Pathogenesis of Woronoff ring in psoriasis.

As a result of ultraviolet light and coal tar therapy, a white ring (Woronoff) may develop in the normal skin adjacent to psoriatic plaques. Injection of prostaglandin E2 (PGE2) 1 cm outside of Woronoff ring produced redness in the ring, demonstrating that vessels within the ring were not unresponsive to PGE2. Whole skin homogenates from Woronoff ring contained an inhibitor of prostaglandin synthesis that was not found in uninvolved skin that was obtained from either psoriatics or normal controls. Prostaglandin E2 levels in the ring were one third of those in uninvolved skin from either psoriatics or normal controls. These findings suggest that the white ring that surrounds ultraviolet-light-treated psoriatic plaques is produced by a local inability to synthesize PGE2 in response to an ultraviolet light stimulus, resulting from the presence of an inhibitor of prostaglandin synthesis.