Pleural effusion, which in pediatric patients most commonly results from an infection, is an abnormal collection of fluid in the pleural space. Pleural effusion develops because of excessive filtration or defective absorption of accumulated fluid. Pleural effusion may be a primary manifestation or a secondary complication of many disorders (see the images below). (See Etiology.)
Complications are uncommon in properly treated parapneumonic effusions. Possible complications include respiratory failure caused by massive fluid accumulation, septicemia, bronchopleural fistula, pneumothorax, and pleural thickening. (See Prognosis, Treatment, and Medication.)
The inner surface of the chest wall and the surface of the lungs are covered by the parietal and visceral pleura, respectively. The small amount of fluid (< 1 mL) between the parietal and visceral pleura in humans forms a thin layer about 10 μL thick.  The protein concentration in the pleura is similar to that of the interstitial fluid. Compared with the interstitial fluid in humans, the pleural fluid has a higher level of bicarbonate, a lower level of sodium and large ̶ molecular-weight proteins (eg, lactate dehydrogenase [LDH]), and a similar level of glucose. 
The cells in pleural fluid in healthy humans are small in number and are mostly macrophages with few lymphocytes and RBCs.  In disease state, these parameters change and large amounts of fluid can accumulate in the pleural space.
The amount of fluid in the pleural space is regulated through a delicate balance between the oncotic and hydrostatic pressures of the pleural space and the capillary intravascular compartments and pleurolymphatic drainage.  Chest wall and diaphragmatic movements enhance absorption of excess pleural fluid, large particles, and cells through preformed stomas. 
The etiologic mechanisms involved in the formation of most pleural effusions include pleural space infection (empyema), abnormal capillary permeability (exudates), increased hydrostatic or decreased oncotic pressure in the setting of normal capillaries (transudates), abnormal lymphatic clearance (exudates), and blood in the pleural space (hemothorax).
In children, infection is the most common cause of pleural effusion. Congestive heart failure constitutes the second most common etiology, followed by malignancy. [3, 4] In a Canadian study of 127 children with pleural effusion, about 50% of effusions were parapneumonic, 17% were caused by congestive heart failure, 10% were caused by malignancy, 9% were caused by renal disease, 7% were caused by trauma, and 6% were associated with other causes. 
In another North American report of 210 children admitted with pleural effusion, Hardie et al showed that 68% of the effusions were parapneumonic (50 of 143 associated with empyema), 11% were caused by congenital heart disease, 5% were caused by malignancy, and 3% were associated with other causes. 
Pleural effusions caused by nonbacterial infectious agents are more common than those caused by bacterial organisms.  Viral effusions are usually asymptomatic and resolve without therapy.
Parapneumonic effusion and empyema are serious complications of bacterial pneumonia. Over the years, the etiologic agents have become more diverse, and their sensitivities to different antibiotics have changed.  In industrialized countries, Streptococcus pneumoniae remains the most common pathogen that causes parapneumonic effusions and empyema in children. [7, 8, 9] .
S pneumoniae reemerged as a more virulent, penicillin-resistant and cephalosporin-resistant organism in the 1980s and 1990s. Penicillin resistance was reported in 26-76% of S pneumoniae isolates from pleural fluid. [10, 11]
Although a marked decrease has been noted in the incidence of invasive pneumococcal disease across all age groups since the introduction of the heptavalent pneumococcal conjugate vaccine (PCV7) in 2000, [16, 17, 18] the empyema hospitalization rate has increased.  Of note, unlike the heptavalent pneumococcal conjugate vaccine, which does not include serotype 1, the current 13-valent pneumococcal conjugate vaccine that replaced the heptavalent pneumococcal conjugate vaccine in 2010 for routine vaccination of children, does include serotype 1.
In developing countries, Staphylococcus aureus is probably the most common infectious agent that causes empyema in children.  However, community-acquired methicillin-resistant S aureus that produces toxins (eg, Panton-Valentine leukocidin) is becoming an increasingly common pathogen in some centers in the United States. [20, 21, 22]
Haemophilus influenzae type B was the predominant etiologic organism for empyema in children in the 1980s.  Current published data show the near disappearance of H influenzae as a major pathogen in children. [4, 24] The decrease in complicated parapneumonic effusion caused by H influenzae is attributed to widespread immunization of infants. 
A wide range of less-common organisms are now recognized as causes of empyema in children. These include coagulase-negative staphylococcus, other streptococcal species (viridans streptococcus, Group A streptococcus, alpha-hemolytic streptococcus), Actinomyces species, and fungi.
Group A beta-hemolytic S pneumoniae with pleural effusion and streptococcal toxic shock syndrome has been described in association with varicella infections in children. 
Anaerobes, including Bacteroides and Fusobacterium species, have been found, particularly in empyema associated with aspiration pneumonia in neurologically impaired children.  Anaerobes have also been found in empyema associated with intraoral and subdiaphragmatic abscesses. 
Pneumocystis jiroveci (previously, P carinii) infection in children with acquired immunodeficiency syndrome (AIDS) can be associated with pleural effusion, with an incidence of about 5%. 
