EPIDERMIS

Epidermal diseases usually show psoriasiform or spongiotic reaction patterns with varying degrees of hyperkeratosis and hyperplasia or acanthosis. The archetypical examples are psoriasis and eczema but the many diseases seen with an epidermal histology reaction pattern are best remembered by the mnemonic PMs PET a small cat called PETAL

P - Pityriasis rosea, P versicolor, P rubra pilaris, P lichenoides,
M - Mycosis fungoides
s - Solar damage, Scabies

P - Psoriasis
E - Eczema
T - Tinea and other fungi
A - Annular erythema
L - Lupus erythematosus, Lues (syphilis). Lichen planus, Light exacerbated Darier's, Linear verrucous epidermal nevus, Lichen striatus,

Some interface pathology diseases can involve the epidermis sufficiently to cause epidermal changes eg Erythema multiforme  and also dermal processes which perforate will cause epidermal changes eg Perforating folliculitis in renal failure, perforating deep fungal infections and TB or elastosis perforans serpiginosa with perforating elastic fibres.

Elastosis perforans serpiginosa

So look at a slide.

Is the pathology Epidermal or Dermal or Both. If Epidermal use the Mnemonic PMs PETAL
If dermal use the mnemonic CUL DVA EVIE (See DERMIS) link opposite.

If Both then use both mnemonics.
Look for any other epidermal or dermal features or clues and then work through the diseases in the mnemonics to see which might best fit the histology in front of you.

If you see Pustules in the epidermis the mnemonic is II

I - Infective  Viral, Bacterial, Fungal, Rickettsial, Protozoa
I Inflammatory eg Drugs, Pustular psoriasis, Rare ( Scabies, Necrolytic migratory erythema, Subcorneal pustular dermatosis, Erosive pustular dermatosis, Acrodermatitis enteropathica)

If you see Vesicles or Blisters the mnemonic is ICI

I - Infective Viral Bacterial Fungal
C - Contact Dermatitis
I - Inflammatory - Drugs, Porphyria, Insect bite, Genetic bullous diseases
I - Immunological - The immune bullous diseases

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