Anogenital lichenoid lesions: clinicopathological features and differential diagnosis

The histopathological diagnosis of lichenoid dermatosis in the anogenital area is challenging. A wide range of conditions can display this pattern, often with overlapping features. Understanding clinicopathological characteristics, key histological clues and potential misleading appearances is crucial for an accurate diagnosis. Common entities like lichen planus or lichen sclerosus can vary in appearance depending on the specific variant or stage of evolution. Additionally, certain pathologies can closely mimic other conditions. For example, syphilis or paraneoplastic pemphigus may present changes identical to lichen planus, while "Mycoplasma pneumoniae-induced rash and mucositis" and paraneoplastic pemphigus can simulate erythema multiforme. Unusual patterns can also arise in cases of lichenoid contact dermatitis or condyloma lata in secondary syphilis. The presence of a plasma cell-rich lichenoid infiltrate serves as a clue for diagnosing plasma cell mucositis (Zoon) and syphilis. However, it is important to note that plasma cells may be outnumbered by lymphocytes in the former or completely absent in the latter. Therefore, in such cases, being aware of other subtle changes and recognizing the characteristics of the clinical presentation is crucial for arriving at the correct diagnosis. This review summarises practical information to empower the diagnostic precision of these challenging conditions.