Papular and Lichenoid Diseases

Abstract

A substantial subset of dermatoses is characterized by small papules. Lichen planus is just the prototype of this peculiar group of diseases. The latter was first described by Sir Erasmus Wilson in his, at that time, fundamental and still fascinating textbook “Diseases of the Skin” (London, Philadelphia 1869). The term lichen originates from the Greek word Λɛιχήνα, which best describes the clinical picture of plaques as a result of aggregation of the primary papules. Today, the basic principles of the pathophysiology have been unraveled, placing this condition on the spectrum of autoimmune diseases largely attributable to a cellular attack against autoantigen-bearing keratinocytes. A wide spectrum of therapies has been developed, but it is still challenging to tackle the harmful and highly chronic manifestations on the mucosa or in the nails and hair follicles. Some other dermatoses can be assigned to this group of papular-lichenoid skin diseases in a strict sense, partly for morphological reasons, but also because of homologies in the pathogenesis: first of all, (chronic) Graft-versus-host disease, lichen nitidus, annular lichenoid dermatosis of adolescence, the Gianotti–Crosti disease of childhood, and, most peculiar and rare, the lichenoid trikeratosis Kaposi–Nékám, nowadays termed keratosis lichenoides chronica.