Tuberculosis is an infectious disease caused by Mycobacterium tuberculosis, and most commonly affect the lungs, but may also involve other organs.
- Primary tuberculosis occurs soon after a person has acquired infection.
- Adult-type or secondary tuberculosis results from the reactivation of latent disease. The extent of lung involvement varies – from small infiltrates to extensive cavitation (destruction of the lung with the formation of cavities).
The primary complex and its complications
Primary tuberculosis typically affects the middle and lower lung lobes because the inspired air that carries the bacteria is typically distributed to these areas. Usually, the lesion known as the Ghon focus is seen peripherally whilst the hilar and paratracheal lymph nodes also appear enlarged due to the dissemination of the bacteria via lymphatic vessels to the nearest lymph nodes, forming the primary complex. In the center of the Ghon focus approximately within 4-8 weeks forms the area of caseation (the immune system kills bacteria by causing inflammation and the formation of granuloma). The lesion tends to heal spontaneously leaving a small calcified nodule (the Ranke complex) in individuals with the adequate immune response. Enlarged lymph nodes may compress the airways and cause the collapse of the lung. Rarely tiny calcific deposits (Simon’s foci) may be observed in other parts of the lung, they appear as round coin-like lesions on the X-ray.
- Rupture of the focus into the space around the lungs (the pleural cavity)
The primary focus is often located close to the pleura – a membrane which lines out the lungs. When the lesion becomes bigger it may involve pleura and cause its rupturing – as a result, bacteria and caseous material gets into the pleural space as a large pleural effusion. Sometimes this effusion may become purulent – tuberculous empyema develops.
- Acute cavitation of the focus
In other cases the large primary focus ruptures and opens into a bronchus, respectively, the caseous material is discharged by coughing. When it happens tuberculosis can spread and affect other parts of the lungs.
Progressive primary tuberculosis
Progressive primary tuberculosis is the advanced disease observed in young children and those who have impaired immunity. It is characterized by the rapid decline in a person’s well-being and the development of tuberculous pneumonia.
Calcified lymph nodes
In hilar and paratracheal lymph nodes affected by the mycobacteria develop caseous lesions which later calcify. Calcified lymph nodes may be detected on X-ray scans in children and adults, who are infected with HIV.
Secondary tuberculosis occurs due to the reactivation of the latent infection. It typically affects the apical and posterior segments of the upper lobes of the lungs. The superior segments of the lower lobes may also be involved. However, the process is usually asymmetrical. Caseation (formation of the cheesy necrosis), fibrosis and cavity formation frequently develop.
- Typically the infection process starts as a patch of pneumonitis which is observed in a part of an upper lobe
- Tuberculous pneumonia is defined as a massive involvement of lung segments or lobes with the coalescence of the lesions.
- Miliary tuberculosis is a rare form of pulmonary tuberculosis characterized by the multiple nodules, distributed throughout all the lungs.
- The extensive cavitary disease develops when the lung is severely damaged. Necrotic areas of the lung form cavities, later the necrotic contents are discharged into the airways causing the spread of bacteria to the other parts of both lungs and leading to the formation of new cavities filled with dead tissues. The disease can progress from a small lesion to the extensive cavitary disease within a few months.
- Rounded lesions which do not cause any symptoms and appear as the encapsulated region of caseous necrosis are called tuberculomas. These lesions usually are stable.
- The lesions heal by fibrosis or scar tissues. Some lesions may later calcify. As time passes the fibrotic lesions shrink, pulling the trachea and hilum to the affected side. In some parts of the lungs, cavities may persist. Sometimes in case of chronic fibroid tuberculosis, these fibrotic lesions still remain highly infectious, although asymptomatic.