Why is Tuberculosis (TB) most likely to occur in the posterior segment of the upper lobe?

From the Guidelines

Tuberculosis Occurrence in the Posterior Segment of the Upper Lobe

Tuberculosis (TB) is most likely to occur in the posterior segment of the upper lobe due to the high oxygen tension and relatively low pH in this region, which creates an optimal environment for Mycobacterium tuberculosis growth 1.

• The upper lobe's high ventilation-perfusion ratio and relatively low lymphatic drainage also contribute to the increased susceptibility to TB infection.
• As a result, the posterior segment of the upper lobe is a common site for TB lesions to develop, often presenting with apical posterior upper lobe or superior-segment lower lobe fibro-cavitary disease and endobronchial spread through the airways 1.
• This pattern is particularly seen in reactivation TB, which often occurs later than 1 year after initial exposure 1.
• Chest radiography has high sensitivity for detecting manifestations of active TB, including lobar pneumonia with associated hilar and/or mediastinal adenopathy or cavitary air space disease involving the apical posterior segments of the upper lobe or superior segment of the lower lobe 1.
• The presence of these radiographic findings, particularly in high-risk patients, should raise concern for TB and prompt further evaluation, including medical evaluation, mycobacteriologic examinations of sputa or tissue, and comparison of current and prior chest radiographs 1.
• Treatment of TB typically involves first-line medications such as isoniazid and rifampicin, with the specific regimen and duration depending on the individual patient's circumstances 1.

From the Research

Tuberculosis (TB) Occurrence in the Posterior Segment of the Upper Lobe

• The occurrence of TB in the posterior segment of the upper lobe can be attributed to several factors, including the high oxygen tension in this area, which favors the growth of Mycobacterium tuberculosis 2.
• The upper lobe, particularly the apical or apicoposterior segments, is a common site for TB due to the high oxygen levels and the presence of a high concentration of lymphatic vessels, which can facilitate the spread of the infection 2.
• The study by 2 found that active pulmonary TB confined to the apical or apicoposterior segments was more common than basal segment TB, suggesting a predilection for the upper lobe.
• Another study 3 mentions that patients with pulmonary TB whose sputum is smear-positive for M. tuberculosis form the main source of infection in communities, which may contribute to the high incidence of TB in the upper lobe.

Comparison of TB in Upper and Lower Lobes

• A comparison of CT findings in apical versus basal involvement of pulmonary tuberculosis found that consolidation, lymphadenopathy, and pleural effusion were more common in basal segment TB than in apical or apicoposterior segment TB 2.
• However, small nodules were less common in basal segment TB than in apical or apicoposterior segment TB, suggesting a difference in the radiological presentation of TB in the upper and lower lobes 2.
• The tree-in-bud sign was a common CT finding in both basal segment TB and apical or apicoposterior segment TB, indicating a similar pattern of infection in both areas 2.

Limitations of Current Research

• While there is evidence to suggest that TB occurs more frequently in the upper lobe, particularly in the apical or apicoposterior segments, the exact mechanisms underlying this predilection are not fully understood 2, 3.
• Further research is needed to elucidate the factors contributing to the high incidence of TB in the upper lobe and to develop more effective treatment strategies for this disease 4, 5.