Topic: glomerulonephritis, tuberculosis, acute osteomyelitis
Question: Discuss the morphological appearance of Primary pulmonary tuberculosis.
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Gross Pathology
– Granulomatous lesions called tubercles in the lung, localized typically to the middle/lower lobes
– Lesions are grey-white to yellow, caseous (cheesy) necrosis in center
– May involve hilar lymph nodes, enlarge and caseate as well
Pathogenesis
– Initial infection is through inhalation of droplet nuclei containing M. tuberculosis
– Bacteria lodges in the distal airspaces of the lung, usually the posterior segments of the upper lobes or the superior segments of the lower lobes
– This leads to the formation of the Ghon focus, which is the initial parenchymal lesion.
Ghon Focus
– Consists of a tubercle with central caseous necrosis surrounded by epithelioid histiocytes, Langerhans giant cells, lymphocytes, and fibroblasts.
– Bacilli are present within and around the lesion.
– The lung attempts to wall off the infection by forming granulomas.
Lymphatic Involvement
– Bacilli drain from the Ghon focus to the regional lymph nodes, typically hilar and paratracheal nodes.
– This leads to enlargement of the nodes and development of central caseous necrosis – known as the Ranke complex.
Healing
– The Ghon focus undergoes fibrosis and calcification, transforming into the Ghon lesion which contains walled-off dead bacilli.
– The involved lymph nodes can also calcify.
– This calcified primary complex is characteristic of past initial TB infection
Histopathology
– Central caseous necrosis surrounded by epithelioid cells, Langerhans giant cells, lymphocytes, plasma cells
– Typical granulomas have necrosis, variable acid-fast bacilli
– Granulomas may be discrete, confluent, or have minimal necrosis early in disease
– Bronchogenic spread causes endobronchial TB with necrosis and edema
– Focal alveolar inflammation with macrophages filling alveoli
– Vasculitis and thrombi in small vessels
Bacterial Findings
– Acid fast staining shows red, beaded M. tuberculosis bacteria
– Grow in culture as dry, crinkled colonies in weeks
– PCR detects M. tuberculosis DNA from specimens
Complexity varies with immune status. HIV coinfection causes poorly-formed granulomas, widespread dissemination. Primary TB in children can present as hilar lymphadenitis or miliary TB. Adults have cavitary pneumonia. Treatment is with standard anti-tuberculous drugs.
