Blisters are accumulations of fluid within or under the epidermis. They have many causes, and a correct clinical diagnosis must be based on a close study of the physical signs.
The appearance of a blister is determined by the level at which it forms. Subepidermal blisters occur between the dermis and the epidermis. Their roofs are relatively thick and so they tend to be tense and intact. They may contain blood. Intraepidermal blisters appear within the prickle cell layer of the epidermis, and so have thin roofs and rupture easily to leave an oozing denuded surface: this tendency is even more marked with subcorneal blisters, which form just beneath the stratum corneum at the outermost edge of the viable epidermis, and therefore have even thinner roofs.
Sometimes the morphology or distribution of a bul-lous eruption gives the diagnosis away, as in herpes simplex or zoster. Sometimes the history helps too, as in cold or thermal injury, or in an acute contact derm-atitis. When the cause is not obvious, a biopsy should be taken to show the level in the skin at which the blis-ter has arisen. A list of differential diagnoses, based on the level at which blisters form, is given in Fig. 9.1.