Differentials
Urticaria
SIGNS / SYMPTOMS
Giant urticaria can be annular with central clearance, but central blistering and target lesions will be absent. While urticaria may recur, resolution of lesions in 24 hours is the norm, but virtually never occurs in EM. Daily new lesions occur in urticaria, but not in EM after the first 72 hours.[3] When urticaria strongly resembles EM due to targetoid appearance, it is termed urticaria multiforme. This entity is seen in children and may have an infectious trigger.
Dermatographism (urtication after pressure) is common in urticaria.
INVESTIGATIONS
Biopsies will distinguish the type of inflammatory infiltrate.
Polymorphous light eruption (PMLE)
SIGNS / SYMPTOMS
A hypersensitivity reaction to sun exposure that occurs often on the distal extremities. It presents with neither target lesions nor mucosal disease. Recurrence annually with the initial sun exposure is common.[23]
INVESTIGATIONS
Biopsy can aid in distinguishing from EM.
Phototesting may elicit typical lesions.
No herpes simplex virus infection found.
Subacute cutaneous lupus erythematosus
Allergic contact dermatitis
SIGNS / SYMPTOMS
Contact allergens can elicit an EM-like illness.
Target lesions should be absent, and atypical sites are likely (not extremities).[29]
INVESTIGATIONS
Patch testing will identify offending allergens in many cases.
Molluscum contagiosum
SIGNS / SYMPTOMS
Id reactions that are annular plaques surrounding a molluscum (pearly papules with central dells) can mimic target lesions. Careful inspection of the central zone to identify the pearly papules with a central dell will aid in diagnosis. Lesions caused by molluscum contagiosum will be both itchy and fixed for extended times, as opposed to lesions caused by EM.[44]
INVESTIGATIONS
Unroofing the central zone of a targetoid lesion will demonstrate cheesy white material in molluscum, or fluid for blisters of EM.
Stevens-Johnson syndrome (SJS)
SIGNS / SYMPTOMS
Severe macular (atypical target) lesions that coalesce, resulting in epidermal blistering, necrosis, and sloughing, with no typical target lesions. Less than 10% total body surface area is affected. May present with only mucosal involvement. Drug-induction is also a clue to the diagnosis of SJS.[2][3][4][5]
INVESTIGATIONS
Biopsy shows minimal inflammation in SJS.
Toxic epidermal necrolysis (TEN)
SIGNS / SYMPTOMS
Severe macular (atypical target) lesions that coalesce, resulting in epidermal blistering, necrosis, and sloughing, with no typical target lesions. Denudation of 30% or more total body surface area, drug triggers, and multiple mucous membrane involvement are typical of TEN.[2][3][4][5]
INVESTIGATIONS
Biopsy shows minimal inflammation in TEN.
SJS-TEN overlap
NK/T-cell lymphoma
SIGNS / SYMPTOMS
Cutaneous lesions may mimic EM.
INVESTIGATIONS
Biopsy and flow cytometry can distinguish from EM.
Secondary syphilis
SIGNS / SYMPTOMS
Scattered scaling papules and plaques.[16]
INVESTIGATIONS
Rapid plasma reagin positive for syphilis.
Paraneoplastic pemphigus
SIGNS / SYMPTOMS
Auto-immune blistering condition associated with the presence of a malignancy (usually lymphoproliferative disease).
INVESTIGATIONS
Biopsy, flow cytometry, and immunofluorescence reveal histological and immunological features.
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