[Skin reaction to dithranol and its modification by the addition of tar (LCD)]

In spite of 70 years' continuous use of dithranol for the topical treatment of psoriasis, there are few reports of contact hypersensitivity reactions to this compound. A male patient with psoriasis had an adverse skin reaction to the traditional topical dithranol treatment; patch tests revealed contact dermatitis in response to 0.02% dithranol in petrolatum, which was characterized by marked erythema and severe bullous reaction to 0.1% dithranol in acetone. A control group of ten volunteers tested under similar conditions did not react with marked erythema until a concentration of 0.1% dithranol in petrolatum was applied. When liquid tar (5% liquor carbonis detergens, LCD) was added to the patch test solutions concentrations that were clearly one or two steps higher were needed before the erythematous skin reaction was induced. Since minimal erythema generally appears in patch tests with greater than or equal to 0.05% dithranol in petrolatum, we believe that in the patient reported here contact hypersensitivity to dithranol was present. The development of large perilesional erythematous areas with accompanying edema during topical dithranol treatment supports this suggestion. It seems that the addition of liquid tar elevates the reaction threshold to dithranol in hypersensitive patients with psoriasis.