Differentials
Pemphigus vulgaris
SIGNS / SYMPTOMS
Difficult to distinguish clinically. Skin lesions are usually erosive and do not form tense bullae. Oral lesions more common.
INVESTIGATIONS
Direct immunofluorescence testing for IgG, C3, or both detects a broad linear band on the surface of epidermal keratinocytes in the suprabasilar region of the epidermis.
Epidermolysis bullosa acquisita
SIGNS / SYMPTOMS
The inflammatory form of epidermolysis bullosa acquisita may mimic bullous pemphigoid closely. Blisters tend to appear both spontaneously and as a result of trauma, predominantly on trauma-exposed body surfaces. Lesions heal with significant scarring.[35]
INVESTIGATIONS
Salt-split skin test for indirect immunofluorescence testing detects linear deposits of IgG at the base of the blister cavity (dermal side). Direct immunofluorescence is also necessary.[35]
Linear IgA dermatosis
SIGNS / SYMPTOMS
Linear IgA dermatosis has a bimodal age of onset, as opposed to the predominating older population in bullous pemphigoid. In linear IgA dermatosis, clear and/or haemorrhagic vesicles or bullae arise on normal, erythematous, or urticarial skin with a characteristic 'string of pearls' appearance.[35]
INVESTIGATIONS
Definitive diagnosis is made by direct immunofluorescence, which reveals linear deposition of IgA along the dermal-epidermal junction.[35] The salt-split skin test for direct immunofluorescence testing, simple to perform, is also necessary to differentiate these diagnoses.
Dermatitis herpetiformis
SIGNS / SYMPTOMS
Dermatitis herpetiformis presents with symmetrical, grouped excoriations; erythematous, urticarial plaques; and papules with vesicles located on the extensor surfaces of the elbows, knees, buttocks, and back. Intensely pruritic, the vesicles are often excoriated to erosions by the time of examination. In bullous pemphigoid, lesions may sometimes appear in the mouth, which is not found in dermatitis herpetiformis.
INVESTIGATIONS
Diagnosis requires direct immunofluorescence of a skin biopsy specimen showing deposition of IgA in a granular pattern in the dermal papillae.[36]
Porphyria cutanea tarda
SIGNS / SYMPTOMS
Patients present with serous or haemorrhagic vesicles or bullae on sun-exposed areas of the body, which include the face, dorsal hands, and extensor forearms. The lesions are often painful and heal slowly, with atrophic scars, milia, and post-inflammatory hyperpigmentation.[2] Lesions in bullous pemphigoid tend to be painless and typically do not present haemorrhagic.
INVESTIGATIONS
The characteristic direct immunofluorescence test reveals C3, IgG, and IgM deposition in the capillary walls and at the dermoepidermal junction. On urinalysis, the porphyrin level is usually >1000 micrograms in a 24-hour period. A reddish-pink colour is seen when the urine is examined with a Wood lamp.[2] Routine histology of a lesion demonstrates the presence of a sub-epidermal blister with little inflammatory infiltrate in the dermis. A haematological examination typically shows elevated levels of iron, ferritin, and transaminases.
Erythema multiforme
SIGNS / SYMPTOMS
Erythema multiforme is an acute self-limited eruption, the hallmark of which is the iris or target lesion. Bullous pemphigoid does not have ocular involvement.
INVESTIGATIONS
Presence of typical target lesions and the histopathological finding of interface dermatitis are diagnostic of erythema multiforme. The direct immunofluorescence test may be negative or only detect clumpy deposits of IgM at the dermal-epidermal junction, reflecting necrotic keratinocytes as a result of the interface dermatitis.
Urticaria
SIGNS / SYMPTOMS
Urticaria is a common, variably pruritic eruption of transient erythematous, oedematous papules and plaques that vary in size and shape. Bullous pemphigoid may initially present with urticarial areas that later develop bullous lesions, but plaques do not appear.
INVESTIGATIONS
There are no routine studies for diagnosis. Biopsy is usually not needed and it would not differentiate from the prodromal, non-bullous phase of bullous pemphigoid, although the latter often has a denser inflammatory cell infiltrate greatly dominated by eosinophils.
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