Development duration and distribution

Several diseases may present with blisters or pustules. There is no common condition that can be used as a "reference point" with which less usual lesions can be compared in the same way as rashes can be compared with psoriasis. A different approach is needed for the assessment of blistering or pustular lesions, based on the history and appearance, and is summarised as the three Ds: development, duration, and distribution.


Was there any preceding systemic illness—as in chickenpox, hand, foot, and mouth disease, and other viral infections? Was there a preceding area of erythema—as in herpes simplex or pemphigoid? Is the appearance of the lesions associated with itching—as in herpes simplex, dermatitis herpetiformis, and eczematous vesicles on the hands and feet?


Some acute blistering arises rapidly—for example, in allergic reactions, impetigo, erythema multiforme, and pemphigus. Other blisters have a more gradual onset and follow a chronic course—as in dermatitis herpetiformis, pityriasis lichenoides, and the bullae of porphyria cutanea tarda. The rare genetic disorder epidermolysis bullosa is present from, or soon after, birth.


The distribution of blistering rashes helps considerably in making a clinical diagnosis. The most common patterns of those that have a fairly constant distribution are shown.

Itching is a very useful symptom. If all the accessible lesions are scratched and it is hard to find an intact blister it is probably an itching rash.

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