Lichen striatus: a model for the histologic spectrum of lichenoid reactions.

Three phases of lichenoid reactions are defined: 1) the primary phase, 2) the established phase; and 3) the senescent phase. Lichen striatus is characterized by a primary pattern that may be apparent in the epidermis, the hair follicles, and rarely the sweat glands and ducts. Focal areas of established lichenoid reaction, that are indistinguishable from lichen planus, are common. They usually are confined to the tips of elongated rete ridges. Established lichenoid patterns are occasionally present in hair follicles and are indistinguishable from those seen in lichen planopilaris. Eccrine hidradenitis, a feature of lichen striatus, is rarely seen in lichen planus. Lichen planus may be an adaptive epidermal response to a clone of aggressive lymphocytes and, as such, may be a manifestation of auto-immunity. In lichen striatus, senescent lichenoid patterns resemble those seen in lichen nitidus.