From Nerves to Nodules: A Case-Based Review of the Progression of Pure Neuritic Leprosy to Lepromatous Leprosy With Erythema Nodosum Leprosum

Leprosy, caused by <i>Mycobacterium leprae</i>, affects the skin and peripheral nerves. Pure neuritic leprosy (PNL) is a rare subtype, characterized by nerve involvement without skin lesions. We report a rare case of PNL progressing to lepromatous leprosy (LL) with erythema nodosum leprosum (ENL), with only one such case previously reported in the literature and reviewed herein. A 35-year-old male diagnosed with PNL on sural nerve biopsy showing acid-fast bacilli (3+) completed multidrug therapy for 12 months. Nine months later, he developed painful erythematous nodules and sensory-motor deficits. Slit-skin smear (bacteriological index 4+) and biopsy confirmed LL with ENL. He was treated with monthly pulses of rifampicin, minocycline, and moxifloxacin, along with thalidomide and corticosteroids. Significant improvement occurred after six months. This case illustrates the intricate clinical challenges associated with leprosy, particularly how PNL can progress into more severe forms such as LL. While Type 1 reactions are documented in PNL, only a single instance of a Type 2 reaction has been recorded in PNL. Persistent bacterial load, immune modulation, host factors, and drug resistance can all contribute to this shift, necessitating vigilant monitoring even after treatment completion. This case underscores the rare transformation of PNL to LL, possibly due to persistent bacilli and reduced cell-mediated immunity, emphasizing the importance of early diagnosis, complete therapy, and careful follow-up.