[Histological differential diagnosis of psoriasis vulgaris and seborrheic eczema of the scalp].

The clinical differential diagnosis between seborrheic dermatitis and psoriasis vulgaris of the scalp can be difficult. We, therefore, tried to elaborate histopathological criteria for a differentiation of the two dermatoses. Forty excisional biopsies were analysed without knowing the clinical diagnosis. The histopathological substrate within the epidermis is characterized in psoriasis by dermatitis-like and in seborrheic dermatitis by psoriasis-like alterations. Therefore, in some cases a definite histopathological diagnosis could not be made. Strong criteria favouring psoriasis are: moderate condensed hyperkeratosis with alternating parakeratosis, PAS-reactive serum inclusions and Munro abscesses within the horny layer, spongiform pustles and neutrophilic leukocytes within the epidermis. Strong criteria for seborrheic dermatitis are: irregular acanthosis with relatively thin condensed orthoor parakeratotic horny layer, spongiosis and spongiotic vesicles, exocytosis of lymphocytes and the lack of any hard criterias for psoriasis. The results may suggest that seborrheic dermatitis of the scalp may transform into psoriasis in patients with a genetical disposition ("psoriatic diathesis", "latent psoriasis") via a Köbner reaction. The existence of the seborrheic dermatitis (Morbus Unna) is not doubted by these investigations.