Thursday, October 23, 2008

Introduction to Differential Diagnosis in Dermatology

There are over 2000 named diseases in dermatology but common things occur commonly. They just sometimes look a bit different! In practice these common conditions can be diagnosed using 4 mnemonics. This approach will not appeal to the purists amongst you but it works. In a busy general practice it gives you a basis on which to approach most of the common rashes you will see.
Click the arrow to start the video below then click the wheel at the base and reset quality to HD. View  on YouTube by clicking the link at the base or Full screen by clicking the outside box at the base.

You just ask yourself the following 3 questions.
1. Is the rash skin coloured or red AND is it scaly or non scaly?
2. Are there pustules or blisters?
3. Is it a funny shape, colour, texture or distribution?
The mnemonics are as follows. The red rashes are the commonest in white people.
1 PMs PET for red scaly diseases
2. CUL DVA EVEI for red non scaly diseases
3. II for pustular diseases
4. ICI for Blistering diseases.
1. Diagnosing skin diseases is not difficult. You look at a rash and decide if it is red and scaly or red and non scaly. If it is red and scaly you use the mnemonic PMs PET (PET is Psoriasis, Eczema and Tinea. This is the Prime Minister's Pet ( I used to always think of Kevin Rudd with a siamese cat called Petal sitting on his lap!) The first P of PM is for Pityriasis rosea or Pityriasis versicolor and the M is for Mycosis fungoides, a T cell lymphoma of the skin.) View red scaly rashes
Now we know that his pet cat is called PETAL. This helps us to remember Psoriasis, Eczema and Tinea but also the less common red scaly diseases of A for Annular erythemas and L for Lupus erythematosus ,Lichen Planus, Light eruption and Lues (an old name for syphilis)

2. If it is red but not scaly consider Cellulitis, Urticaria, Lupus, Light eruption, Drug reaction, Viral exanthem or Annular erythema .The mnemonic is C U L at the Department of Veterans Affairs Evei (your girlfriend Evei) (CUL DVA EVEI) where EVEI stands for Erythema multiforme, Vasculitis and Erythema nodosum and Infiltrates View the red non scaly rashes
3. If there are Pustules then the mnemonic is II
(aye aye) Infective( viral, bacterial, fungal) or Inflammatory eg psoriasis or a pustular drug reaction or Sweet's syndrome. Common causes include Staph folliculitis , modified fungal infection or if the vesicles are grouped herpes simplex. Pustules on the face are Acne, Rosacea, Staph folliculitis or H Simplex if grouped. View the pustular rashes

4. If there are Blisters or vesicles The mnemonic is ICI(Imperial Chemical Industries) Inflammatory including Immunological, Contact dermatitis and Infective.
Inflammatory causes can include drugs but remember Immunological causes in the elderly particularly bullous pemphigoid. Contact dermatitis usually gives smaller vesicles rather than blisters but individual vesicles can join up into blisters. If blisters are linear and itchy it is probably a Plant contact dermatitis. Infective blisters are usually bullous impetigo due to a staph infection. If in a dermatomal distribution blisters are likely to be Herpes Zoster. View the blistering rashes

5. If Skin Coloured and Scaly The mnemonic is ( I am coming Don't go away) Ichthyosis, Acanthosis nigricans, Confluent and Reticulate papillomatosis, Dariers, Grovers and Acrokeratosis verruciformis) but these are mostly uncommon conditions.

6. Skin coloured and Non scaly  No good mnemonic except ICS (Infiltrates of cells or substances)     Most cases are Infiltrates of cells or substances  eg Granuloma annulare, Sarcoidosis, Mucinosis, Scleromyxedema, Scleredema, Scleroderma,  Metastases,

For other morphologies such as funny shape, colour or distribution, click on the link in Labels opposite to go to the appropriate section for sample images of relevant conditions and a discussion on how to diagnose them. If you need help with skin disease terminology try this tutorial from Logical Images.

The following links are for General Practitioners who have access to the Skin Consult website. If you are a GP and wish access to this site, please register at and provide proof of status as requested ie email a copy of your letterhead.
You need to log into before clicking the links below.

Rash on Face- Under Breasts and Flexures - Hands and Feet - Genitalia and Anus - Lower Legs and Arms - Hair and Scalp Problems - Mouth and Lips- Nails- Light and Dark Patches on Skin- Itch Localised and Generalised- Ringworm Like Rashes - Red All Over Patient- Unusual Rashes in Children- Unusual Rashes in Adults

For expert diagnosticians have a look at the Diagnosis Page of Global Skin Atlas and search using the morphology function alone or combine morphology and site.