erythema multiforme

WKP

PO

Definition : Erythema multiforme (EM) is a skin condition of unknown cause; it is a type of erythema possibly mediated by deposition of immune complexes (mostly IgM-bound complexes) in the superficial microvasculature of the skin and oral mucous membrane that usually follows an infection or drug exposure.

It is an uncommon disorder, with peak incidence in the second and third decades of life.

The disorder has various forms or presentations, which its name reflects (multiforme, "multiform", from multi- + formis). Target lesions are a typical manifestation.

Two types, one mild to moderate and one severe, are recognized (erythema multiforme minor and erythema multiforme major).

Microscopy

 subepidermal bullae

• basement membrane in bullae roof due to dermal edema

 dermal inflammatory infiltrate (includes lymphocytes, histiocytes)

• eosinophils may be present.
• neutrophils are sparse or absent.

 Overlying epidermis :

• epidermal liquefactive necrosis
• epidermal degeneration
• dyskeratotic keratinocytes / dyskeratosis

 May also have dermoepidermal bullae with basal lamina at floor of bullae
 Variable epidermal spongiosis and eosinophils
 No leukocytoclasis, no microabscesses, no festooning of dermal papillae

 Erythema multiforme may have variable histologic changes from toxic epidermal necrolysis to dermal disturbances.

Differential diagnosis

 acute graft versus host disease / acute cutaneous VHD

• basal layer vacuolization and necrosis
• spongiosis
• apoptosis
• acantholysis
• chronic inflammation of upper dermis with perivascular lymphocytic infiltrate and intraepidermal lymphocytes

 fixed drug reaction

• eosinophils
• marked vascular wall thickening

 Steven-Johnson syndrome or toxic epidermal necrolysis

• full thickness epidermal necrosis with separation of epidermis from dermis
• necrotic keratinocytes at edge of bullae

 subacute cutaneous lupus erythematosus

• fibrinoid necrosis at dermoepidermal junction with liquefactive degeneration and atrophy of epidermis / epidermal atrophy