Postprimary (Reactivation/Reinfection) Tuberculosis

Usually postprimary MTb occurs as a result of previously latent infection, but genotyping of MTb now suggests that reinfection appears to be more common than previously recognized, especially in areas of the world with high MTb incidence [1]. During the initial infection, organisms may be transported by the blood stream to the apical and posterior segments of the upper lobes and to the superior segments of the lower lobes. Later, reactivation of infection in these regions, which is favored by the relatively high oxygen tension in these lung segments, tends to occur when host defenses become impaired. The latent organisms then become active, inflammation with necrosis occurs, and overt infection develops. Unlike the healing that commonly occurs with primary MTb infection, postprimary MTb infection is often associated with progressive disease. As the inflammation mounts, tissue destruction occurs and caseous material liquefies and may acquire communication with the tracheobronchial tree, producing the characteristic pathologic and radiologic finding of postprimary MTb: cavitation. The presence of cavitation tends to promote worsening infection by allowing more oxygen to reach the inflammatory focus, and also creates the opportunity for endobronchial spread of infection and communication of infection to other persons.

If host defenses triumph, cavities in postprimary MTb heal with scar formation. Bronchiectasis, volume loss, and areas of emphysema are common sequelae. Chronic cavities, often very thin-walled, may persist. Typical clinical manifestations of postprimary MTb include failure to thrive, fatigue, night sweats, weight loss, and low-grade fever. Hemoptysis may occur, and is commonly due to bronchiectasis, although the presence of this symptom has been associated with active disease. The presence of chest pain may indicate spontaneous pneumothorax, and shortness of breath may herald the presence of extensive tuberculous bronchopneumonia or developing ARDS.

Imaging Findings

The most typical finding of postprimary MTb is that of poorly-defined areas of consolidation favoring the apical and posterior segments of the upper lobes (figure 1), and to a lesser extent the superior segments of the lower lobes. Opacities may also be found in other segments, but the dominant foci of consolidation tend to be in the apical and posterior segments of the upper lobes.

 

 

 

 

 

Figure 1A. Postprimary MTb Infection

Frontal chest radiograph shows extensive consolidation within the right upper lobe.

 

 

 

 

Figure 1B. Lateral Radiograph

The lateral radiograph shows the apical and posterior distribution of the consolidation to advantage.

Often small, poorly-defined opacities, or satellite nodules, are seen at the periphery of the dominant foci of consolidation. On HRCT, such nodules characteristically show centrilobular, branching, linear patterns, or so-called "tree-in-bud" opacities (figure 2). These opacities represent peribronchiolar granulomatous inflammation and obstruction of lumens by exudate or fibrosis.

Figure 2. "Tree-in-bud" Pattern

HRCT in postprimary MTb infection shows small nodules with branching configurations, representing "tree-in-bud" opacities.

Areas of cavitation may be seen in up to 45% of patients with active postprimary MTb on chest radiographs (figure 3) [2], but small cavities are more easily appreciated with CT and HRCT (figure 4). Cavities may be thick- or thin-walled; air-fluid levels are relatively uncommon. Occasionally, poorly-defined nodules ranging in size from 2-10 mm may be seen in a patchy distribution, spatially separated from areas of cavitation: these nodules often represent endobronchial spread of MTb infection (figure 3). Endobronchial spread of infection usually occurs as a result of spillage of caseous material from cavities, although rarely it can occur following rupture of an infected lymph node into a bronchus; the latter mechanism may allow an endobronchial spread pattern of infection to occur in primary MTb infection.

Figure 3. Cavitation and Endobronchial Spread of Infection in Postprimary MTb Infection

Frontal chest radiograph shows upper lobe cavitation (more easily seen on right side) in a patient with MTb infection proven by sputum smear. Smaller, poorly-defined nodules are present (most easily seen within the left lower lung), representing endobronchial dissemination of caseous material.

 

Figure 4A. Small Cavity in a Patient with Postprimary MTb Infection

Frontal chest radiograph shows a poorly-defined, left upper lobe subpleural opacity. Cavitation is difficult to appreciate.

 

 

 

4B. Axial CT Image Shows That the Lesion is Cavitary

Note surrounding lung nodules, representing endobronchial dissemination of caseous material.

Lymphadenopathy is uncommon in postprimary MTb, as are pleural effusions. When effusions occur, they are often discovered in elderly patients, and a parenchymal focus of infection is frequently evident.

References: To return to reference section after viewing abstract, click here before clicking on "abstract".

1. Barnes P, Cave M. Molecular epidemiology of tuberculosis. N Engl J Med 2003; 349:1149-1156.

2. Woodring J, Vandiviere H, Fried A, Dillon M, Williams T, Melvin I. Update: the radiographic features of pulmonary tuberculosis. AJR 1986; 146:497-506. Abstract

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