Erythema nodosum is a common form of panniculitis that characteristically involves the lower legs.
Mechanism: hypersensitivity reaction to a variety of stimuli (infection, pregnancy, other)
(1) The usual finding consists of tender erythematous subcutaneous nodules or plaques over both lower legs, especially the pretibial area (shins).
(2) Similar lesions may appear elsewhere.
(3) Systemic findings of fever, malaise and arthralgias may occur, depending on the underlying precipitating factor.
(4) Older lesions may appear violaceous or resemble bruising.
(1) There is a septal panniculitis, with variable extension into the lobule.
(2) Early lesions show neutrophils with increasing numbers of lymphocytes and histiocytes.
(3) Later lesions show granulomas and foreign body giant cells
(4) Miescher's granulomas are small collections of histiocytes in the septa arranged around a central cleft.
(5) Perivascular inflammation or patchy vasculitis is present but may require multiple sections to identify.
(6) Hemorrhage into lobules and/or septae is common.
(7) Focal necrosis with foamy histiocytes may be present.
(8) The granulomas are negative under polarizable light.
(9) Resolving lesions show septal fibrosis.
transient (resolve in a few weeks)
intermittent ""crops" of new lesions over a few months
months or years
(1) foreign body granulomas
(2) infection with mycobacteria or deep fungal pathogens
(3) nodular vasculitis or erythema induratum
(4) traumatic panniculitis
(5) alpha-1-antitrypsin deficiency
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