Panniculitis: definition of terms and diagnostic strategy.

Inflammation of the subcutaneous tissue represents a dynamic process that shows different histopathologic findings at different stages of development; therefore, the stage of evolution of a lesion at the time of biopsy influences the microscopic appearance significantly. Furthermore, location and type of inflammation may vary among different examples of the same panniculitis independent of the stage of evolution. For these reasons, the histopathologic diagnosis of panniculitides is often difficult. Currently, the most common approach to diagnosis is differentiation between predominantly septal and predominantly lobular panniculitis, followed by the distinction between lesions with and without vasculitis. Although these criteria are important for diagnosis, they are often insufficiently specific. To determine an alternative method of diagnosis, 329 cases of panniculitis were histopathologically analyzed using the following parameters: location and type of inflammatory infiltrate within and around the subcutaneous tissue, presence or absence of fat necrosis, type of necrosis, presence or absence of vascular changes, and presence or absence of associated findings (e.g., hemorrhage, sclerosis). On the basis of the results of this study and of an extensive review of the literature, tables of histopathologic findings for the diagnosis of panniculitides are presented.