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Introduction
Amidst the broad spectrum of inflammatory dermatoses, panniculitis remains one of the most challenging areas of dermatopathology. Most pathologists are accustomed to examining and evaluating the epidermis and superficial dermis, given that the vast majority of skin biopsies do not show subcutaneous tissue. Thus, when faced with a biopsy showing inflammation of the subcutaneous tissue as the primary disease process, it is important to have a method to systematically evaluate the sample.
Many panniculitides present with the physical finding of subcutaneous nodules in common, with or without pain and overlying cutaneous changes. There may be other features from the patient history that can be combined with the features on biopsy to identify a specific diagnosis, including anatomic distribution, vascular compromise, fever, weight loss, arthralgia, drug use, systemic illnesses including autoimmune diseases and immunosuppression.
In order to identify specific pathological findings, the subcutaneous tissue should be adequately sampled through a generous incisional or excisional biopsy including skin and subcutaneous fat. Although a punch biopsy (minimum 6 mm) may provide some subcutaneous adipose tissue, dermal thickness varies by anatomic site and this method may capture only the most superficial portion of the panniculus. A tunnelled biopsy (a series of two punch biopsies to reach subcutis) often results in poor orientation at the time of processing and may not adequately represent the clues harboured in the epidermis and dermis. A portion of the biopsy can be sent for microbiological cultures (if suspected to be infectious) or flow cytometry (if suspected to be lymphoma).
The basic approach to histological evaluation of panniculitis starts with the pattern of inflammation-septal-predominant versus lobular-predominant. 1 2 Once a predominant pattern is selected, the next steps are to identify the presence of vasculitis and calibre of involved vessels, the predominant cell type (neutrophils, lymphocytes, histiocytes) and any specific histological features associated with the various disease entities. 3-5 At this point, clinicopathological correlation is often helpful in confirming the diagnosis. Herein, we focus on the challenges of lymphocyte-predominant panniculitis and provide an algorithmic approach to obtaining a specific diagnosis ( figure 1 ).
Septal-predominant lymphocytic panniculitis
Septal-predominant panniculitis can often be recognised when the inflammatory infiltrate results in septal widening visible on low magnification. Inflammatory changes may spill over into the lobules but...