Borderline Lepromatous Leprosy Simulating Sarcoidosis on Histopathology: A Case Report.

Leprosy shows a broad spectrum of clinical and histopathological features depending on host immunity and disease stage. The characteristic "leprosy pattern" involves superficial and deep dermal infiltrates in perivascular, peri-appendageal, and perineural locations. While tuberculoid leprosy and cutaneous sarcoidosis may occasionally have overlapping histopathology, borderline lepromatous (BL) leprosy demonstrating sarcoidal granulomas is uncommon. We report a 50-year-old female presenting with multiple infiltrated erythematous and hypoesthetic plaques, and asymmetrically thickened peripheral nerves. Skin biopsy from a clinically downgraded lesion revealed sarcoidal granulomas (discrete, non-caseating, with sparse lymphocytic rims); however, the presence of interspersed foamy histiocytes, a distinct Grenz zone, and acid-fast bacilli on Wade-Fite stain, together with clinicopathologic correlation, confirmed a diagnosis of BL leprosy. Ancillary investigations helped rule out sarcoidosis. This case presents the diagnostic challenge created by overlapping granulomatous patterns, and reinforces the importance of detailed clinicopathologic correlation and ancillary testing including mycobacterial stains, especially since slit-skin smears are often negative in borderline cases. Recent studies have shown that changes in immune cell composition within granulomas can influence transition along the leprosy spectrum and may possibly give rise to atypical morphologies (like sarcoidal patterns). Early diagnosis and treatment remain central to leprosy control, and awareness of such presentations can help avoid diagnostic delay in endemic regions.