Borderline Tuberculoid Leprosy Masquerading as Granuloma Annulare: A Clinical and Histological Pitfall.

Introduction: Leprosy is a chronic granulomatous disease caused by Mycobacterium leprae, an intracellular acid-fast bacillus that tends to infect the skin and peripheral nerves. Because of the wide array of cutaneous manifestation, diagnosis is not always straightforward, especially in nonendemic regions of the world such as the United States.

Case Report: The authors report an unusual case of borderline tuberculoid leprosy in an 80-year-old white woman from California. Clinical examination revealed multiple nonscaly annular plaques, with central clearing and absence of lesional anesthesia, distributed over the trunk and extremities initially clinically diagnosed as granuloma annulare (GA). After 2 years of unsuccessful treatment with topical corticosteroid, biopsy revealed a palisaded and interstitial granulomatous infiltrate with necrobiosis, without discrete granulomas, compatible with GA. However, the presence of perineural lymphocytes in the reticular dermis prompted a Fite stain, which revealed acid-fast bacilli within the Schwann cells of a small peripheral nerve, pathognomonic for leprosy.

Conclusion: This is the first reported case of leprosy masquerading both clinically and histologically as GA. Dermatopathologists should be aware of the possibility of leprosy given the presence of perineural lymphocytes amidst any pattern of granulomatous infiltrate and obtain a Fite stain.