Primary pulmonary tuberculosis. Primary TB complex morphology. Complications. Progressive hematogenous tuberculosis - generalised or pulmonary
Tuberculosis is a communicable chronic granulomatous disease caused by mycobacterium tuberculosis, it commonly involves lungs but can affect any part of the body, causing tuberculous granulomas which undergo caseous necrosis.
Primary TB is the form of the disease that develops in previous unexposed (and therefore unsensitized) patients, it is usually asymptomatic, and progressive disease mainly affects the immunocompromised. TB causes a hypersensitivity reaction which is the cause of the tissue destruction. Macrophages are activated after the consumption of the bacteria, here they survive and proliferate. This macrophage goes on to trigger T-cells which in turn activate more macrophages and cause the formation of the hypersensitivity granuloma.
Primary TB almost always begins in the lungs, as sensitization develops, a gray-white inflammatory region known as Ghon focus develops, this focus then undergoes caseous necrosis, in combination with the parenchymal lesion and nodal involvement we know this as Ghon complex. During the first few weeks there is then dissemination to other parts of the body. After this, the Ghon complex undergoes fibrosis and calcification (known as Ranke complexes), generally no secondary lesions occur outside of the lungs in primary TB.
Ghon focus: a subpleural nodule of cheesy, yellow-white caseous necrosis just beneath the pleura (bottom).
Image: “Tuberculosis - Sub-pleural primary (Ghon) focus (6596011395)” by Yale Rosen from USA, CC BY-SA 2.0, via Wikimedia Commons.
Remember: the Ghon focus lies subpleurally, in the lower part of the upper lobe or the upper part of the lower lobe (the best-ventilated zones). Ghon complex = Ghon focus + caseating hilar lymph nodes (with the lymphatics joining them); once healed and calcified it is the Ranke complex.
Histology of the active sites show granulomatous inflammation with caseating and noncaseating granulomas with giant cells. In rare cases, primary TB can develop to
progressive primary TB, where there is massive hematogenous dissemination to organs such as the spleen and liver in what is known as miliary TB, but this is rare in the primary form and can also complicate secondary TB, see below, also look at the typical image of a tuberculoma.
In less than 5% of cases we get progressive primary TB, this includes the following:
- Primary caseous pneumonia - local extension of Ghon focus to entire lobe/ segment, it appears yellow-gray with caseous necrosis and can turn to liquefaction