Vesicles or Blisters

PATHOLOGY OF VESICLES

When you consider the mnemonic for the clinical diagnosis of vesicles you remember it is ICI, I for infective both viral, bacterial and fungal, C for contact dermatitis and the second I for immunological and inflammatory.  Histologically a vesicle will look like a space in the epidermis.  The epidermal covering to the vesicle may be thin or may be thick depending on the level at which the vesicle is formed in the epidermis and the area of the body in which it has formed because the thickness of the epidermis and stratum corneum varies across body sites 

Vesicles can form because of keratinocyte necrosis particularly secondary to a herpesvirus infection, but they can also form because of an accumulation of fluid between the keratinocytes as in a spongiotic dermatitis, particularly pompholyx eczema where you have the thick overlying wall of the stratum corneum and epidermis on the fingers, the palms and the soles.  

Vesicles may subsequently become pustules if the clear fluid is infiltrated by neutrophils.  In acute dermatitic vesicles the intraepidermal vesicle is surrounded by some spongiosis in the adjacent epidermis.  There may also be exocytosis of lymphocytes.  Spongiotic vesicles can also be seen in pityriasis rosea, in miliaria crystallina and in some dermatophyte infections and candida and rarely as part of an erythema annulare centrifugum, but again this is just secondary to a large degree of spongiosis. Vesicles and blisters can also occur because of Acantholysis either due to congenital deficiency of adherent molecules such as desmogliens in Hailey Hailey and Darier's disease, or acquired in Pemphigus or Grovers syndrome

If a vesicle has a lot of eosinophils in it then consider Incontinentiapigmenti.  Excess of eosinophils in the epidermis can also be seen in an allergic contact dermatitis, arthropod bite reactions and rarely in both pemphigoid and pemphigus vulgaris and foliaceous.

Vesicles caused by herpes virus are due to damage and lysis of virally invaded keratinocytes.  This results in large multi-nucleated keratinocytes and is seen not only in herpes simplex but also in herpes zoster and also in some common warts or varicella

Occasionally intraepidermal vesicles may occur in erythema multiforme but usually in this condition the damage is at the dermoepidermal junction and any blisters that form are sub-epidermal blisters.

If we now go back and look at a mnemonic of ICI then the infective causes of vesicles are mainly the herpes viruses but also Orf and even Hand, Foot and Mouth disease.  There is also the fungal vesicle variant due to dermatophytes or candida.  Bacteria themselves usually cause blisters rather than vesicles and the split is subcorneal.  It is rare for a bacterial infection to cause an intraepidermal vesicle.

The C is for contact dermatitis. These vesicles may join to form large blisters which are commonly seen in plant contact dermatitis, but it is a mild intraepidermal form that is seen first.

Clinically sometimes subepidermal fluid accumulation can look as if it is causing vesicle formation.  This phenomenon is particularly seen in a severe type of erythema multiforme with vacuolisation of the basal layer and subepidermal bulla formation.  A similar picture can be seen in Sweet's syndrome, again with the severe papillary oedema seen in this condition.  Really it can also be a feature of a polymorphous light eruption.

Dermatitis herpetiformis may also give apparent vesicles, but again the oedema is found in the sub basement membrane layer of the papillary dermis.