Pulmonary Infections in the Normal Host

Chapter 3 Pulmonary Infections in the Normal Host

Pneumonia ranks sixth among the causes of death in the United States and is the leading cause of death due to infection. The factors responsible for this high mortality rate include an increasing elderly population, immunocompromised hosts in greater numbers, new etiologic agents of pneumonia, antibiotic-resistant organisms, and unusual organisms acquired from international travel. The etiologic agent can reach the lungs by several routes. The most common is inhalation of airborne droplets, followed by aspiration of nasopharyngeal organisms, hematogenous spread to the lungs from other extrathoracic sources of infection, direct extension from a localized site of infection, and infection from penetrating wounds.

Clinical features are important in the determination of the etiologic agent of pneumonia (Table 3-1). Community-acquired pneumonias occurring in previously healthy individuals are caused by Streptococcus pneumoniae in 50% to 75% of cases and by Mycoplasma pneumoniae, viral organisms, or Legionella pneumophila. Nosocomial pneumonias (i.e., acquired in the hospital by patients who are already ill) typically are caused by gram-negative organisms or Staphylococcus aureus. Certain preexisting conditions are associated with pneumonias due to specific organisms. For example, patients with altered states of consciousness or those in coma are more likely to develop aspiration and subsequently develop infections due to mouth organisms (i.e., gram-negative organisms and anaerobes). S. aureus infection can occur after influenza pneumonia; in patients with chronic obstructive pulmonary disease (COPD), Haemophilus influenzae infection is common. S. aureus and Pseudomonas aeruginosa organisms are common superinfectants in patients with cystic fibrosis.

Table 3-1 Clinical Clues to the Cause of Pneumonia

Clinical Circumstance Likely Causative Organisms
Previously well, community-acquired 50% to 75% due to Streptococcus pneumoniae (pneumococcus), Mycoplasma pneumoniae, virus, or Legionella pneumophila
Hospital-acquired, otherwise ill Gram-negative organisms, including Pseudomonas aeruginosa, Klebsiella pneumoniae, Escherichia coli, and Enterobacter species; Staphylococcus aureus; less commonly, S. pneumoniae and Legionella
Alcoholism S. pneumoniae most common; gram-negative organisms, anaerobes, and S. aureus frequent causes
Diabetes mellitus Suspect gram-negative organisms and S. aureus
Altered consciousness, coma Gram-negative organisms and anaerobes
Drug addiction If not an AIDS patient, suspect Staphylococcus and gram-negative organisms
After influenza S. aureus
Chronic bronchitis with exacerbation Haemophilus influenzae (common)
Cystic fibrosis Mucoid, P. aeruginosa

From Woodring JH: Pulmonary bacterial and viral infections. In Freundlich IM, Bragg DG (eds): A radiologic approach to diseases of the chest. Baltimore, Williams & Wilkins, 1992.


The pathologic classification of pneumonia is based on the anatomic localization of the disease process. Categories include lobar pneumonia, bronchopneumonia or lobular pneumonia, hematogenous bacterial infection, and acute interstitial pneumonia.

Lobar Pneumonia

Radiographic Features

This type of pneumonia produces a pattern of confluent opacification, often with air bronchograms (Fig. 3-1). The entire lobe may be involved, but more frequently because of early use of antibiotics, the pneumonia involves only one or more segments within a lobe (i.e., sublobar form). A lobar pneumonia may result in expansion of the lobe due to voluminous edema, which is usually caused by infection with K. pneumoniae (Fig. 3-2). The enlargement of the lobe can be recognized radiographically by bulging of the interlobar fissures. Necrosis, cavitation, and development of a unique complication, pulmonary gangrene, may ensue.

The computed tomography (CT) features of lobar pneumonia are similar to those seen on standard radiography (Fig. 3-3). There is usually evidence of confluent opacification with air bronchograms. The air bronchograms are often more easily visualized with CT examination. Table 3-2 summarizes the radiographic clues to the cause of pneumonia.

