Photoallergic contact dermatitis to 8-methoxypsoralen in Ficus carica

Photocontact dermatitis to Ficus carica is induced by furocoumarins present in sap. These substances are generally considered to cause phototoxic reactions.

We conducted a patch test and histopathological study of patients with phytophoto contact dermatitis from the fig tree to evaluate the mechanism underlying the photoreaction.

Patch and photopatch testing with serial dilutions of two natural furocoumarins [5-methoxypsoralen and 8-methoxypsoralen (8-MOP)] contained in plant sap were performed in 47 patients. A synthetic furocoumarin, 4,5',8-trimethylpsoralen, was also tested. Histopathological analyses were made of some positive photoreactions.

Positive photopatch tests reactions to 8-MOP were obtained in 12 of 47 patients, in 4 of them down to a concentration of 0.0001%. Patch tests and photopatch tests to the other two furocoumarins were negative. Histopathological findings on biopsies from positive photopatch tests to 8-MOP showed a dermatitis.

Allergic photoreactions induced by contact with plants containing coumarins are generally regarded as chance findings. This study has demonstrated that phytophoto allergic contact dermatitis resulting from furocoumarins is not an exceptional finding, and should be suspected in subjects with diffuse clinical manifestations in photo-exposed but also non-exposed sites. To differentiate allergic from toxic photoreactions, patch tests need to be performed with serial dilutions of furocoumarins. Histological analysis of a biopsy sample from a positive test site will reveal alterations compatible with a photoallergic contact dermatitis.