The panniculitides represent a group of heterogeneous inflammatory diseases that involve the subcutaneous fat. The specific diagnosis of these disorders requires histopathological study because different panniculitides usually show monotonous clinical appearance, namely subcutaneous erythematous nodules on the lower extremities. Histopathological study of panniculitis is also challenging because of an inadequate clinicopathological correlation and the evolutionary nature of the lesions. Often, biopsy specimens are taken from late‐stage lesions, which results in non‐specific histopathological findings. In addition, large‐scalpel incisional biopsies are required. However, with adequate biopsy samples a histopathological differential diagnosis between a mostly septal and a mostly lobular panniculitis is straightforward and with adequate clinicopathological correlation, a specific diagnosis may be rendered in most cases of panniculitis. Mostly septal panniculitides with vasculitis include superficial thrombophlebitis and cutaneous polyarteritis nodosa (cutaneous arteritis). Septal panniculitides with no vasculitis may appear as the consequence of dermal inflammatory processes extending to the subcutaneous fat, such as necrobiosis lipoidica, scleroderma, subcutaneous granuloma annulare, rheumatoid nodule and necrobiotic xanthogranuloma, whereas in other cases the inflammatory process involves primarily the connective tissue septa of the subcutis with no participation of the overlying dermis. The most frequent septal panniculitis is erythema nodosum. In contrast, the most common lobular panniculitis with vasculitis is erythema induratum of Bazin (nodular vasculitis). Mostly lobular panniculitides without vasculitis comprise a large list of disorders, including sclerosing panniculitis (lipodermatosclerosis), subcutaneous fat necrosis of the newborn, panniculitis associated with connective tissue diseases, pancreatic panniculitis,
1
‐antitrypsin deficiency‐associated panniculitis, infective panniculitis and factitious panniculitis. In recent years, several cases of both septal and lobular panniculitis have been described as a consequence of the administration of new drugs, including immune checkpoint inhibitors and
BRAF
inhibitors used as treatment of metastatic melanoma, tyrosine kinase and Bruton tyrosine kinase inhibitors used for treatment of leukemia and other haematological malignancies and tumour necrosis factor inhibitor drugs.