Table 3-2 Radiographic Clues to the Cause of Pneumonia

Radiographic Finding Likely Causative Organisms
Round pneumonia Suspect Streptococcus pneumoniae (pneumococcus)
Complete lobar consolidation S. pneumoniae, Klebsiella pneumoniae, and other gram-negative bacilli; Legionella pneumophila and occasionally Mycoplasma pneumoniae
Lobar enlargement K. pneumoniae, pneumococcus, Staphylococcus aureus, Haemophilus influenzae
Bilateral pneumonia (bronchopneumonia) S. pneumoniae still common, but suspect others, including S. aureus, streptococci, gram-negative bacilli, anaerobes, L. pneumophila, virus, and aspiration syndromes
Interstitial pneumonia Virus, M. pneumoniae, and occasionally H. influenzae, S. pneumoniae, and other bacteria
Septic emboli Usually S. aureus; occasionally gram-negative bacilli, anaerobes, and streptococci
Empyema or bronchopleural fistula S. aureus, gram-negative bacilli, anaerobes, and occasionally, pneumococcus; mixed bacterial infections common
Contiguous spread to chest wall Actinomycosis; occasionally other bacteria or fungi
Cavitation S. aureus, gram-negative bacilli, anaerobic bacteria, and streptococci; cavitation uncommon with S. pneumoniae or L. pneumophila
Pulmonary gangrene K. pneumoniae, Escherichia coli, H. influenzae, Mycobacterium tuberculosis, S. pneumoniae, anaerobes, or fungi
Pneumatoceles S. aureus, gram-negative bacilli, H. influenzae, M. tuberculosis, and measles; S. pneumoniae rare
Lymphadenopathy M. tuberculosis, fungi, virus, M. pneumoniae, common bacterial lung abscess, and rarely plague, tularemia, and anthrax
Fulminant course with acute respiratory distress syndrome (ARDS) Virus, S. aureus, streptococci, M. tuberculosis, and L. pneumophila

From Woodring JH: Pulmonary bacterial and viral infections. In Freundlich IM, Bragg DG (eds): A Radiologic Approach to Diseases of the Chest. Baltimore, Williams & Wilkins, 1992.


Radiographic Features

The radiographic appearance of bronchopneumonia pneumonia is most frequently that of multiple, ill-defined nodular opacities that are patchy but that may eventually become confluent and produce consolidation with airspace opacification (Fig. 3-4). The opacification may be multifocal and involve several lobes, or it may be diffuse. As the disease progresses, segmental and lobar opacification develops, similar to the pattern of a lobar pneumonia. Early necrosis and cavitation can occur. The nodular opacities of bronchopneumonia can be identified with facility on CT scans. The small nodules, usually less than 1 cm in diameter, represent peribronchiolar areas of consolidation or ground-glass opacity. They are called acinar or airspace nodules, but these nodules histologically are found in a peribronchiolar location. They are ill-defined and may be of homogenous soft tissue opacity and obscuring vessels, or they may be hazy and less dense so that adjacent vessels are clearly seen (i.e., ground-glass opacity). These nodules usually have a centrilobular location because of their proximity to small bronchioles.

Hematogenous Spread of Infection

Radiographic Features

Septic infarcts tend to be multiple and peripheral and to abut the pleural surface. They occur more frequently in the lower lobes. These nodules or wedge-shaped opacities may show evidence of cavitation (Fig. 3-6). CT often demonstrates a vessel connected to the area of infarction. On CT, the septic infarcts appear as wedge-shaped, peripheral opacities abutting the pleura. They may contain air bronchograms or rounded lucencies of air, sometimes referred to as pseudocavitation. True cavitation is common. Occasionally, septic bacterial infection may result in diffuse massive seeding of the lungs with a miliary pattern (i.e., very small nodular pattern), although this is much more common with hematogenous dissemination of granulomatous infections.


Box 3-1 outlines the complications of pneumonia.


Necrosis of lung parenchyma with cavitation (Fig. 3-7) may occur in pneumonia, particularly that produced by virulent bacteria, including S. aureus, streptococci, gram-negative bacilli, and anaerobic bacteria. If the inflammatory process is localized, a lung abscess will form. It is usually rounded and focal, and it appears to be a mass (Fig. 3-8). With liquefaction of the central inflammatory process, a communication may develop with the bronchus; air enters the abscess, forming a cavity, which often contains an air-fluid level. The walls of the cavity may be smooth, but more often, they are thick and irregular.

Multiple, small cavities or microabscesses may develop in necrotizing pneumonia (Fig. 3-9). They are recognized as multiple areas of lucency within a consolidated lobe or segment. A similar appearance may be produced by consolidation superimposed on areas of preexisting emphysema. If the necrosis is extensive, arteritis and vascular thrombosis may occur in an area of intense inflammation, causing ischemic necrosis and death of a portion of lung. This is a particular complication of Klebsiella pneumonia and other pneumonias producing lobar enlargement. The radiographic features include multiple areas of cavitation, often with air-fluid levels. Portions of dead lung may slough and form intracavitary masses.

Pneumatocele Formation

Pneumatoceles are usually associated with pneumonia caused by virulent organisms; the classic offender is S. aureus (Fig. 3-10). They usually form subpleural collections of air, which result from alveolar rupture. Radiographically, they appear as single or multiple, cystic lesions with thin and smooth walls. They may show rapid change in size and location on serial radiographs.

