Primary vesicular-bullous skin lesions include vesicles and bullae. Secondary lesions may include scale, crust, milia, and scarring. These conditions are classified by depth of skin involvement.
Vesicles are circumscribed, fluid-filled epidermal elevations <1 cm in diameter that: 
Contain serous exudates or serum mixed with blood, or are seropurulent in character
Are discrete, grouped (e.g., herpetic infection), irregularly distributed, or linear as in Rhus dermatitis (e.g., poison ivy)
Are short-lived, either breaking spontaneously or evolving into bullae through enlargement or coalescence.
Bullae are blisters >1 cm in diameter containing serous or seropurulent fluid. When bullae are located within the epidermis, they present with thin and flaccid walls, which rupture resulting in erosions, scale, and crust. Erosions are the result of loss of the superficial layers of the epidermis and, in the absence of secondary infection, typically heal without scarring.
The fragility of a blister is often related to anatomical location (e.g., intraoral blisters typically present as erosions due to the fragility of mucosa, irrespective of localisation of the bullae within or below the epidermis).  Friction blisters occur below the corneal layer, but present as tense bullae due to the thick stratum corneum of volar skin.
Bullae located in the epidermis are flaccid and rupture easily. The Nikolsky's sign may be demonstrated in a patient with intraepithelial bullae by the shearing off of the epithelium with lateral pressure on intact skin. For bullae developing in the basement membrane, the Asboe-Hansen sign is often elicited by the extension of the blister into unblistered skin when pressure is applied to the top of the blister.  These signs serve to demonstrate that in some diseases (e.g., pemphigus vulgaris and bullous pemphigoid) the extent of blistering is greater than observed by inspection.
Subepidermal bullae are tense. Subepidermal blisters may result in ulceration and scarring following rupture. Ulcers occur when both the epidermis and superficial dermis are lost, either as a consequence of the blistering process (e.g., epidermolysis bullosa), or resulting from secondary infection and/or trauma. Pigment is lost in the ulcer base as the melanocytes are shed in the blister roof.
Clinical features that should be noted when evaluating a patient with a blistering process are the presence of scale, crust, milia, and scarring.
Scale is laminated masses of keratin.  It may indicate a superficial blistering process such as pemphigus, infection-mediated desquamations, and drug eruptions (e.g., toxic epidermal necrolysis).
Crust is dried exudate composed of serum, cells, and often bacteria. It may be found overlying erosions in pemphigus.
Milia are white keratinous cysts 1 mm to 4 mm in diameter. When present in a blistering process they may suggest epidermolysis bullosa or porphyria cutanea tarda.
Scarring is due to a blister developing at either the dermal-epidermal junction (e.g., basement membrane zone) or the dermis. The presence of urticarial papules, plaques, and/or excoriations, even in the absence of blisters, may be an early indicator of autoimmune blistering diseases (e.g., bullous pemphigoid or dermatitis herpetiformis).
Blisters occur at various depths in the epidermis or dermis and may be classified based on the depth of the skin involved. A broad classification divides blisters into those arising within the epidermis (intraepidermal, which includes subcorneal) and those that develop below the epidermis (subepidermal, which includes the basement membrane zone).
Department of Dermatology
Uniformed Services University of the Health Sciences
JHM declares that he has no competing interests.
Dr Jon H. Meyerle would like to gratefully acknowledge Dr Keira L. Barr, a previous contributor to this topic.
Professor of Dermato-Epidemiology
Center of Evidence Based Dermatology
Nottingham University Hospitals
Queen's Medical Centre
HCW declares that he has no competing interests.
Head of Department
Department of Dermatology
St George Hospital
University of New South Wales
DFM declares that she has no competing interests.
Associate Professor and Clinical Director
Section of Dermatology
University of Chicago
VPR declares that she has no competing interests.
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