Active and inactive edges of psoriatic plaques: identification by tracing and investigation by laser--Doppler flowmetry and immunocytochemical techniques.

In plaque psoriasis it is likely that biochemical and ultrastructural changes precede the appearance of the typical plaque that is recognizable clinically. Currently, no technique exists by which the very early changes in psoriasis can be investigated. We report a method in which plaques of psoriasis are serially traced to identify their advancing edge. Eight-two untreated plaques from 15 patients and 38 treated plaques from 6 patients were traced over a three-week period; 65% of untreated and 57% of treated plaques showed consistent asymmetrical movement, allowing identification of an active and an inactive edge of each plaque. Using this technique, the active edge of two or more plaques was identified in each of ten patients. Blood flow measured by laser Doppler flowmetry indicated a 2.5-to-4.5-fold increase in cutaneous blood flow at the active edge compared with the inactive edge of each plaque. Punch biopsies from the sites investigated by laser Doppler flowmetry were examined by routine histology and monoclonal antibody immunohistology, but revealed no epidermal change and no T lymphocytic excess when the two areas were compared. We infer from these findings that the earliest change in a developing plaque is an increased blood flow, probably associated with a diffusable, and possibly humoral, initiating factor that accumulates at the active edge, stimulating transformation of normal skin to psoriatic plaque.