A 77-year-old woman originally from India presented with diminished sensation in her distal extremities, and a diffuse rash, which progressed into annular plaques (panel A) and nodular lesions (panel B). Electromyography and nerve conduction studies revealed diffuse sensorimotor peripheral neuropathy. Punch biopsy of a nodular lesion revealed numerous bacilli (panel C) consistent with multibacillary (lepromatous) leprosy and scattered neutrophils and small vessel vasculitis representing erythema nodosum leprosum (ENL) (panel D). Leprosy is a chronic granulomatous disease caused by Mycobacterium leprae, which affects primarily the skin and peripheral nervous system. Classification of leprosy is based on the wide spectrum of host cellular immune responses to the M. leprae antigen, ranging from tuberculoid (TT) to lepromatous leprosy (LL)1. ENL is a poorly understood, but common immune complication of LL that affects approximately 50% of patients with LL and causes significant disability and morbidity in endemic regions2. The most common clinical symptoms include pain, nodular and papular skin lesions, edema, neuropathy, and arthralgias, but can also affect other organs including the eye, liver, spleen, and kidney1, 2. Untreated, ENL can result in permanent neurological damage and disability. Management of ENL aims to control acute inflammation and prevent the onset of recurrent episodes. ENL is treated with corticosteroids alongside anti-mycobacterial agents (Fig. 1).

Figure 1
figure 1

Panel a Painless, annular plaques on right gluteal region. Panel b Erythematous, nodular lesions on bilateral shins. Panel c Right leg punch biopsy with Fite stain demonstrates numerous acid-fast bacilli (black arrow), consistent with multibacillary lepromatous leprosy. Panel d Punch biopsy with H&E demonstrates perivascular (red arrow), perineural (black arrow), and interstitial infiltrate of lymphocytes, plasma cells, histiocytes, and scattered neutrophils consistent with erythema nodosum leprosum.