Infectious panniculitides: an update

Very few areas in the realm of diagnostic dermatopathology may be so challenging both for the dermatologist and the histopathologist as are those related to panniculitis, because of their frequent overlapping microscopical patterns and uncertain etiology. Classically, a dicotomic taxonomy key has been proposed according the prevalent involvement of subcutaneous septa or lobules of the inflammation, presence or absence of vasculitis and type of vessel involved, but exceptions to this approach do occur and overlapping forms are sometimes encountered. Infectious panniculitides have also been traditionally approached according to this schema, and their microscopic diagnosis may be even more complex when the causative agent is unknown and underrepresented in the specimen. Many types of pathogens are capable to evoke protean clinical manifestations, which range from organism-specific to aspecific pictures. For this reason a tissue biopsy is always mandatory to ascertain the type of lesion and differentiate an infectious process from its many other mimickers essentially represented by reactive-based panniculitides. Recognition of morphologic changes which characterize distinct infections in the subcutis often needs a complete clinical history, physical examination and laboratory studies, especially when few microorganisms if any are found. This review will be focussed on the pathophysiology of the adipose tissue in relation to immunity and mechanisms of host reaction. The most frequent infectious panniculitides will then be discussed with special reference to their microscopic pictures, to provide clues to their specific diagnosis and the use of immunohistochemistry and molecular biology techniques as ancillary techniques.