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Panniculitis is an inflammation of the subcutaneous fat. Skin biopsy plays a critical role in the differential diagnosis of panniculitis. The most common approach to diagnosis relies on the differentiation between predominantly septal or lobular panniculitis, followed by the distinction between lesions with and without vasculitis. It is also very important to submit a part of the skin biopsy for microbiological analysis and for T-cell clonal expansion if T-cell lymphoma is suspected. Erythema nodosum, the most frequent septal panniculitis, has many causes and in its typical clinical presentation, does not require skin biopsy. In other panniculitis, diagnosis is based on the integration of the clinical and histological data, which renders a deep cutaneous biopsy necessary. Periarteritis nodosa, a septal panniculitis with vasculitis characterized by subcutaneous nodules and livedo racemosa, can be associated with systemic involvement. Nodular thrombophlebitis needs search for associated coagulopathy, Behçet\'s disease, periarteritis nodosa or Buerger\'s disease. Lobular panniculitis are classified according to the nature of cells present in the inflammatory infiltrate. If there is a lymphocytic infiltration, lupus panniculitis is difficult to differentiate from subcutaneous panniculitis-like T-cell lymphoma. If there are histiocytes, it can be a sarcoidosis, a cytophagic histiocytic panniculitis or, in newborn, a subcutaneous fat necrosis. Neutrophilic panniculitis needs careful clinic-pathologic correlation. Treatment of panniculitis can be challenging and is based on the histopathological findings. Frequently, the precise cause of panniculitis cannot be established from the outset, so it is important to follow-up patients and not hesitate to repeat the skin biopsy.

Panniculitis is a rare cutaneous manifestation of dermatomyositis (DM). The appearance of panniculitis during treatment with methotrexate (MTX) is exceptional and has only been described in 3 cases. We report a case of a 50-year-old woman suffering from DM since 1997 who was treated with corticosteroids showing favorable clinical and biological evolution. When a relapse occurred 2 years later, she was treated with higher-dose of corticosteroids in combination with a 7,5 mg weekly dose of methotrexate. The evolution was rapidly favorable. Eighteen months later, the patient had multiple subcutaneous nodules on limbs and buttocks. Anatomopathological examination showed panniculitis. There was no evidence supporting progression in DM. Prednisone dose was increased to 0.5 mg/kg/day, always in combination with MTX, without any clear signs of improvement. MTX treatment was stopped and the cutaneous lesions completely disappeared in 2 months without any relapse. This objective response lasted for 42 months. Our observation is particular given the occurrence of panniculitis in a patient undergoing treatment for dermatomyositis with methotrexate and illustrates the difficulties in the diagnosis. This entity must be known despite its exceptional nature since cutting off MTX treatment generally induces the disappearance of subcutaneous nodules.

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