Recognizing Pneumonia

General Considerations

• Pneumonia can be defined as consolidation of the lung produced by inflammatory exudate, usually as a result of an infectious agent.

• Most pneumonias produce airspace disease, either lobar or segmental. Other pneumonias demonstrate interstitial disease and some produce findings in both the airspaces and the interstitium.

Case Quiz 8 Question

This is a chest x-ray immediately following a diagnostic test the patient was undergoing. While not a pneumonia, this case illustrates prior knowledge of the different radiographic densities and a mechanism for developing a pneumonia described in this chapter. What might explain this unusual appearance? The answer is at the end of the chapter.

• Most microorganisms that produce pneumonia are spread to the lungs via the tracheobronchial tree, either through inhalation or aspiration of the organisms. In some instances, microorganisms are spread via the bloodstream and in even fewer cases, by direct extension.

• Because many different microorganisms can produce similar imaging findings in the lungs, it is difficult to identify with certainty the causative organism from the radiographic presentation alone. However, certain patterns of disease are very suggestive of a particular causative organism ( Table 8.1 ).

  TABLE 8.1

  Patterns that Might Suggest a Causative Organism

  Pattern of Disease	Likely Causative Organism
  Upper lobe cavitary pneumonia with spread to the opposite lower lobe	Mycobacterium tuberculosis (TB)
  Upper lobe lobar pneumonia with bulging interlobar fissure	Klebsiella pneumoniae
  Lower lobe cavitary pneumonia	Pseudomonas aeruginosa or anaerobic organisms (Bacteroides)
  Perihilar interstitial disease or perihilar airspace disease	Pneumocystis carinii (jirovecii)
  Thin-walled upper lobe cavity	Coccidioides (coccidiomycosis), TB
  Airspace disease with effusion	Streptococci, staphylococci, TB
  Diffuse nodules	Histoplasma, Coccidioides, Mycobacterium tuberculosis (histoplasmosis, coccidiomycosis, TB)
  Soft-tissue, finger-like shadows in upper lobes	Aspergillus (allergic bronchopulmonary aspergillosis)
  Solitary pulmonary nodule	Cryptococcus (cryptococcosis)
  Spherical soft-tissue mass in a thin-walled upper lobe cavity	Aspergillus (aspergilloma)
  Bilateral, peripheral, lower lobe ground-glass opacities	SARS-CoV-2 (COVID-19)

• Some use the term “infiltrate” synonymously with pneumonia, but many diseases, from amyloid to pulmonary fibrosis, can infiltrate the lung.

General Characteristics of Pneumonia

• Because pneumonia fills the involved airspaces or interstitial tissues with some form of fluid or inflammatory exudate, pneumonias appear denser (whiter) than the surrounding, normally aerated lung.

• Pneumonia may contain air bronchograms if the bronchi themselves are not filled with inflammatory exudate or fluid (see Fig. 5.5 ).

  • Air bronchograms are much more likely to be visible when the pneumonia involves the central portion of the lung around the hilum. Near the periphery of the lung, the bronchi are usually too small to be visible ( Fig. 8.1 ).

    

  • Remember that anything of fluid or soft-tissue density that replaces the normal gas in the airspaces may also produce this sign so an air bronchogram is not specific for pneumonia (see Chapter 4 ).

• Pneumonia that involves the airspaces appears fluffy and its margins are indistinct.

  • Where pneumonia abuts a pleural surface , such as an interlobar fissure or the chest wall, it will be sharply marginated.

• Interstitial pneumonia, on the other hand, may produce prominence of the connective tissues, vessels, and bronchi that make up the interstitium of the lung but also may spread to adjacent airways and resemble airspace disease.

• Except for the presence of air bronchograms, airspace pneumonia is usually homogeneous in density ( Fig. 8.2 ).

  

• In some types of pneumonia (i.e., bronchopneumonia), the bronchi as well as the airspaces contain inflammatory exudate. This can lead to atelectasis associated with the pneumonia.

• Box 8.1 summarizes the keys to recognizing pneumonia.

  BOX 8.1

  Recognizing a Pneumonia: Key Signs

  • More opaque than surrounding normal lung.

  • With airspace disease, the margins may be fluffy and indistinct except when they abut a pleural surface, like the interlobar fissures, where the margin will be sharp.

  • Pneumonia tends to be homogeneous in density.

  • They may contain air bronchograms.

  • Interstitial pneumonias may produce reticular prominence in the lungs.

  • Pneumonias may be associated with atelectasis in the affected portion of the lung.

Patterns of Pneumonia

• Pneumonias may be distributed in the lung in several patterns described as lobar, segmental, interstitial, round, and cavitary ( Table 8.2 ).

  TABLE 8.2

  Patterns of Appearance of Pneumonias

  Pattern	Characteristics
  Lobar	Homogeneous consolidation of affected lobe frequently with air bronchograms
  Segmental (bronchopneumonia)	Patchy airspace disease frequently involving several segments simultaneously; no air bronchogram; atelectasis may be associated
  Interstitial	Reticular interstitial disease usually diffusely spread throughout the lungs early in the disease process; may progress to airspace disease
  Round	Spherically shaped pneumonia usually seen in the lower lobes of children; may resemble a mass
  Cavitary	Produced by numerous microorganisms, chief among them being Mycobacterium tuberculosis

• Remember, these are terms that simply describe the distribution of the disease in the lungs; they aren’t diagnostic of pneumonia because many other diseases can produce similar patterns in the lung.