[Osteomalacia as an apparently rare side effect of oral fumaric acid therapy. Secondary DeToni-Debré Fanconi syndrome in the adult].

A case of fully reversible tubular toxicity with consecutive metabolic osteopathy following systemic fumaric acid therapy is reported. This secondary effect of oral fumaric acid therapy obviously occurs very rarely, never having been described before. A 46-year-old female patient with a long history of recurrent palmoplantar psoriasis underwent oral treatment with fumaric acid and its esters in accordance with the Schäfer method, preceding attempts at curative treatment with conventional antipsoriatic agents having proved unsatisfactory. Two months later, the patient started to experience arthralgia, back pain in the early hours of the morning and myalgia with increasing frequency, progressing to disablement in moving and walking and, finally, to total immobility. Not until 9 months later was the reason for these severe disabilities found: they stemmed from hypophosphataemic osteomalacia as a result of a complex disturbance of the renal tubular system. The clinical symptoms and the results of laboratory chemistry tests returned to normal as soon as fumaric acid medication was discontinued. Two attempts at reexposure confirmed the causal relationship. Oral fumaric acid medication should never be administered without clinical and chemical controls.