[Apparent worsening of psoriasis lesions revealing methotrexate overdosage]

Methotrexate (MTX) is an antimetabolite drug used in the treatment of cancers and autoimmune diseases and frequently in dermatology for cutaneous and/or arthritic psoriasis. Toxicities due to MTX overdosage are mainly cutaneous, hepatic and hematologic. Herein, we report a case of MTX overdosage presenting as an erosive and an inflammatory flare of preexisting psoriatic plaques and with new palmar lesions.

A 51-year-old male with a 6-year history of plaque psoriasis resistant to topical corticosteroids was started for the first time on MTX 20mg weekly. One week later, he presented with fever, general weakness and mucocutaneous ulcerations. Physical examination revealed inflammatory, erythematous and partially erosive annular plaques strictly confined to preexisting psoriatic lesions, along with keratotic psoriatic palmar plaques. Further questioning indicated that the patient was taking MTX 20mg daily. Investigations revealed neutropenia (1040/mm³) and skin histology showed prominent dystrophic keratinocytes and confirmed the diagnosis of methotrexate toxicity. Clinical and biological improvements were observed after cessation of MTX and treatment with folinic acid, IV hydration and urine alkalization.

Skin lesions due to acute MTX toxicity are rare, but they herald later-onset pancytopenia. Identification of these cutaneous lesions might enable earlier treatment initiation. The predilection of MTX toxicity for preexisting lesions or the de novo appearance of palmoplantar pustules should not lead to the erroneous diagnosis of psoriasis flare.