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[|I absolutely agree with the
statement of the value of biopsies in psoriasis.|auteur195]

[|However, the classical histopathological picture as described in the
chapter on histopathology is seen only in a minority of the biopsies taken from the psoriatic
plaque. In particular, the Munro micro abscesses and micro pustules of
kogoj can be difficult to find. Acute psoriatic papules and the margin zone of
spreading psoriatic plaques more frequently show the complete histopathological
picture as described above. Therefore, for diagnostic purposes biopsies have to
be taken from the margin zone or from psoriatic papules.|auteur197]

[|The eight major characteristics, together with micrographs, are well-described. In addition to the difficulty in distinguishing psoriasis from eczema on the palms and soles, I also do feel that erythrodermic psoriasis, with or without pustulation, also is frequently non-specific with overlap features between eczema and psoriasis. Certainly, I believe that immunohistochemistry may frequently be of more value in distinguishing psoriasis from its clinical and histological “look alikes”, eg eczema, cutaneous T-cell lymphoma, etc.|auteur215]

The typical histological appearance of psoriasis is well known (photo 40). It is characterized by:

  • parakeratosis,
  • presence of Munro microabscesses (photos 41 and 42),
  • absence of a granular layer except on the nail, where conversely hypergranulosis exists,
  • epidermal acanthosis pertaining solely to the interpapillary (rete) ridges,
  • elongation and edema of the dermal papillae,
  • thinning of the superjacent malpighian stratum,
  • increase in the number of mitoses,
  • major proliferation of the endothelial cells with hypertrophy and vasodilatation of the papillary capillarie

Photo 40. Photo 41. Photo 42. Pustular psoriasis is characterized by subcorneal multilocular pustules.

This typical histological appearance will only be found in clinically typical lesions (photo 43). When it comes to the palms of the hands and the soles of the feet, a spongiosis is practically always observed. Histological examination will not allow eczema to be differentiated from psoriasis in these localisations.

Photo 43. Thus, histological examination in psoriasis is only useful in two contexts: for securing histological evidence in an illness whose management will be reflected over the long term, and for ruling out other illnesses: parapsoriasis en plaque, lichen and so on.

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