Abstract

Background: Several forms of acquired macular pigmentary disorders have been described in the literature, including ashy dermatosis, erythema dyschronicum perstans, and lichen planus pigmentosus(LPP). While considered by some to be on a spectrum, they are increasingly recognized as distinct clinicopathologic entities(1). LPP inversus is a rare variant of LPP with prominent flexural involvement. Here we report a case of LPP inversus with accompanying histologic features highlighting the importance of clinicopathologic correlation in the diagnosis of acquired pigmentary disorders. Case: A 58 year old woman with a history of lichen planus presented with 1 year history of new pruritic rash. On exam there were grey-purple patches without overlying surface change involving the bilateral axillae, inframammary folds, antecubital fossae, as well as scattered patches on the forearm, flexural wrist, and lower legs. A punch biopsy from the axilla revealed a thinned epidermis with sawtooth rete ridge pattern with an underlying band-like lymphocytic infiltrate and presence of Civatte bodies. There was focal necrosis of the basal layer with prominent pigment incontinence. Given the clinical distribution, a diagnosis of LPP inversus was rendered. Discussion: Lichen planus pigmentosus inversus is a rare clinical variant of LPP characterized by acquired grey-purple macules with a flexural predominance. While overlap exists between LPP and other acquired pigmentary disorders, a recent global consensus study has defined LPP as a distinct entity(1). A recent histopathologic review suggests that features favoring a diagnosis of LPP include an active lichenoid dermatitis, a moderate to severe lymphocytic infiltration, and more prominent pigmentary incontinence(2). Given clinical overlap, histology is useful in diagnosing disorders of acquired macular pigmentation. Here we present a case of LPP inversus, demonstrating characteristic clinicopathologic features of an otherwise rare disorder.

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