5 - The psoriasiform reaction pattern

Abstract

The psoriasiform reaction pattern is defined morphologically by the presence of epidermal hyperplasia with elongation of the rete ridges in a regular manner. This definition encompasses a heterogeneous group of dermatological conditions. A principal feature of the psoriasiform tissue reaction is said to be the formation of a suprapapillary exudate secondary to the intermittent release of serum and leukocytes from dilated papillary dermal blood vessels. In psoriasis, the increased epidermal mitotic activity resulting in elongated rete ridges is believed to be secondary to the release of mediators from these dilated vessels. In lichen simplex chronicus, the epidermal hyperplasia may be related to chronic rubbing and irritation, while in Bowen's disease there is increased mitotic activity of the constituent cells. In many of these conditions, the exact pathogenesis of the psoriasiform hyperplasia remains to be elucidated. The major psoriasiform dermatoses considered in this chapter are psoriasis, psoriasiform keratosis, pustular psoriasis, Reiter's (now reactive arthritis) syndrome, pityriasis rubra pilaris, parapsoriasis and its variants, and lichen simplex chronicus.