Abstract

Leprosy is a chronic infectious disease caused by Mycobacterium leprae that primarily affects the skin and peripheral nerves. The clinical disease spectrum is classified by the strength of the host immune response, with tuberculoid leprosy representing strong, effective host immunity and lepromatous leprosy arising in the setting of poor, ineffective host immunity. The histologic findings are similarly classified, with rare or even absent organisms seen in tuberculoid leprosy (i.e., paucibacillary disease) and many organisms (e.g., globi) seen in lepromatous leprosy. We present a case of a 27-year-old Micronesian male who presented with a generalized rash and painful nodules on his extremities in 2017, and was ultimately diagnosed with leprosy on the basis of smears from six distinct cutaneous lesions. He was treated with a WHO Dosepak regimen for three years, with many gaps in the treatment program. The patient then returned to clinic in 2021 with a firm, tender 8mm violaceous nodule on the left proximal dorsal forearm. Histologic examination revealed a well-circumscribed dermal spindle cell proliferation with peripheral collagen trapping. Immunohistochemistry revealed positive CD163 expression and absent CD34 expression, confirming the diagnosis of dermatofibroma. The deeper edge of the lesion showed some foamy macrophage. Rare mycobacteria were identified on Fite special stain, including some within a small caliber nerve. The overall findings are supportive of a dermatofibroma arising in a chronic regressive lesion of cutaneous leprosy following polychemotherapy treatment. Dermatofibromas arising in the setting of cutaneous leprosy is a rarely reported phenomenon and understanding the immunologic response to Mycobacterium leprae may lend insight into the pathogenesis of reactive fibrohistiocytic proliferations. For clinicians and pathologists, maintaining a high index of suspicion is essential in such cases, especially when only rare organisms are present.

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