick and forms a stretchable serous membrane over the lung and chest wall. The visceral pleura also is composed of collagen and elastin connective elements, through which travels its vascular supply, rendering it thicker than the parietal pleura. Once formed, the visceral pleura adheres tightly to the lung parenchyma and interlobar fissures. The parietal pleura also is firmly anchored to the ribs, intercostal muscles, and central diaphragm, and it is tightly adherent as it reflects over the descending aorta, esophagus, and pericardium. Because of the structures that the pleura invests, pressures within the pleural space are important determinants of the transmural pressure of the heart, esophagus, and lungs.
Normally no communication occurs between the left and right pleural cavities, but fluid may enter the pleural space from the peritoneal cavity in some children through pores in the diaphragm. In healthy children, fluid enters the pleural space from the capillaries, lymphatics, and interstitial spaces of both pleurae. Approximately 0.01 mL/kg/hour of fluid enters the space. In the healthy state, this fluid amounts to approximately 0.25 mL/kg body weight per pleural space and creates a film 10 μm thick between the parietal and visceral pleurae. The fluid lining allows for direct mechanical coupling between the lungs and the diaphragm, intercostal muscles, and other muscles of the chest wall. Mechanical coupling via the pleural space transmits to the lungs the forces generated by the diaphragm and other muscles of inspiration. Any widening of the pleural space reduces the efficiency of the inspiratory muscles. In addition to coupling to inspiratory-force generators, fluid in the pleural space permits the pleurae to slide over one another during the respiratory cycle. Glycoproteins within the matrix formed by microvilli on mesothelial cells also reduce friction during breathing.
Research on animals with pleurae similar in thickness to those of humans shows that the visceral pleura, because of its relative thickness, plays a limited role in fluid resorption in both health and disease. The parietal pleura, though more leaky, also plays a larger role in fluid uptake. Most of the fluid is resorbed by lymphatics in the parietal pleura, with an apparent maximal rate of absorption of 0.20 mL/kg/hour, which tends to minimize the amount of fluid in the pleural space. The fluid equilibrium for the pleural space is defined by permeability of pleural mesothelial cells, hydrostatic pressure differences between the parietal and visceral capillaries and lymphatics, and the oncotic pressure of blood compared with pleural fluid.
The normal cell population within the pleural space is small—1,500 to 4,500/μL—and is composed of 75% percent macrophages, with the balance lymphocytes. Mesothelial macrophages are active phagocytes and may have enhanced antigen-presenting capacity.
Free radicals of nitric oxide (NO), produced from activity of inducible NO synthase within mesothelial cells, likely are the first line of defense against infection of the pleural surfaces. Mesothelial cells also secrete proinflammatory (tumor necrosis factor-alpha) and antiinflammatory (interleukin-10) cytokines in abundance when stimulated. During inflammation, neutrophils enter the pleural space under the “direction” of mesothelial cells via mechanisms similar to those described for vascular endothelial cells and involving intercellular adhesion models, selectins, and integrins, whose production is up-regulated within the mesothelial cells. Early in inflammation, all types of cells aggregate around openings in the parietal lymphatics to form “pleural tonsils,” called Kampmeir foci. During inflammation, these foci are found most commonly on the dorsal, caudal, and mediastinal parietal pleura. The foci appear on scanning electron microscopy as mounds of lymphocytes, histiocytes, neutrophils, plasma cells, and swollen and metabolically active mesothelial cells.
Three clinically recognized stages occur during inflammation. First, dry or plastic pleurisy reflects ingress of inflammatory cells, with minimal fluid. Second, pleurisy with effusion indicates that the inflammatory process has increased the permeability of the mesothelial cells, and fluid enters the pleural space at rates exceeding its removal. Third, organizing pleural disease is reached only with bacterial or fungal parapneumonic effusions. The effusion becomes fibrinous, and the accumulating pleural fluid no longer flows freely in the space. Instead, pockets of fluid are loculated between gelatinous adhesions. If frank pus is present in the pleural space, the effusion can correctly be called an empyema. Because of widespread confusion about the correct meaning of the term empyema, it should be used only after a clear statement of the intended meaning is made.
CLINICAL FINDINGS IN DISEASES OF THE PLEURA
Findings Caused by Excess Fluid and Inflammation
Regardless of cause, excess fluid accumulates in the pleural space when production exceeds resorption. Systemic diseases increasing visceral fluid hydrostatic pressures or decreasing plasma oncotic pressure cause thin, transudative pleural effusions. Thicker, exudative effusions (protein concentrations greater than or equal to 50% of serum) result from diseases, usually inflammatory, involving the pleural surfaces themselves. Inflammation increases the permeability of mesothelial cells and the permeability of pleural capillaries to protein, and may decrease parietal lymph resorption.
Underlying cardiac, hepatic, or renal diseases usually cause transudative effusions. Ventilatory function may be impaired directly by the underlying disease (e.g., pulmonary edema). A large pleural effusion also partially uncouples the lung from the muscles of inspiration, deforms the diaphragm and chest wall, and compresses the lung. Consequently, worsening tachypnea, cyanosis, and retractions with diminished breath sounds and dullness to percussion usually accompany large transudative effusions (see Hydrothorax). Dyspnea associated with transudation into the pleural space, in the absence of inflammation, may be related to mechanical inefficiencies caused by mechanical uncoupling of the lungs and the muscles of respiration and by compression of lung neurogenic receptors connected to vagal and sympathetic fibers within the lung.
