(in Pathology of the Skin, Philip McKee )
The histopathological features of dermatitis include dermal and epidermal changes, their relative proportions varying to some extent with the subtype but, more importantly, with the stage of evolution of the disease. It is essential not to consider the changes of dermatitis as static; different features are seen at different stages. There is little gain in attempting to distinguish the various clinical subtypes using histological methods. Instead, once the disorder has been recognized as dermatitic in nature, clinical examination is a much more satisfactory method of determining the particular variant.
The histological hallmark of dermatitis is the presence of intercellular oedema or spongiosis (L, Gr spongia, sponge). Slight degrees of intracellular oedema may also be evident. In the early stages of development, spongiosis results in widening of the intercellular spaces, thus rendering the intercellular bridges conspicuous. Further accumulation of fluid leads to the eventual development of an intraepidermal vesicle. An extreme degree of spongiotic vesiculation is sometimes called reticular degeneration. A constant finding in association with the oedema is lymphocytic infiltration of the epidermis (exocytosis). In severe acute reactions the lesions very often become traumatized and may show marked crusting.
Concomitant with these changes are varying degrees of epithelial proliferation, ranging from mild acanthosis in early acute dermatitis through to a psoriasiform epidermal hyperplasia in chronic variants. Parakeratosis is frequently seen overlying spongiotic foci whilst hyperkeratosis is a usual accompaniment of chronic dermatitis.
The dermis is often congested and oedema is usually marked in active lesions. The vessels of the superficial vascular plexus are surrounded by a mixed inflammatory cell infiltrate composed of lymphocytes, neutrophils and occasional eosinophils.
Traditionally, dermatitis is subclassified histologically into acute, subacute and chronic variants. In acute lesions vesiculation is marked and bullae are frequent. Subacute dermatitis is also characterized by marked spongiosis with vesiculation but in a much less degree, and acanthosis is usually present. In chronic dermatitis, although spongiosis is evident, vesicles are rare. Epithelial proliferation is marked and often shows psoriasiform change.
Stasis dermatitis shows, in addition to the epithelial changes, marked haemosiderin deposition in the dermis accompanied by fibrosis and new blood vessel formation.
The features of seborrhoeic dermatitis deserve special mention as they can so easily be confused with psoriasis. This variant of dermatitis is typified by parakeratosis and a psoriasiform hyperplasia of the epidermis. Occasionally, intraepidermal polymorphs are present. In contrast to psoriasis, however, seborrhoeic dermatitis shows foci of spongiosis.
Whilst spongiosis is a characteristic feature of dermatitis, it is also seen in many other inflammatory dermatoses, particularly superficial dermatophytoses (a periodic acid‑Schiff reaction is always of value in spongiotic lesions) and pityriasis rosea, and occasionally in herpes gestationis and erythema multiforme. It is typical of miliaria rubra and is a feature of polymorphic eruption of pregnancy.
Lichenification and, hence, the appearance of chronic dermatitis, may be a feature of any pruritic, excoriated disorder. Thus, whilst it is obviously seen in all chronic forms of dermatitis, it is also a complication of many other conditions, such as psoriasis and lichen planus.