ABSTRACT
Panniculitis means inflammation of the subcutaneous tissue. The subcutaneous fat is roughly organized in lobules separated by thin fibrous septa. The blood vessels in the septa are usually larger than the small capillaries seen among the adipocytes. The fat is completely devoid of inflammatory cells, and any amount of inflamma tion in the subcutaneous tissue is abnormal. The hallmark of panniculitis is inflam mation. Other findings are necrosis of the fat, which, on occasions, can be incomplete necrosis, without inflammation such as in the case of lupus panniculitis. Most of the time necrosis of the fat is complete and results in a marked inflammatory reaction with histiocytes, some with foamy cytoplasm, giant cells, lymphocytes, plasma cells and neutrophils. Typically panniculitis has been classified as being predominantly septal or lobular depending on which one of these compartments was primarily involved. The typical example of septal panniculitis is erythema nodosum, while for the lobular panniculitis the classic example is erythema induratum. In reality, if the process has been present for some time, both compartments are involved, and it is difficult to decide if an individual case is more septal than lobular. Another impor tant histologic parameter is the presence or absence of vasculitis. Vasculitis, charac terized by the presence of neutrophils in the wall of the blood vessels, with reactive endothelial hyperplasia and focal necrosis, may involve arteries, veins or venules.
