|Pulmonary Pathology Online
Pathology of Infectious Granulomas of the Lung
Infections are a common cause of granulomatous lung disease.
Granulomas may present as a nodular mass of the lung but may be spread diffusely through the lung.
Infectious agents causing granulomatous lung diseases are as follows:
Certain features may be present which support a diagnosis of infection:
- Most important feature is necrosis, but less so if the "necrosis" is no more than some degenerative change in collagen.
- If the necrotic center contains neutrophils, the index of suspicion for infection should be especially high .
Pathologists should search for fungal infection.
- Most necrotizing granulomas are related to infection.
- Special Stains: In a few cases the organisms may be visible on the H & E stained section.
Special stains like Ziehl- Neelsen (ZN) stain for microbacteria and Grocott’s Methenamine Silver (GMS) stain for fungi.
Grocott’s Methenamine Silver (GMS): Advantage and disadvantage of GMS: GMS is much more reliable than PAS for staining fungi.
Black/lung debris/pigment are usually distinguishable by being more heterogenous than fungal spores and may confuse the picture on the GMS.
In this case PAS is helpful.
- Tissue necrosis and a vasculitis are commonly seen in infections. [Differential diagnosis:- Pulmonary angiitis / granulomatosis (Wegener’s granulomatosis etc ].
Such vasculitis is usually non-necrotising, showing mural/intimal infiltration by lymphocytes and other mononuclear cells.
- In Chronic Granulomatous Disease, an inherited group of conditions characterized by abnormal phagocytic cell function, necrotizing granulomas occur with a number of infectious pathogens.
- Microorganisms are generally found most frequently in an extracellular location within the debris of the necrotic granuloma center, and not in the cellular rim.
- In mycobacterial infection, microorganisms may be relatively few and far between, requiring thorough searching of one or two adequately stained sections.
Despite the higher index of suspicion and endless staining/searching nothing may be found.
- A significant proportion of infectious granulomas (proven some other way), and almost one third of radiographically solitary granulomatous lesions, will be negative on special staining.
Therefore a lack of organisms does not exclude an infectious etiology.
In such circumstances, the pathologist can describe the lesion, and convey the suspicion, but indicate the lack of stainable microorganisms.
- If granulomatous inflammation, especially in association with necrosis, is found in an otherwise typical nodular fibrous lesion of silicosis , the index of suspicion of a complicating mycobacterial infection should be very high.
- Obviously, in any case, if fresh material is available (Autopsy /Open/ Thoracoscopic lung biopsy), and the possibility of infection is suspected , tissue should be sent for microbiological examination for appropriate culture.
Serology, particularly in some of the fungal infections, may also provide useful information.
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