Pneumatocele 

Pulmonary pneumatoceles can be single emphysematous lesions but are more often multiple, thin-walled, air-filled, cystlike cavities. Most often, they occur as a sequela to acute pneumonia, commonly caused by Staphylococcus aureus. However, pneumatocele formation also occurs with other agents, including Streptococcus pneumoniae, Haemophilus influenzae, Escherichia coli, group A streptococci, Serratia marcescens, Klebsiella pneumoniae, adenovirus, and tuberculosis. Pneumatoceles are generally observed soon after the development of pneumonia but can be observed on the initial chest radiograph.

Noninfectious etiologies include hydrocarbon ingestion, trauma, and positive pressure ventilation.

In premature infants with respiratory distress syndrome, pneumatoceles result mostly from ventilator-induced lung injury.[1, 2, 3]

In most circumstances, pneumatoceles are asymptomatic and do not require surgical intervention.[4] Treatment of the underlying pneumonia with antibiotics is the first-line therapy. Close observation in the early stages of the infection and periodic follow-up care until resolution of the pneumatocele is usually adequate treatment. The natural course of a pneumatocele is slow resolution with no further clinical sequelae. Invasive approaches should only be reserved for patients who develop complications.

Since the 1950s, multiple theories have been proposed as to the exact mechanism of pneumatocele formation; however, the exact mechanism remains controversial.

Carrey suggested that the initial event is inflammation and narrowing of the bronchus, leading to the formation of an endobronchial ball valve.[5] Ultimately, this bronchial obstruction leads to distal dilatation of the bronchi and alveoli. In 1951, Conway proposed that a peribronchial abscess forms and subsequently ruptures its contents into the bronchial lumen.[6] This also acts similarly to a ball-valve obstruction in the bronchus and leads to distal dilatation. In 1972, Boisset concluded that pneumatoceles are caused by bronchial inflammation that ruptures the bronchiolar walls and causes the formation of "air corridors."[7] Air dissects down these corridors to the pleura and forms pneumatoceles, a form of subpleural emphysema.

Traumatic pneumatocele has a different pathophysiology from the infectious type,[8] developing in a 2-step process. Initially, the lung is compressed by the external force of the trauma, followed by rapid decompression from increased negative intrathoracic pressure. A "bursting lesion" of the lung occurs and leads to pneumatocele formation.

Although no particular genetic predisposition is recognized, pneumatocele formation is associated with hyperimmunoglobulin E (IgE) syndrome (Buckley-Job syndrome).[9, 10]  Because of immunodeficiency, individuals with this syndrome are predisposed to infection with staphylococcal pneumonia, with the known complications of abscess and pneumatocele formation.

Infectious etiologies associated with pneumatocele formation include the following:

• S aureus

• S pneumoniae

• H influenzae

• K pneumoniae

• S marcescens

• E coli

• Group A streptococci

• Mycobacterium tuberculosis

• Pseudomonas aeruginosa

• Adenovirus

Noninfectious etiologies include the following:

• Trauma

• Hydrocarbon ingestion

• Positive pressure ventilation (especially among premature infants)[11]

International statistics

Incidence of postinfectious pneumatocele formation ranges from 2-8% of all cases of pneumonia in children.[12] However, the frequency can be as high as 85% in staphylococcal pneumonias.

Race-, sex-, and age-related demographics

No specific racial predilection is observed for pneumatocele formation. Because pneumatoceles are usually a complication of pneumonia, the predilection is based on susceptibility for infection.

No sex predilection is known.

Infants younger than 1 year account for three fourths of the cases of staphylococcal pneumonia. Because pneumatoceles commonly develop as a complication of staphylococcal pneumonia, pneumatoceles are found more frequently in infants and young children. One study reported that 70% of pneumatoceles occurred in children younger than 3 years.[13]

In general, an uncomplicated pneumatocele carries an excellent prognosis. Complete resolution of the pneumatocele is the most common outcome.

Rare complications, including tension pneumatocele, can lead to death from respiratory or cardiovascular collapse from progressive enlargement of the pneumatocele. However, this is rare and, if detected promptly, can be properly treated.

