Erythema Nodosum

 

Mechanism: hypersensitivity reaction to a variety of stimuli (infection, pregnancy, other)

 

Clinical findings:

(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.

 

Histologic findings:

(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.

Duration	Term
transient (resolve in a few weeks)	typical
intermittent ""crops" of new lesions over a few months	acute recurrent
months or years	chronic

 

Differential diagnosis:

(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