Panniculitis

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Panniculitis

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

• 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)

• 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).

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.

Pancreatic panniculitis

• Necrosis of the fat lobule with calcium soap outlining necrotic lipocytes

Pancreatic panniculitis may occur in the setting of pancreatitis or pancreatic neoplasm.

Subcutaneous fat necrosis of the newborn

• Necrosis of the fat lobule with crystallization of fat (solid fat cannot coalesce like “chicken soup”)

• Radial crystals in lipocytes

• Granulomatous inflammation

Differential Diagnosis

• Steroid-induced fat necrosis (aka post-steroid panniculitis) looks identical

• Sclerema neonatorum has identical crystals, but no inflammation

Fig 16-6 Subcutaneous fat necrosis of the newborn

Eosinophilic panniculitis

• Eosinophils in septum and lobule

• Necrosis variable

Fig 16-7 Eosinophilic panniculitis

Causes

• Churg–Strauss syndrome

• Helminthic parasite

• Hypereosinophilic syndrome

Suppurative and granulomatous panniculitis

• Lobular panniculitis

• Suppurative and granulomatous infiltrate

• Usually infectious etiology

• May be factitial

Predominantly granulomatous lesions may be associated with lipodystrophy or connective tissue disease.

Nodular vasculitis/erythema induratum (EI) of Bazin

• Necrosis of the fat lobule (“chicken soup”)

• Multinucleated giant cells

• Nodular neutrophilic vasculitis often present in septum

• Caseation necrosis common

EI commonly involves the calves. Liquefied fat may drain from the lesions. Some authorities use the term “nodular vasculitis” when patients are purified protein derivative (PPD) negative, and EI when they are PPD positive. Polymerase chain reaction has been used to identify mycobacterial antigens within lesional tissue.

Alpha₁-antitrypsin deficiency

• Neutrophilic infiltrate

• Fat necrosis and granulomatous infiltrate variable

• Liquefactive necrosis of the septum may be seen

Differential Diagnosis

Infectious etiologies should be excluded. Neutrophilic panniculitis may also occur in myelodysplastic syndromes.

Fig 16-8 Erythema induratum

Fig 16-9 Alpha₁-antitrypsin deficiency

Lipodermatosclerosis (stasis panniculitis)

• Necrosis of the fat lobule (“chicken soup”)

• Lipomembranous change (“frost on the windowpane”)

• Overlying stasis change

Lipomembranous change may be noted in many forms of lobular panniculitis, including lupus panniculitis.

Cytophagic histiocytic panniculitis

• Lobular panniculitis

• “Beanbag cells”

Cytophagic histiocytic panniculitis may follow infection or may be induced by lymphoma. The patient often dies of a hemorrhagic diathesis. Beanbag cells are histiocytes that have engulfed inflammatory cells and erythrocytes.

Fig 16-10 Lipodermatosclerosis

Fig 16-11 Cytophagic histiocytic panniculitis

Traumatic fat necrosis (“mobile encapsulated lipoma”)

• Resembles lobular panniculitis

• Fibrous capsule

Subcutaneous panniculitis-like lymphoma

• Resembles lobular panniculitis

• May closely resemble lupus panniculitis

• Atypical lymphocytes rimming lipocytes

• Natural killer and gamma–delta phenotypes may be seen

Fig 16-12 Subcutaneous panniculitis-like lymphoma

Further reading

Geraminejad, P, DeBloom, JR, 2nd., Walling, HW, et al. Alpha-1-antitrypsin associated panniculitis: the MS variant. J Am Acad Dermatol. 2004; 51(4):645–655.

Gonzalez, EG, Selvi, E, Lorenzini, S, et al. Subcutaneous panniculitis-like T-cell lymphoma misdiagnosed as lupus erythematosus panniculitis. Clin Rheumatol. 2006; 26(2):244–246.

Heymann, WR. Panniculitis. J Am Acad Dermatol. 2005; 52(4):683–685.

Ma, L, Bandarchi, B, Glusac, EJ. Fatal subcutaneous panniculitis-like T-cell lymphoma with interface change and dermal mucin, a dead ringer for lupus erythematosus. J Cutan Pathol. 2005; 32(5):360–365.

Massone, C, Kodama, K, Salmhofer, W, et al. Lupus erythematosus panniculitis (lupus profundus): clinical, histopathological, and molecular analysis of nine cases. J Cutan Pathol. 2005; 32(6):396–404.

McBean, J, Sable, A, Maude, J, et al. Alpha1-antitrypsin deficiency panniculitis. Cutis. 2003; 71(3):205–209.

Phelps, RG, Shoji, T. Update on panniculitis. Mt Sinai J Med. 2001; 68(4–5):262–267.

Requena, L, Sánchez Yus, E. Panniculitis. Part II. Mostly lobular panniculitis. J Am Acad Dermatol. 2001; 45(3):325–361.

Schneider, JW, Jordaan, HF. The histopathologic spectrum of erythema induratum of Bazin. Am J Dermatopathol. 1997; 19(4):323–333.

Sutra-Loubet, C, Carlotti, A, Guillemette, J, et al. Neutrophilic panniculitis. J Am Acad Dermatol. 2004; 50(2):280–285.

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