Clinical and pathological differential diagnosis of localised granulomatous skin lesions

Granulomas are focal infiltrates consisting mainly of macrophages or macrophage-derived cells (epithelioid, giant cells), chiefly in the case of impossibility or slow degradation of specific antigens. The variability of manifestations complicates the clinical and pathological diagnosis of granulomatous skin diseases due to individual patient reactivity and the specifics of often unidentified triggering factors. The mini-review analyses possible approaches to differentiating the most likely localised granulomatous lesions (granuloma annulare, sarcoidosis, tuberculides, leprosy, and lupus miliaris disseminates faciei) by providing recommendations on possible key clinical and histopathological characteristics. The review is illustrated by a case of a localized granulomatous lesion with features that are atypical but possible for some of the diseases discussed, with the most likely diagnosis of granuloma annulare (clinical course, localization, vertical infiltrates, destruction of elastic fibers, accumulation of mucin, solitary eosinophils). Pathologists need to consider different diagnostic approaches for different types of histological diagnoses, which may require opposite therapies. Therefore, the possibility of infection or foreign material in all types of granulomatous inflammation should be considered and PAS staining and polarized light microscopy should be recommended as basic steps in the examination. Special techniques such as Ziehl-Nielsen or Grocott methenamine silver should be also applied to identify the pathogen if necessary. It is essential to have enough tissue for histological examination, including immunohistochemical staining and polymerase chain reaction. Pathologists should not hesitate to ask for a larger tissue sample early in the disease if necessary. Keywords: tuberculoid granuloma, palisade granuloma, ring-shaped granuloma, granulomatosis of the skin, histochemistry, biopsy.