Morbidity/mortality

Although mortality from the initial pneumonia can be significant, mortality associated with pneumatoceles is quite low. Complete resolution without long-term sequelae is typical; however, rare complications can occur, including the following:

• Tension pneumatocele

• Pneumothorax

• Secondarily infected pneumatocele

Complications

A tension pneumatocele can develop if airtrapping continues and the pneumatocele expands. This complication occurs most frequently with positive pressure ventilation. If severe, the lesion can cause compression of adjacent structures, with hemodynamic instability and severe airway obstruction. If unrecognized and untreated, this can result in respiratory failure and death.

Pneumothorax can occur from a pneumatocele rupturing into the pleural space. This can lead to collapse of the lung, requiring evacuation of the pleural air to reexpand the lung. A bronchopleural fistula can result as a complication of the pneumothorax.

A pneumatocele can become secondarily infected, usually by a different bacterium from the one that caused the primary pneumonia. Some advocate percutaneous drainage of infected pneumatoceles, especially if fluid- or pus-filled to prevent the development of severe lung abscess that may require surgical excision. Drainage can be both diagnostic and therapeutic. If drained, the fluid should be cultured for bacteria and fungus.

Children present with typical features of pneumonia, including cough, fever, and respiratory distress. No clinical findings differentiate pneumonia with or without pneumatocele formation.

Mild, moderate, or severe respiratory distress may be present, with tachypnea, retractions, grunting, and nasal flaring. Fever is almost always present and may be as high as 40-41°C.

Lung examination findings vary depending on the stage of the pneumonia. Auscultation of the chest reveals focal or bilateral decreased breath sounds. Inspiratory crackles are frequently heard. As the pneumonia resolves and the pneumatocele persists, the lung examination findings can be normal or focal decreases in breath sounds can be present, depending on the size of the pneumatocele.

In most children admitted to the hospital, the average time from admission to the development of the pneumatocele is 4-7 days. Occasionally, pneumatoceles are present on the initial radiograph.

If findings are positive, blood culture helps to guide antibiotic therapy in patients with pneumatocele. If sputum is available, this is a good noninvasive method to discover potential pathogens. If effusion is present, culturing pleural fluid from thoracentesis can be a direct method to identify the causative organism. Tests for bacterial antigen detection can be performed on blood, urine, and pleural fluid.

Initial chest radiography often reveals pneumonia without evidence of a pneumatocele. Parapneumonic effusion or empyema can be present. Radiographic evidence of a pneumatocele most often occurs on day 5-7 of hospitalization. Rarely, it may be visible on the initial chest radiograph. Chest radiograph findings are shown in the images below.

Usually, chest CT scanning with contrast is not necessary to diagnose a pneumatocele, but CT scanning occasionally helps to differentiate an abscess from a pneumatocele (see the image below).

Rarely, CT-guided needle aspiration of the pneumatocele can relieve compression from a large and/or tension pneumatocele.

Percutaneous catheter drainage should only be considered for a significant tension pneumatocele or a secondarily infected pneumatocele. In these rare situations, drainage has been reported to dramatically improve the patient's cardiovascular status.[14, 15]

Pathology is not commonly observed because most pneumatoceles resolve without surgical resection. However, a few reports documented necrotic material around the pneumatocele. Cavity walls can contain organized inflammatory cells with focal collections of multinucleated giant cells. In 1972, Boisset reported the presence of air corridors between the bronchiolar lumen and the interstitial space.[7]

Medical care for pneumatocele is treatment of the underlying condition. In most circumstances, this involves administration of broad-spectrum antibiotics to treat the pneumonia. Therapy should be directed against the most common bacterial organisms in children, including S aureus and S pneumoniae.

Positive pressure ventilation can result in a sudden increase in size and tension of a pneumatocele. Therefore, careful monitoring is essential in patients receiving positive pressure ventilation when pneumatoceles have been documented.

