Chapter 3 Pulmonary Infections in the Normal Host
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.
CLASSIFICATION
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.
COMPLICATIONS OF PNEUMONIA
Box 3-1 outlines the complications of pneumonia.
Cavitation
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.
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.
PNEUMONIAS CAUSED BY GRAM-POSITIVE BACTERIA
Streptococcus pneumoniae
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.
PNEUMONIAS CAUSED BY GRAM-NEGATIVE AEROBIC ORGANISMS
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.
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.
ASPIRATION PNEUMONITIS AND ANAEROBIC PNEUMONIA
Box 3-9 Aspiration Pneumonitis and Anaerobic Pneumonia
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).