Discussion

Figure 1 shows variably sized red nodules on the lower extremities. The patient described that these were painful on palpation. Figures 2-4 show subcutaneous fat necrosis, saponification and lobular panniculitis, with unremarkable dermis and epidermis.  The described clinical scenario is characteristic of acute pancreatitis. Classic clinical manifestations include severe upper abdominal pain radiating to the back associated with nausea and vomiting. The CT-findings of peripancreatic fluid collection reflect the parenchymal edema and enzymatic peripancreatic fat necrosis characteristic of the early phase of this process.  Amylase and/or lipase levels ≥ 3 times above the reference range are considered diagnostic of acute pancreatitis. The most common predisposing conditions include alcohol abuse and biliary obstruction.  In 2-3% cases of pancreatic disease (neoplastic and non-neoplastic) the increased serum levels of lipase lead to fat necrosis in areas not adjacent to the pancreas.  Subcutaneous fat necrosis in this context is called pancreatic panniculitis (PP). Clinically, it is characterized by the development of erythematous tender subcutaneous nodules that may ulcerate, usually in the lower extremities, but may also affect breasts, buttocks and abdomen. Histologically, it is characterized by lobular panniculitis and fat necrosis.

Differential diagnosis: The differential diagnosis includes subcutaneous abscess/infectious panniculitis. Infection should be suspected in all cases of panniculitis, and especially in immunosuppressed patients, children and older individuals. Pertinent microorganism stains, cultures and correlation with the clinical scenario are necessary to exclude this possibility. Histologically, in an infectious panniculitis the inflammatory cells are usually proportional to the tissue necrosis and the inflammation shows a mixed septal and lobular distribution. Bacterial infections will show a predominantly neutrophilic infiltrate, while fungal and mycobacterial infections will show a significant granulomatous component. By contrast, in PP cases the degree of fat necrosis is out of proportion compared to the inflammatory infiltrate. The inflammatory cells consist of neutrophils at the periphery of the necrosis, and mononuclear cells within the fat lobules.  Lupus panniculitis (lupus profundus) may show identical histology, however it is not associated with pancreatic disease. The inflammatory cells usually include a component of plasma cells.  The epidermis and dermis usually show the characteristic changes associated with lupus (follicular plugging, vacuolization of the basal layer, epidermal atrophy, increased dermal mucin, perivascular/periadnexal monuclear cells). Erythema nodosum, an inflammatory panniculitis secondary to infections, drugs and autoimmune disorders, will have a similar clinical presentation, but histologically it is characterized by septal, not lobular panniculitis. The inflammatory infiltrate may be neutrophilic or mononuclear depending on the age of the lesion. Erythema induratum is an immunologic reaction most commonly to tuberculosis infection. It presents as tender nodules usually affecting the calves. Histologically it is characterized by a lobular panniculitis associated with vasculitis. In contrast to PP, erythema induratum shows a well developed granulomatous pattern, and in the cases where necrosis is present it is usually of the caseating type.  In areas of low prevalence of tuberculosis its presence should prompt evaluation for HIV infection.

References:

García-Romero D, Vanaclocha F. Pancreatic panniculitis. Dermatol Clin. 2008; 26(4):465-70.

Requena L, Sanchez Yus, E. Panniculitis. Part ii. Mostly lobular Panniculitis. J Am Acad Dermatol. 2001; 45(3):325-361

Rosai and Ackerman’s Surgical Pathology, 10th edn; 108–111, 1007-1011

Ronald P Rapini. Practical Dermatopathology, 2005: 213-219