Classic findings of early pleuritis are pain in the chest or shoulder (implying that the parietal pleura of the central diaphragm is involved), guarding of the affected side, upper quadrant abdominal pain (pleura on costal diaphragm involved), a pleural friction rub, and grunting, shallow respirations. Pain fibers are present in the parietal but not the visceral pleura. Therefore, pleuritic pain reflects extension of inflammation to the chest wall early in the course of the thoracic process. Because inflammatory effusions cause pain and are exudative, the presence of pain indicates that the effusion quite likely is an exudate. An inflammatory effusion that is becoming large reduces these signs quickly, except for cough and rapid, shallow respirations and, occasionally, chest wall hyperesthesia. Later, the child has fever, cough, and dyspnea. Thus, a child with a large inflammatory effusion appears ill, with fever and dyspnea. Children with immunodeficiencies and those on corticosteroids may have large pleural effusions with few clinical findings. In immunocompetent children, if the inflammation is triggered by an infection with anaerobic bacteria, the findings of pleural involvement may be less dramatic than is typical for infections caused by pathogens such as Streptococcus pneumoniae, Staphylococcus aureus, Haemophilus influenzae, or Streptococcus pyogenes (group A beta-hemolytic streptococcus). Whether the effusion is exudative or transudative, intercostal spaces may bulge outward and the mediastinum may be pushed to the contralateral side, if the effusion is massive. There can be “e” to “a” changes on auscultation; breath sounds are diminished in intensity and often are “tubular” or resembling the sound of a drummer’s brush sliding across a drum.
Important conditions in the differential diagnosis of pleuritic chest pain in children include costochondritis, chest pain associated with asthma or gastroesophageal reflux, herpes zoster, and occult trauma.
Air in the Pleural Space
If the air leak is small and the child has little antecedent lung disease, the only symptom of a pneumothorax may be chest pain. The mechanism for this pain is unclear. If the pneumothorax is large or the child has severe underlying disease, pain, cough, dyspnea, and cyanosis may be present. Breath sounds usually are reduced on the side of accumulation of air in the pleural space. If very large, the trachea and cardiac impulse are displaced contralaterally. In a small infant, a large pneumothorax causes subcostal fullness.
Imaging of the Pleura and Pleural Space
The most common and important process involving the pleura in children, a pleural effusion, usually is detected first on the chest radiograph. Skillful use of chest radiography, with occasional help from thoracic ultrasound and computed tomography (CT), allows identification of pleural disease, characterization of an effusion as free or loculated, and distinction between intrapleural processes and peripheral parenchymal disease of the lung.
The usual chest radiographic projections used to evaluate pleural disease are the posteroanterior or anteroposterior, lateral and lateral decubitus (Box 243.1).
Ultrasound has proven to be the imaging modality of choice in pediatrics in localizing pleural fluid and in evaluating peripheral densities abutting the pleura. If a pleural effusion likely is present but decubitus views suggest that it is not free in the pleural space, thoracic ultrasound may identify a loculation that can be aspirated under direct ultrasound guidance. Even when a CT image suggests that a pleural fluid accumulation is homogeneous, the ultrasound often correctly shows that the pleural process is variegated and that large quantities of fluid cannot be removed from one thoracentesis. In these cases, the ultrasound image may show areas of pleural thickening, with fibrinous adhesions separating areas containing relatively thin fluid from others filled with thick pus. Other advantages of ultrasound for use in children are that it is portable and that it does not require separation from parents or a controlled breathing pattern.
BOX 243.1. Important Points to Keep in Mind When Interpreting Radiographs
The periphery of pleural masses and pleural loculations generally makes an obtuse angle with the chest wall. Pleural lesions can be distinguished from peripheral lung lesions because the latter usually give rise to an image that meets the chest wall at an acute angle.
Free pleural fluid causes diffuse hazy opacity on the supine anteroposterior radiograph, whereas on upright films, it causes a meniscus that alters the costophrenic angle.
Pulmonary consolidation usually follows a lobar pattern and thus is distinguished from free pleural fluid, which does not respect lobar boundaries.
If an effusion is probable, both left and right lateral decubitus radiographs may be requested to detect fluid moving freely within the pleural space.
Pleural fluid may be seen as densities in interlobar fissures. If loculated, interlobar fluid may create a rounded density.
Lucencies within a pleural effusion before thoracentesis suggest a bronchopleural fistula or, less often, an anaerobic infection.
If the hemithorax is partially opacified, obtaining lateral decubitus radiographs is preferable first. If the entire hemithorax is opacified, ultrasound is the first imaging choice.
CT scan images offer the promise of more precise portrayal of pleural disease. However, they can be misleading when used to image inflammatory processes like parapneumonic effusions. They show the effusion in worrisome detail that often hastens consideration of unnecessary invasive remedies (see Fibrothorax). CT scans do help in distinguishing between lung abscesses touching the visceral pleura and loculated pus in the pleural space. Lung abscesses rarely require more than antibiotic therapy, and not tube thoracostomy or débridement. Pus in the pleural space, which often requires drainage or débridement, makes an obtuse angle with the chest wall, and an abscess makes an acute angle. CT scans also help detail parenchymal and hilar adenopathy and calcifications accompanying pleural inflammation and may, therefore, help clarify the etiology of the effusion in puzzling cases. High resolution CT scanning should provide more detailed images of how pleural processes affect the parietal pleura and intercostal muscles. Magnetic resonance imaging (MRI), though useful in other aspects of thoracic imaging, particularly of the mediastinum, usually is not required to evaluate pediatric pleural disease because primary and metastatic malignancies of the pleura are such uncommon findings in children. Digitally stored images, created using x-rays because of their image enlargement and clarification capabilities, should prove useful when evaluating pneumothoraces, in particular.