Pleural effusion occurs in 2-38% of all cases of pulmonary tuberculosis in children.  Tuberculous pleural effusions can be either primary or the result of reactivation disease. Primary tuberculous pleural effusion results from direct hematogenous invasion of the pleural space by Mycobacterium tuberculosis; it is usually unilateral and is often found in the absence of pulmonary parenchymal disease. Tuberculous pleural effusion due to reactivation disease is typically associated with focal parenchymal disease.  Tuberculous pleural effusion commonly occurs in adolescents and is uncommon in the preschool-aged child. 
Congestive heart disease
Congestive heart disease is a less-common cause of pleural effusion in children than it is in adults. It occurs primarily as a result of elevated left atrial or pulmonary capillary wedge pressure.  Effusions are usually bilateral and transudative.
Lymphoma is the most common of all childhood malignancies that is associated with pleural effusion.  Other childhood malignancies, such as leukemia, neuroblastoma, chest wall sarcoma, Wilms tumor, and hepatoma, rarely cause pleural effusions. 
In malignancies, effusion can result from direct pleural invasion by the tumor, obstruction of the lymphatic pathway, or pneumonia or atelectasis that results from bronchial obstruction either by the tumor or by accompanying lymphadenopathy. The pleural effusion is usually unilateral and bloody or chylous in nature.
Chylous effusion is a rare cause of pleural effusion in children, although it is the most common cause of pleural effusion in the first week of life.  Chylothorax may be congenital or acquired. It arises from the leakage of chyle into the pleural space as a result of damage to the thoracic duct by rupture, laceration, tear, or compression.  (See the images below.)
A higher incidence of chylothorax is seen in children with Down syndrome, Noonan syndrome, extralobar sequestration, diaphragmatic hernia, hydrops fetalis, and/or pulmonary hypoplasia.
Hemothorax should be suspected if pleural fluid hematocrit is more than 50% of peripheral blood. It mostly occurs as a result of trauma. Other causes of hemothorax include malignancy, pulmonary infarction, rupture of pulmonary sequestration or arteriovenous malformation, spontaneous intrathoracic vessel rupture, and postpericardiotomy syndrome.
Other causes of transudative effusions include hypoalbuminemia, nephrosis, hepatic cirrhosis, and iatrogenic causes (eg, a misplaced central line or a complication of ventriculopleural shunt).
Other, rare causes of pleural effusion include pancreatitis (effusions are usually hemorrhagic, unilateral and left sided), rupture of a pulmonary hydatid cyst into the pleural space, and Lemierre syndrome (postpharyngitis anaerobic sepsis with thrombophlebitis of the internal jugular vein).
Parapneumonic effusions and empyema are more common in boys than girls.  In addition, parapneumonic effusions and empyema are more commonly encountered in infants and young children than in older children. In a Spanish study, children younger than 5 years had a higher incidence of empyema than did children aged 5-17 years. 
Occurrence in the United States
Pleural effusion in children is usually a manifestation of an underlying disorder, and its prevalence mirrors that of the underlying disease. Empyema was reported in about 0.6-2% of children with bacterial pneumonia. [34, 35] The prevalence of pleural infections appears to be increasing in some industrialized countries. In the United States, the empyema-associated hospitalization rate increased 70% between 1997 (2.2 per 100,000) and 2006 (3.7 per 100,000 children). 
Byington et al reported that a significant increase in the incidence of empyema in children, from 1 case per 100,000 children to 14 cases per 100,000 children, occurred in Utah between 1993 and 2003. [37, 13]
Information on the incidence of pleural effusion in children is limited. As in the United States, infectious agents are the most common cause of pediatric pleural effusion internationally. The distribution of pleural effusion depends on the population studied.
In Spain, the incidence of parapneumonic effusion in children younger than age 5 years increased from 1.7 per 100,000 (in 1999) to 8.5 per 100,000 (in 2004).  In France, the incidence of empyema increased from 0.5 per 100,000 (in 1995) to 13 per 100,000 (in 2003). 
Transudative, chylous, and hemorrhagic pleural effusions respond to treatment of the underlying condition, and their prognosis is identical to that of the underlying cause. Viral and mycoplasmal effusions usually resolve spontaneously, while most patients recover well from parapneumonic effusion or empyema if appropriately treated.
Empyema has a complicated course if not treated and drained early, especially in children younger than 2 years. In a systemic review, Avansino et al reported a higher mortality rate for children treated with antibiotics and chest tubes (3.3%) compared with those treated with fibrinolytic therapy, video-assisted thoracoscopic surgery (VATS), or thoracotomy (0%).  The mortality rate is higher in children younger than age 2 years. 
Most tuberculosis effusions completely resolve with the use of proper antituberculous agents. Residual pleural thickening can occur in 50% of patients. 
A malignant cause worsens the prognosis for patients with pleural effusion, depending on the underlying tumor.
Empyema causes significant acute morbidity. However, death from empyema in previously healthy children in the industrialized world is uncommon. The mortality rate is higher for children younger than age 2 years.  In a series of 74 children with pneumococcal empyema, 5% died, 5.5% had hemolytic uremic syndrome, 38% had bacteremia, and 51% were admitted to intensive care. 
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