[Correctly adDRESS the cause of hemophagocytic lymphohistiocytosis].

Hemophagocytic lymphohistiocytosis (HLH) is a rare and severe syndrome usually associated with a cytotoxicity deficiency, which leads to an excess of immune response driven by activated macrophages and cytotoxic T cells. In children, HLH can be genetic, as part of a familial lymphohistiocytosis, or secondary: the most frequent causes are systemic-onset juvenile idiopathic arthritis, hematological malignancies, and severe infections, especially with Ebstein-Barr virus or leishmaniosis. We report on the case of a 3-year-old girl with no past medical history, who presented inaugural Pseudomonas aeruginosa maxillary osteitis, with secondary HLH. The rarity of this osteitis, the characteristics of the pathogen, and the onset of HLH oriented the diagnosis toward primary immunodeficiencies, malignancies, or systemic diseases. Steroids were initiated at 2mg/kg/day and were very effective in improving the systemic symptoms. Antibiotic therapy was continued unchanged. A few days after discontinuation of steroids, while the patient was still under antibiotics, she presented with erythroderma. Skin biopsy revealed eosinophil infiltrate in line with the diagnosis of a drug reaction with eosinophilia and systemic symptoms (DRESS), even though we only observed very transient eosinophilia, up to 0.98G/L, during HLH. Stopping antibiotics normalized the symptoms without using systemic corticosteroids. Patch tests confirmed an allergy to piperacillin. These atypical manifestations of DRESS underline that causative diagnosis of HLH is challenging, and DRESS syndrome should be considered.