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.

Skin Morphology refers to terms used to describe the skin and skin diseases. 
We look at 
Colour - Red, Normal skin colour, Hyperpigmentation, Hypopigmentation,  
Surface texture - Scaly, Non scaly, Oozing, Sweating or Dry, Hard (indurated)
Pustules
Vesicles
Blisters

We describe
Flat macules or patches or raised papules or plaques
We describe
Nodules
Erosions
Ulcers
Eschars
Bruising and Petechiae and Palpable purpura

We look at the Distribution of a rash
Sun exposed skin
Unilateral dermatomal 
Following embryonic lines of Blaschko
Acral Palms and Soles

We then synthesise these features into a diagnosis. For the experienced dermatologist much of this is done subliminally and through pattern recognition a diagnosis is made very rapidly. 
However how do you approach this when you are a neophyte? 

You do the following

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 PETAL for red scaly diseases (The PMs PET a little cat named PETAL)

2. CUL DVA EVIE for red non scaly diseases (See You Later at the Dep of Veterans Affairs EVIE where EVIE is your girlfriend)

3. II for pustular diseases ( II or Aye Aye if it helps)

4. ICI for vesicular or Blistering diseases. (Imperial Chemical Industries)

Diagnosis boils down to this -

What do I see?
What mnemonic will I use?
What are my likely differential diagnoses?
Where else will I examine or what tests will I do to arrive at the likely diagnosis?
What now is my preferred diagnosis?
Hence what is my treatment?

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 PETAL (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 rarely Pityriasis rubra pilaris and Pityriasis lichenoides. This P can also stand for Pharmaceutical because drugs can cause red scaly rashes and the M is for Mycosis fungoides, a T cell lymphoma of the skin.) The little s is for Solar, Scabies, Syphilis and Syndromes (eg Dariers and Grovers and CIE Congenital ichthyosiform erythroderma) that can rarely present as a red scaly rash. Never forget solar in Australia particularly Porokeratoses or SCC in situ on the legs looking like psoriasis. 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 (See you later at the Dept of Veterans Affairs EVIE)  C U L at the Department of Veterans Affairs Evie (your girlfriend Evie) (CUL DVA EVIE) where EVIE stands for Erythema multiforme, Vasculitis , Infiltrates and Erythema nodosum .  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 or Erosive pustular dermatosis scalp. 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) Infective, Contact dermatitis and Inflammatory including Immunological .
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 and it is not a mnemonic I use much. Easier just to learn to recognise these 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, texture 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. This website is available to Members of the Australian Institute of Dermatology.

You need to log into www.skinconsult.com.au 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.