The systemic effect of dithranol treatment in psoriasis.

We have investigated the systemic effect of local treatment with dithranol for one week in psoriasis by a combination of subjective assessment of the severity of individual plaques and more objective assessment of blood flow (measured by laser-Doppler flowmetry) of the centre of the plaque, and at the active edge of the plaque. There is both subjective and objective evidence of an improvement in untreated plaques of psoriasis when dithranol is used on plaques elsewhere on the body. Blood flow falls at the active edge and at the centre of the plaques that are untreated. These findings indicate a systemic effect of local treatment that is more likely to be due to circulating factors, possibly T cells, rather than a direct effect of circulating dithranol. They also suggest that within patient comparisons of topical treatment in psoriasis may be inaccurate.