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Inflammatory diseases of the subcutaneous fat are a heterogenous group of conditions with quite different etiology and pathogenesis. Panniculitis may develop due to infectious organisms, a drug reaction, thermal or other physical injury, an autoimmune phenomenon, or as a sign of metabolic disease. The clinical presentation of panniculitis in the form of infiltrated subcutaneous nodules is often difficult to diagnose with specificity. For example, differentiation of panniculitis from vasculitides involving medium-sized vessels of the subcutaneous fat is clinically challenging. Therefore, interpretation of a biopsy specimen has always played an integral and important role in the diagnosis of inflammatory diseases of the subcutaneous fat (Table 11-1).

TABLE 11-1Evaluation of Panniculitis

However, the study and classification of inflammatory processes that involve the adipose tissue is one of the more difficult subjects in all of dermatopathology.1-5 Why does panniculitis pose such a challenge to dermatopathologists? The main reasons for that are the following: (1) in general, panniculitis is relatively uncommon, (2) biopsies often are too superficial and do not sample the affected subcutaneous tissue adequately, (3) the clinical findings for many panniculitides are fairly similar so that clinicopathological correlation may not help to solve a particular case. Based on these points, dermatopathologists have always been at risk to ascribe too much specificity to morphologic observations made on few samplings taken from small numbers of patients.

It must be emphasized that the response of adipose tissue to injury is limited, so that panniculitides of different etiology and even vasculitides of medium-sized blood vessels may share considerable histomorphologic features (eg, foamy macrophages, cystic fat necrosis, calcification, sclerosis). Moreover, just as with all other inflammatory skin conditions, morphologic features in panniculitides are highly variable in the course of their evolution over time.5 Lastly, many of these conditions have not been studied at the molecular level in detail, and thus inadequate information is available to date concerning their etiology and pathogenesis.

To gain a better understanding of pathologic processes in the subcutaneous fat, it is necessary to be aware of (1) the normal histology of adipose tissue and (2) the reactions ...

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