Pleural Effusions and Empyema

Pleural effusion is a common complication of pneumonia, occurring in about 40% of cases (Fig. 3-11). Most effusions are parapneumonic, but infection of the pleural space with empyema requiring drainage is an important but uncommon complication of some pneumonias. Empyemas can be recognized by the presence of gross pus within the pleural space, by a white blood cell count in the pleural fluid of greater than 15,000 cells/mm3, by the presence of bacteria within the pleural fluid, or by a pH less than 7.2. Chapter 18 provides more detail on the pleural complications of pneumonia.

Parenchymal necrosis in an underlying pneumonia may produce a fistula between the bronchus and the pleural space (i.e., bronchopleural fistula), and this results in an empyema with an air-fluid level. Further discussion of these entities can be found in Chapter 18.


The most common gram-positive bacteria causing pneumonia include S. pneumoniae (pneumococcus), S. aureus, and Streptococcus pyogenes.

Streptococcus pneumoniae

S. pneumoniae (Box 3-2) is responsible for one third to one half of community-acquired pneumonias in adults. These infections occur more frequently in the winter and early spring. Pneumococcal pneumonia occurs in healthy people, but it is much more common in alcoholic, debilitated, and other immunocompromised individuals.

The radiographic features include consolidation that is usually unilateral, although it may be bilateral, and it typically affects the lower lobes (see Fig. 3-1). Although it is a lobar pneumonia, it is uncommon for the lobe to be completely consolidated. Cavitation is rare, and large pleural effusions are uncommon. When present, they suggest the development of empyema. Sometimes, especially in children, the pneumonia may have a rounded, masslike appearance (Fig. 3-12). This is called a round pneumonia; it results from centrifugal spread of the rapidly replicating bacteria by way of the pores of Kohn and canals of Lambert from a single primary focus in the lung.

Staphylococcus aureus

S. aureus (Box 3-3) is a gram-positive coccus, and the spherical organisms occur in pairs and clusters. This pneumonia rarely develops in healthy adults, but it is sometimes a complication of viral infections and is much more common in infants and children. In infants, unilateral or bilateral consolidation involving the lower lungs is the most frequent radiographic presentation. Pneumatoceles, thin-walled cysts filled with air or partially filled with fluid, may develop and occasionally rupture into the pleural space, resulting in pneumothorax. In adults, the disease is usually bilateral and is preceded by an atypical pneumonia such as influenza. Cavitation is a common feature, and the cavities may be multiple, thick walled, and irregular (Fig. 3-13). There is a high incidence of large pleural effusions, and empyema resulting from bronchopleural fistula is a common complication. Methicillin resistant staphylococcus aureus (MRSA) pneumonia usually occurs as a nosocomial infection in health care centers particularly in older, immunocompromised or intensive care unit patients.

Staphylococcal infection in the lungs may occur by way of the hematogenous route. This is usually the result of septic emboli, which arise in the central veins or as vegetations on cardiac valves, particularly in intravenous drug abusers and patients with indwelling intravenous catheters. The radiographic appearance is that of multiple nodular masses with or without cavitation, as previously described.


Pneumonias caused by gram-negative organisms usually are nosocomial pneumonias that affect hospitalized patients. These pneumonias tend to occur in patients maintained on artificial ventilators or in those who have intravenous catheters or a variety of other ancillary support systems. The incidence of gram-negative pneumonia acquired in the community is increasing, which may be related to the overgrowth of resistant organisms because of widespread use of broad-spectrum antibiotics.

Klebsiella pneumoniae

Klebsiella pneumonia (Box 3-5) usually occurs in middle-aged or elderly patients, in those with underlying chronic lung disease, and in alcoholic individuals. Radiographic features consist of an upper lobe consolidation. Cavitation is common, and the lobar consolidation may lead to an expanded lobe with bulging interlobar fissures (see Fig. 3-2). If necrosis is extensive, pulmonary gangrene may develop.

Escherichia coli

E. coli pneumonia (Box 3-6) may be caused by direct extension from the gastrointestinal or genitourinary tract across the diaphragm or result from bacteremia. As is true of most of the gram-negative pneumonias, it is frequently characterized by the development of necrosis and multiple cavities. The lower lobes are more frequently involved.

Pseudomonas aeruginosa

P. aeruginosa pneumonia (Box 3-7) usually occurs in hospitalized patients, particularly those with debilitating disease (see Fig. 3-9). Organisms that affect the lungs often result from contamination of suction and tracheostomy devices. Radiographic features include a lower lobe predilection. However, the consolidation may spread rapidly to affect both lungs. Pleural effusions are uncommon. Multiple, irregular nodules may develop and are usually associated with bacteremia. These nodules may cavitate.

Haemophilus influenzae

H. influenzae pneumonia (Box 3-8) usually develops in patients with COPD. The appearance is typically that of a bronchopneumonia with homogeneous segmental opacities, usually in the lower lobes. Cavitation and pleural effusions are rare.


