Abstract

Leprosy (Hansen’s disease) is a chronic infectious disease caused by Mycobacterium leprae or lepromatosis. There is a large clinical and histologic spectrum of leprosy. Lepromatous leprosy is characterized by widely distributed erythematous macules, papules and nodules, and microscopic findings of diffuse dermal infiltrates of foamy macrophages. Here we report a case of lepromatous leprosy with an exaggerated epithelial inductive phenomenon. A 74-year-old man presented with multiple circumscribed, hyperpigmented  and anesthetic nodules on his face. The clinical impression was concerning for neoplasm of uncertain behavior versus atypical pigmented lesions.  Biopsy of the facial lesions revealed a diffuse dermal infiltrate of foamy histiocytes. The overlying epidermis was hyperkeratotic and hyperplastic with basaloid induction, papillary mesenchymal bodies, trichohyalin granules and increased pigmentation along the basal layer. The histologic differential of epidermal and histiocytic proliferations, such as verruciform xanthoma and infectious etiologies (e.g. leprosy, leishmaniasis, atypical mycobacterial infection) was entertained. Fite-Faraco stain highlighted a diffuse proliferation of bacilli within the foamy histiocytes (lepra or Virchow cells), and a diagnosis of lepromatous leprosy was rendered. The pathogenesis of the epidermal induction phenomenon is unclear. The dermal proliferation of histiocytes is reminiscent of dermatofibroma and may share mechanisms which converge on epidermal induction. Further studies to interrogate this process as it relates to Hansen’s disease may yield targetable pathways relevant for diagnosis and treatment.