Imaging in Primary Tuberculosis

Primary tuberculosis is said to occur when clinically manifest infection follows the first exposure to the organism. Ultimately, the ability of MTb to cause human infection is related to the organism's ability to survive dormant within host macrophages for long periods of time, and to incite a T-cell-mediated, delayed hypersensitivity response by the infected host. Under normal circumstances, the host will sequester the MTb organism by forming granulomas. Usually, these granulomas show caseous necrosis, a pattern characteristically, but not exclusively, associated with tuberculous infection. The initial infection site has been termed the Ghon focus. Shortly after the infection occurs, organisms may spread through the lymphatics to hilar and mediastinal lymph nodes. Both sites usually heal with fibrosis and calcification. The combination of the lung parenchymal and lymph node MTb infection sites has been termed the Ranke or Ghon complex (figure 1).

Figure 1. Ranke (Ghon) Complex

Frontal chest radiograph shows a calcified right lung nodule with associated calcified hilar lymph nodes.

Organisms within the active Ghon focus often gain access to the bloodstream and may disseminate to extrathoracic organs; but, usually, host defenses are sufficient to prevent overt infection from developing in extrathoracic sites. It is important to remember that, although the pulmonary, lymphatic, and extrathoracic foci of infection are usually inactive at this point, organisms remain viable and may serve as the nidus for reactivation of disease when circumstances become favorable. Primary MTb infection in children is usually asymptomatic, and may be detected only with the conversion of skin tests. When symptoms occur, cough and fever are most common. In contrast, adults with primary MTb infection are usually symptomatic, and may present with weight loss, failure to thrive, fever, cough, and hemoptysis. Night sweats may also occur.

Patients with primary MTb most often show no radiologic abnormalities. If overt infection occurs, the pattern is usually one of air-space consolidation (figure 2), often involving an entire lobe. The right lung is more commonly affected than the left, although no definite zonal predominance is seen. Cavitation in primary MTb is unusual, and miliary dissemination is similarly uncommon.

 

 

Figure 2. Primary MTb

Frontal chest radiograph shows left lower lobe consolidation with a small left pleural effusion. MTb organisms were recovered from sputum cytology.

 

 

Atelectasis is often encountered in children with primary MTb (figure 3), and may be related to airway compression by enlarged lymph nodes. Less commonly, rupture of an infected lymph node into an adjacent bronchus may cause endobronchial dissemination of infection associated with atelectasis. Adults with primary MTb uncommonly present with pulmonary atelectasis.

Figure 3. MTb Infection Causing Bronchial Stenosis

Right upper lobe atelectasis in a child with right upper lobe bronchial compression due to MTb adenitis.

Radiographic abnormalities in primary MTb infections are often slow to resolve, even with the institution of prompt treatment. Air-space opacities may take more than 6 months to clear, and lymphadenopathy may take even longer to resolve. Lymphadenopathy commonly occurs in children with primary MTb infection. Usually hilar lymph nodes are involved, and mediastinal lymph nodes, particularly in the right paratracheal region, may be enlarged, as well. Unilateral lymphadenopathy is more often seen than is bilateral disease, and occasionally lymph node enlargement may be the only radiographic finding present. Lymphadenopathy is uncommon in adults with primary MTb, unless they are immunocompromised (see later). Lymph nodes actively infected with MTb quite commonly show central low attenuation, representing necrosis, on contrast-enhanced CT. Pleural effusion may occur in patients with primary MTb infection. Often, when tuberculosis is discovered as the cause of pleural effusion, no parenchymal focus of disease is radiographically evident; this pattern is considered characteristic of primary MTb pleural infection. Usually such effusions are small and unilateral.

Progressive Primary Tuberculosis

Rarely, a parenchymal focus of primary MTb infection becomes rapidly progressive. Extensive consolidation and cavitation develop, either at the site of the initial pulmonary parenchymal focus of infection or in the apical and posterior segments of the upper lobes. Thus, progressive primary MTb infection may closely resembles postprimary MTb infection.

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

Kim H, Song K, Goo J, Lee J, Lee K, Lim T. Thoracic sequelae and complications of tuberculosis. Radiographics 2001; 21:839-858; discussion 859-860. Text

Lee K, Im J. CT in adults with tuberculosis of the chest: characteristic findings and role in management. AJR 1995; 164:1361-1367. Abstract

Primack S, Logan P, Hartman T, Lee K, Muller N. Pulmonary tuberculosis and Mycobacterium avium-intracellulare: a comparison of CT findings. Radiology 1995;194:413-417. Abstract

Leung A. Pulmonary tuberculosis: the essentials. Radiology 1999; 210:307-322. Text

Lee J, Lee K, Jung K, Han J, Kwon O, Kim J, Kim T. Pulmonary tuberculosis: CT and pathologic correlation. J Comput Assist Tomogr 2000; 24:691-698. Abstract

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