Pulmonary aspiration (Box 3-9) is a common clinical problem. Many conditions predispose persons to aspiration, including reduced levels of consciousness, alcoholism, drug addiction, esophageal disease, periodontal and gingival disease, seizure disorders, and nasogastric tubes.

Aspiration of particulate matter or foreign bodies may produce different clinical syndromes, depending on the size of the aspirated material and the level of airway obstruction. Large food particles or foreign bodies may be aspirated into the larynx and upper trachea, resulting in the so-called café coronary syndrome, which is caused by acute upper airway obstruction. These patients exhibit respiratory distress and aphonia.

Results of chest radiographs are usually normal for patients who have aspirated foreign bodies. If the foreign body is opaque, it may be visible in the airways. Air trapping may occur if the foreign body causes airway obstruction of one of the major bronchi. This can be demonstrated by inspiratory and expiratory radiographs, decubitus views, or chest fluoroscopy. Occasionally, complete obstruction of the bronchus results in atelectasis and, if the foreign body is unrecognized, in the development of distal pneumonitis or bronchiectasis.

Ninety percent of aspiration pneumonias and lung abscesses are caused by anaerobic organisms. The pathogens include Prevotella, Bacteroides, Fusobacterium, and Peptostreptococcus. Because of the presence of oxygen in the lung, the progression of anaerobic infection is slow, beginning in the dependent lung zones. If the patient is in a supine position when the aspiration occurs, the superior segments of the lower lobes are most commonly affected, with the right side affected more frequently than the left (Fig. 3-14). Aspiration can also affect the posterior segments of both upper lobes. Chronic or recurrent aspiration, particularly in patients who are in the upright position, usually results in consolidation involving the basilar segments of the lower lobes. The middle lobe and lingula are uncommon sites for aspiration pneumonia. Aspiration is the most common cause of a primary lung abscess (see Fig. 3-8).

A primary lung abscess refers to a focal, walled-off area of anaerobic pneumonia with central liquefaction necrosis. It is most commonly identified in the superior segments of either lower lobe. Lung abscesses have a fairly thick wall and may or may not have an air-fluid level. A rounded, masslike lesion may precede the development of cavitation.

Occasionally, aspiration of nontoxic material that contains insufficient bacteria to produce an infection or insufficient volume to produce atelectasis may occur. The radiographic appearance usually consists of basilar patchy opacities resembling atelectasis, and these areas clear within several days. Mendelson’s syndrome is a specific form of aspiration that results from the aspiration of gastric acid. This event produces a chemical pneumonitis and acute lung injury. The radiographic manifestations of gastric aspiration are similar to those of noncardiogenic pulmonary edema. The distribution is usually diffuse.


Atypical pneumonia syndrome (Box 3-10) describes pneumonias that do not respond to usual empiric antimicrobial therapy or do not have clinical features distinctive from the usual bacterial pathogens responsible for community-acquired pneumonias. Originally, these atypical pneumonias were thought to be caused by viruses. However, other treatable organisms have emerged as important causes of atypical pneumonia, including M. pneumoniae, L. pneumophila, and Chlamydia. These nonviral, atypical pneumonias are for the most part readily treatable with antibiotics.

Most patients with atypical pneumonia present with a nonspecific syndrome consisting of fever, usually without shaking chills, and nonproductive cough, headache, myalgias, and some degree of dyspnea. This contrasts with the classic presentation of bacterial pneumonia, which is characterized by abrupt onset with fever, shaking chills, and purulent sputum, often with chest pain. Patients with the latter signs and symptoms usually have a bacterial pneumonia attributable to pneumococci, group A streptococci, Klebsiella, S. aureus, or H. influenzae. Many of the atypical pneumonias are associated with extrapulmonary manifestations. For example, diarrhea is a prominent part of Legionella and Mycoplasma infection.

Mycoplasma pneumoniae

M. pneumoniae (Box 3-11) accounts for approximately 20% of all cases of pneumonia. It usually occurs during the winter months in enclosed populations, such as students in college dormitories. The incubation period is 2 to 3 weeks, and the onset is often insidious, with low-grade fever and nonproductive cough. Extrapulmonary manifestations may include otitis, nonexudative pharyngitis, and diarrhea.

The radiographic features are usually those of a fairly diffuse, interstitial, fine reticulonodular pattern. This may evolve to patchy airspace consolidation, particularly in the lower lobes (Fig. 3-15). Hilar adenopathy is seen in approximately 20% to 40% of patients. The radiographic appearance is very similar to that of many viral infections. The diagnosis is made by serologic evaluation.

Feb 28, 2016 | Posted by in RESPIRATORY IMAGING | Comments Off on Pulmonary Infections in the Normal Host

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