[Panniculitis : Patterns and clues]

Inflammatory infiltrates of the subcutis are a particular challenge in routine histopathology as they occur relatively rarely in submitted specimens and often only superficial portions of adipose tissue are captured in biopsies. On the other hand, panniculitides are very heterogeneous in etiology. They can arise as a reaction to physical stimuli, in metabolic disorders, as a drug reaction, in autoimmune diseases or due to pathogens. The correct classification of infiltrate patterns of adipose tissue and knowledge of associated features and diagnostic clues are essential for narrowing down the differential diagnosis. This is illustrated using more common panniculitides (lupus profundus, erythema nodosum, lipodermatosclerosis). In addition, rarer adipose tissue diseases (alpha‑1 antitrypsin deficiency, pancreatic panniculitis, morphea profunda, eosinophilic fasciitis) and vasculitides that should be considered in the differential diagnosis are discussed.