Histological spectrum of pure neuritic leprosy with atypical clinical presentation at a tertiary care centre

Introduction: Pure neuritic leprosy (PNL) is a rare disease and is characterised by isolated involvement of one or more peripheral nerves by Hansen's disease (HD) in the absence of skin involvement. The aim of this study was to assess the histological spectrum of PNL cases with atypical clinical presentation. Material and Methods: A retrospective analysis of all biopsy proven PNL cases (total 23) over the past 16 years was done. Detailed histopathological examination of the nerve was performed. Myelin and axonal status were evaluated using luxol fast blue/ Periodic acid Schiff (LFB/PAS) stain and immunohistochemistry (IHC) for neurofilament protein (NFP) respectively. Results: Clinically a diagnosis of HD was suspected in 70%, mononeuritis multiplex in 12%, vasculitis and demyelination in 9% cases each. Moderate to severe epineurial and endoneurial inflammation were seen in 91% and 96% of cases respectively. Granulomatous inflammation was seen both in the endoneurial (70%) and epineurial (17%) location. Foam cell infiltration was more common in the endoneurial (60%) than the epineurial (22%) location. Occasional cases showed necrosis (4%) and vasculitis (13%). Severe myelin and axonal loss was seen in 74%. Leprosy bacilli were identified in 12 (52%) cases. One case showed normal morphology although leprosy bacilli were present. Conclusion: Although PNL is known to cause endoneurial inflammation, epineurial inflammation or even granulomas can be seen. Necrosis and vasculitis are rarely seen in PNL. Myelin and axonal loss are almost universal. Even if morphologically the biopsy is normal, staining for leprosy bacilli should be performed on all suspected PNL cases.