Pneumatoceles almost never require surgical resection. As mentioned above, percutaneous catheter drainage of a pneumatocele that involves more than 50% of hemithorax with severe atelectasis, tension pneumatocele, bronchopleural fistula, or an infected pneumatocele is rarely required. Video-assisted thoracoscopy has been used successfully to treat enlarging multicystic pneumatoceles.[16]  Muniraman et al reported that bedside ultrasound-guided chest tube drainage was used to decompress a large pneumatocele in an unstable preterm infant.[17]

Traumatic pneumatoceles commonly resolve with observation without additional therapy. Indications for surgical intervention with a traumatic pneumatocele are similar to those of a postinfectious pneumatocele (ie, development of tension pneumatoceles, a secondary infection of the pneumatocele, and cardiovascular compromise).

Consider consulting a surgeon in the presence of an infected pneumatocele or a tension pneumatocele and/or in the presence of a persistent bronchopleural fistula.

Diet

No special dietary requirements are indicated.

Activity

Caution patients with pneumatocele against skydiving or exposure to very high altitudes because of an increased risk of pneumothorax. Scuba diving must be avoided until the pneumatocele completely heals.

Class Summary

Intravenous antibiotics should be directed against the most likely bacterial pathogens, including S aureus and S pneumoniae. Other considerations should include antibiotic coverage for K pneumoniae,E coli, and group A streptococci. Most often, an appropriate single agent can be used, but combined antibiotic therapy can be considered, especially if a specific organism is not identified. As community-associated methicillin-resistant S aureus (MRSA) has increasingly been identified, reconsideration of empiric choice of antistaph beta-lactam for seriously ill patients with suspected MRSA should be carefully considered.[18] The following antimicrobials may be used to target an underlying pneumonia.

Oxacillin (Bactocill)

A very effective antibiotic for treating S aureus as well as S pneumoniae.

Ampicillin and sulbactam (Unasyn)

Drug combination of beta-lactamase inhibitor with ampicillin. A very effective antibiotic for treating S aureus as well as S pneumoniae. Also effective for many anaerobic infections.

Cefuroxime (Zinacef)

Very effective antibiotic for treating S aureus and S pneumoniae.

Vancomycin (Lyphocin, Vancocin, Vancoled)

Very effective antibiotic for treating methicillin-resistant S aureus as well as for treating penicillin-resistant S pneumoniae.

To avoid toxicity, current recommendation is to assay vancomycin trough levels before fourth dose. Use CrCl to adjust dose in patients with renal impairment.

Clindamycin (Cleocin)

Very effective antibiotic for treating S aureus as well as S pneumoniae. Lincosamide for treatment of serious skin and soft tissue staphylococcal infections. Also effective against aerobic and anaerobic streptococci (except enterococci). Inhibits bacterial growth, possibly by blocking dissociation of peptidyl tRNA from ribosomes, causing RNA-dependent protein synthesis to arrest.

Ciprofloxacin (Cipro)

Very effective antibiotic for treating S aureus as well as S pneumoniae. A fluoroquinolone with activity against Pseudomonas, streptococci, MRSA, S epidermidis, and most gram-negative organisms but no activity against anaerobes. Inhibits bacterial DNA synthesis and consequently growth.

Most pneumatoceles resolve completely in a few weeks to months. However, in some healthy children, pneumatoceles persist as long as 16 months. Therefore, intermittent outpatient monitoring of chest radiographs is appropriate until resolution. Some recommend chest CT imaging after the findings on plain radiography are clear to ensure complete resolution. However, no clearly recognized radiological or clinical signs help to predict progression of the pneumatocele.

Findings on pulmonary function studies frequently are abnormal initially because of a restrictive defect and, at times, an obstructive defect. Over time, these abnormalities improve and, most often, return to normal predicted ranges. These should not be routinely performed during the acute stages. The increased pressures in spirometry may increase the risk of rupture.

No preventative therapy is available.

After starting appropriate intravenous antibiotic therapy, perform chest radiography to monitor improvement of pneumonia and progression of the pneumatocele.

If significant pleural effusion is present or develops, consider thoracentesis and/or chest tube drainage.

Consider transfer to an intensive care unit when a large tension pneumatocele is causing cardiovascular compromise or significant airway obstruction. Similarly, consider transfer to an intensive care unit if a rupture of the pneumatocele causes a pneumothorax.