Panniculitis

Dirk M. Elston

The inflammatory infiltrate in septal panniculitis spills over into the lobule. The inflammation in lobular panniculitis often involves the septum, therefore, septal and lobular panniculitis are best differentiated by the architecture of the lobule. In septal panniculitis, the lobule is intact; lipocytes are similar in size and shape, and the scan appearance resembles a bowl of toasted oat cereal. In lobular panniculitis, the lobule is necrotic; lipophages are common, and in most instances free lipid accumulates in pools that vary in size and shape. The scan appearance resembles the surface of a bowl of chicken soup. Important exceptions to this rule include conditions that result in solidification of fat (sclerema, subcutaneous fat necrosis of the newborn, and steroid-induced fat necrosis). In each of these conditions, the solidified fat cannot coalesce. Pancreatic panniculitis results in calcification of the necrotic membranes, and also prevents coalescence of lipid.

Fig 16-1 Septal panniculitis

Fig 16-2 Lobular panniculitis

Septal panniculitis

Erythema nodosum is the major form of septal panniculitis. Infections can occasionally produce necrosis and inflammation within the septum, without necrosis of the fat lobule. Alpha₁-antitrypsin deficiency can occasionally result in liquefactive necrosis of the septum without necrosis of the lobule.

Erythema nodosum

Key features

• Septal panniculitis (little to no necrosis of lobule)

• Neutrophils in septum in acute phase

• Mononuclear cells and granulomatous inflammation in chronic phase

• Miescher’s granuloma: central cleft within granuloma

Fig 16-3 Erythema nodosum

Lobular panniculitis

Lupus panniculitis (lupus profundus)

Key features

• Necrosis of fat lobule (“chicken soup”)

• Fibrin and hyaline rings around fat

• Nodular lymphoid infiltrates

Fig 16-4 Lupus panniculitis

Fig 16-5 Pancreatic panniculitis

Although connective tissue disease can produce less specific patterns of granulomatous panniculitis, the features noted above are highly characteristic of lupus panniculitis. Plasma cells are common in the nodular lymphoid foci. Overlying features of lupus erythematosus may be present. The overlying dermis may be sclerotic. Lipomembranous changes may be present (eosinophilic “frost on the windowpane” or “ferning” pattern at the edge of the lipid vacuole).

Pearl

Subcutaneous panniculitis-like lymphoma may have an identical histologic appearance. Features that favor lupus include: interface dermatitis, lymphoid follicles with reactive germinal centers, mixed cellular infiltrate with plasma cells, and lack of T-cell receptor-gamma gene rearrangement. It should, however, be noted that interface change and dermal mucin have been noted in some patients with lymphoma.