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Septal Panniculitis
Erythema Nodosum
If attention is given solely to histologic features, panniculitides generally are difficult to subclassify.
There is some utility in dividing the panniculitides into septal and lobular categories, depending on the distribution of the inflammatory infiltrates. The distinctions, however, are relative; septal lesions may
show minor lobular components and lobular lesions commonly have septal components. In the category of lobular patterns, if the infiltrates are prominently lobular, then this component generally should be given
primacy, regardless of the character of the septal component. In practice, it seems likely that most of the panniculitides are actually vasculitides of the subcutaneous tissue (even though most of the infiltrates
are lymphohistiocytic in character). Physiologic mechanism may influence the expressions of inflammation in subcutaneous fat; in cold weather, blood may be shunted to the lobules away from the dermis. Hot weather
may influence the vascular plexus in the opposite manner with increased circulation to the dermis and the septa, and a decrease in the blood flow to the lobules. Similarly, the metabolic activity of adipose tissue
may influence the vascularity of the fat and, in turn, influence the distribution of inflammation in response to injury.
Included in the category of septal panniculitides are those disorders generally classified as erythema nodosum.
Some lobular disorders, clinically in a distribution that is typical of erythema nodosum, may be nodular, but to include lobular disorders in the septal category would defeat the utility of the histologic category.
In spite of the clinical characteristics, these lobular variants are better classified as something other than classic erythema nodosum. It, perhaps, is more appropriate to reserve the classical designations as
clinically descriptive terms; at the histologic level, histologic characterizations seem to be preferable.
The early changes of lymphohistiocytic septal panniculitis (erythema nodosum) are characterized by little
alteration in the physical dimensions of the affected fibrous septa. Inflammation is mostly perivascular in the septa and lymphohistiocytic in character. A rather characterisitic, but uncommon lesion of early
erythema nodosum, is a minute palisaded granuloma. Small histiocytes cluster radially around a small angulated cleft in the inflamed septum; this distinctive lesion is one of Miescher’s granulomas (Meischer
described a variety of granulomas) of erythema nodosum (S10,C3,P1-1 & 2).
Better developed lesions of lymphohistiocytic septal panniculitis (erythema nodosum-like) show widening of the
septa with fibrosis, edema, fibrin deposits, and perivascular and interstitial infiltrates of lymphocytes and histiocytes. The fibrous tissue and the infiltrates extend from the septa into the neighboring adipose
tissue; in this manner, the septa are widened by the process of substitutive fibrosis (newly formed fibrous tissue creeps from the periphery toward the center of affected lobules). Small granulomas and
multinucleated giant cells collect in the fibrous septa and, even more prominently, they collect in the newly formed fibrous tissue which extends into the fat. The granulomatous reaction, in part, may be a response
to damaged adipose tissue (S10,C3,P1-3 & 4). Old, chronic lesions with both marked septal changes and
prominent granulomatous changes are sometimes characterized as subacute migratory panniculitis.
Requena L, Yus ES: Panniculitis. Part 1. Mostly septal panniculitis. J Am Acad Dermatol 2001; 45:163-83.
Tarroch X, et al: Subcutaneous nodules in Whipple’s disease. J Cutan Pathol 2001;28: 368-70.
Lobular Panniculitis
(lipogranulomatous reaction)
In evaluating patterns of lobular inflammation in adipose tissue, the basic reaction of fat to injury must be a
consideration. Damaged lipocytes may degenerate but, in the act, may leave behind pools of free lipids. The pools of lipids appear as rounded, empty defects; they are symmetrically rounded, but vary in size.
The lipid material evokes a histiocytic response that may be lipophagic, or frankly granulomatous. In the granulomatous category, a clue to the nature of the granuloma is the presence of symmetrically rounded
defects in the center of some of the granulomas. These combined patterns of rounded defects and granulomatous components qualify as a lipogranulomatous response (S10,C4,P2-1-4). In the past, this type of reaction was given recognition as a specific category of lipogranulomatous panniculitis (Rothman and Makai). In some examples of lipogranulomatous reaction, the defects come to be outlined by convoluted membranes (a peculiar reaction of histiocytes to free lipids in the tissue. These variations may include irregular deposits that bulge into the lumens of the defects; the bulges and convolutions may be mistaken for parasites (S10C4aP2a-1-5).
Diffuse, Neutrophilic, Lobular Panniculitis
An uncommon pattern of lobular panniculitis is characterized by diffuse, interstitial (among lipocytes)
infiltrates of neutrophils. In contrast to a cellulitis or fasciitis, the infiltrating cells do not tend to break down to produce necrosis and suppuration (S10, C5,P3-1-3). In some examples as a lesion evolves, histiocytes replace the neutrophils as the preponderant component of the
interstitial infiltrates (S10C6P4-1-3). Some aspects (suppuration not being one of them) of these reactions might be
compared to the interstitial infiltrates of neutrophils that are a basic part of the interstitial reaction in the “leukocytoclastic angiitides,” or Sweet’s syndrome of the dermis. This type of reaction is seen in
early lesions in the setting of so-called Weber-Christian disease. In later stages, lipogranulomatous changes (a basic reaction of adipose tissue to injury) may be the chief histologic feature.
Calcifying Panniculitis
A distinctive, lobular panniculitis is occasionally a feature of patients with renal failure who are on
dialysis. It is characterized by both septal and lobular alterations. The lobular changes tend to be in the nature of coagulative necrosis with preservation of ghost outlines of lipocytes (increased acidophilia and
loss of nuclear staining). Lipocytes degenerate to form lipid-filled cysts; in turn, the cysts evoke an lipogranulomatous response. Small vessels of the lobules contain fibrin thrombi. Calcification of the media of
small, muscular vessels is an important feature of the reaction; it is often associated with a delicate, fibrous hyperplasia of the intima and narrowing of the lumen of the affected vessels (S10C5P3-4-8). Calcification of elastic fibers in patterns which resemble the changes of pseudoxanthoma elasticum may also be a feature (S10C5aP3a-1-8). The thrombotic process may involve vessels of the dermis and lead to areas of necrosis and
ulceration. The process is associated with abnormal levels of calcium and phosphorus in the blood. I would favor the interpretation that thrombosis of vessels is the primary insult. The calcification of muscular
vessels is indicative of an abnormality in the blood level of calcium and phosphorus; it is evidence of a basic affinity of elastica for deposits of calcium.
Walsh J, Fairley JA: Calcifying disorders of the skin. J Am Acad Dermatol 1995;33:693-706.
Granulomatous Lobular Panniculitis
Granulomatous patterns are common in inflammatory processes affecting adipose tissue. Lipogranulomatous
reactions are discussed above. Some examples are relatively pure granulomas with little evidence of either necrosis, or non-specific infiltrates of inflammatory cells. For such lesions, if special stains are
negative, sarcoidosis should be mentioned in the differential diagnosis. Some examples are associated with non-specific infiltrates of inflammatory cells and are necrotizing and destructive; vasculitic changes may
be represented (S10C7P5-1-6). For such examples, special stains are indicated; an infectious process should be
ruled out. If stains and cultures (and molecular probes) are negative, such patterns might then suggest the possibility of erythema induratum (another clinical designation of dubious value in histologic
classifications), a necrotizing, granulomatous lobular panniculitis.
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