Does histoid leprosy represent a locally hyperimmune variant of lepromatous leprosy?

Background: Histoid leprosy is a subtype of leprosy with distinct clinical presentation and histologic features. It accounts for <4% of leprosy cases. The exact location of histoid leprosy along the immune spectrum and its relation to other subtypes is unclear.

Aim: To evaluate the local immune phenomenon which define histoid leprosy.

Design: Parallel skin biopsies from histoid lesions and from unremarkable skin in patients with histoid leprosy were evaluated and the histologic findings compared.

Methods: Formalin fixed, paraffin embedded tissue sections from lesional and non-lesional biopsies were assessed for changes in epidermis and dermis; type and extent of infiltrate, presence or absence of pseudocapsule and associated reactions. Bacillary index was evaluated using Wade Fite stain for lepra bacilli.

Results: Amongst 208 leprosy cases, six cases of histoid leprosy were identified (2.88%). The cases showed presence of nodules, patches and plaques overlying clinically unremarkable skin. Fourteen skin biopsies were evaluated of which the lesional biopsies showed equal proportion of fusocellular, fusocellular epithelioid and fusocellular-vacuolated histology. A greater circumscription was noted in lesional biopsies; however the cellular content of the infiltrate was similar in lesional and non-lesional biopsies. A case of erythema nodosum leprosum in histoid leprosy was also seen.

Conclusions: Ours is the first study comparing normal and lesional skin in histoid leprosy. Though the histoid lesions appear to have a derivation from lepromatous leprosy, the local histologic and clinical alterations may be a result of heightened local immunity or reactive local modifying factors.