When we are looking at histopathology of the skin in terms of inflammatory skin diseases or even including some of the skin malignancies, the correct way to do it is to look at the mnemonics that we use for the diagnosis of skin disease and consider these when looking at the histological specimen. For instance the red scaly diseases are all going to have histopathological changes in the epidermis of one sort or another. In general terms these are going to be either psoriatic or spongiotic. If you have a tumour that is involving the epidermis then again you are going to have some degree of scale and usually the changes that you are going to see are going to be acanthosis when there is thickening of the epidermis, some hyperplasia of the epidermis and hyperkeratosis when there is thickening of the stratum corneum.
If you look at the mnemonic for the red scaly disease PMs PETAL then you are considering diseases such as psoriasis, eczema, tinea. The A is for annulare erythema, the L for lupus or lichen planus, for some forms of light eruption and for lues or syphilis. It also can be part of pityriasis lichenoides, although that also comes under the initial Ps. If you look at the first P in the mnemonic then it is pityriasis rosea, pityriasis versicolor, pityriasis alba, pityriasis lichenoides, pityriasis rubra pilaris. The M if for mycosis fungoides which is a T-cell lymphoma and the small 's' is due to solar damage and rarely scabies! In mycosis fungoides the tumour is a lymphocyte but the epidermis is involved because the lymphocytes invade the epidermis and when they irritate it then you start to get some scale associated with it.
If you look at the mnemonic for the red scaly disease PMs PETAL then you are considering diseases such as psoriasis, eczema, tinea. The A is for annulare erythema, the L for lupus or lichen planus, for some forms of light eruption and for lues or syphilis. It also can be part of pityriasis lichenoides, although that also comes under the initial Ps. If you look at the first P in the mnemonic then it is pityriasis rosea, pityriasis versicolor, pityriasis alba, pityriasis lichenoides, pityriasis rubra pilaris. The M if for mycosis fungoides which is a T-cell lymphoma and the small 's' is due to solar damage and rarely scabies! In mycosis fungoides the tumour is a lymphocyte but the epidermis is involved because the lymphocytes invade the epidermis and when they irritate it then you start to get some scale associated with it.
Hence, when you come to look at a histological slide, you look initially at the epidermis and see if it is normal or abnormal. You look at the stratum corneum and see if there is any thickening there. See whether there is orthokeratosis or parakeratosis. You tend to get parakeratosis when you get hyperproliferative disorders such as psoriasis or pityriasis rubra pilaris or malignancies such as SCC in situ. Orthokeratosis can be seen in a variety of conditions and alternating also with parakeratosis as a feature of pityriasis rubra pilaris.
A complicating factor in this is that psoriasis can sometimes be itchy and you can get it rubbed and if you get it rubbed you can get eczematous features superimposed on it. This is particularly the case in the lower legs. So everything is not black and white, there are shades of grey, but certain features will predominate and it is those predominating features that allow you to make the diagnosis. For instance, if there is a lot of spongiosis it is more likely to be a dermatitic process than it is going to be a psoriatic process. If you have other epidermal features that are in keeping with psoriasis then a little bit of spongiosis shouldn't put you off that diagnosis.
A complicating factor in this is that psoriasis can sometimes be itchy and you can get it rubbed and if you get it rubbed you can get eczematous features superimposed on it. This is particularly the case in the lower legs. So everything is not black and white, there are shades of grey, but certain features will predominate and it is those predominating features that allow you to make the diagnosis. For instance, if there is a lot of spongiosis it is more likely to be a dermatitic process than it is going to be a psoriatic process. If you have other epidermal features that are in keeping with psoriasis then a little bit of spongiosis shouldn't put you off that diagnosis.
So, have a look at the slide. Look to see if there are changes in the epidermis and if there is, then consider these possibilities.
Other changes that can occur in the epidermis though may be due to damage from a Type 2 immune reaction, often a drug reaction. There you get features of epidermal necrolysis but you also get a lot of damage at the dermal epidermal junction and you get a much more inflammatory infiltrate. Though there still may be scale or the like, it is debatable where the primary pathology actually is and which mnemonic you should use.
