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, Annular Erythema and Lupus. The first P of PM is for Pityriasis rosea or Pityriasis versicolor, Pityriasis Rubra Pilaris, Pityriasis Lichenoides and the M is for Mycosis fungoides, a T cell lymphoma of the skin.) The only additional problem we have in Australia is the extent of our
solar damage which can masquerade as a red scaly rash and even be non scaly. So add solar damage after the mnemonics for both the red scaly and non scaly rashes.
View GlobalSkinAtlas for Images
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.
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 and
Lichen Planus.
In this group there are five conditions that are common. These are
psoriasis, eczema, tinea , pityriasis rosea and pityriasis versicolor. Other less common conditions include mycosis fungoides, lupus erythematosus, parapsoriasis, lichen planus and several other pityriasis conditions such as pityriasis rubra pilaris and pityriasis lichenoides
The more common disorders will be outlined first and representative photographs of them will be shown.
Step 1 Once you have decided that a rash is red and scaly
look to see if the scale is broken and if there are cracks in the surface with any oozing or weeping. If there are, then it is a form of eczema, probably discoid eczema, rather than psoriasis or one of the other red scaly disorders. If it is not, then
Step 2 scratch it first of all and see the nature of the scale. If you get the silvery wax like scale of psoriasis you are home and hosed. If you do not, then
Step 3 the first thing you should do before you even look elsewhere is to
take some scrapings for fungal culture. You should do this before you treat any red scaly rash rather than afterwards. Pityriasis rosea you will diagnose because of the herald patch, oval shaped lesions and the distribution along the rib lines on the trunk and Pityriasis versicolor because the lesions will either be whiter than the surrounding tanned skin or pinker than the surrounding white skin, depending on whether it is summer or winter!
Psoriasis
This is a composite image of psoriasis.
For more information on psoriasis
try this reference.
Additional Diagnostic Features In psoriasis he skin grows 8 to 10 times faster than normal so you get this thick build up of scale that is easily scraped off. The lesions are usually quite
sharply defined, the colour is often a
salmon pink colour, except in the more inflammatory irritated types of psoriasis where it can be much redder. You should look at the more typical areas where you would expect to see psoriasis such as the
elbows, the knees, the scalp. Look at the nails and see if there is any evidence of pitting. Look under the flexures and the elbows or breasts or in the groin area or especially around the anus to see if there is the relatively
smooth non scaly lesions that occur in these areas.
Psoriasis on the lower legs can have an eczematous look to it as well because of stasis features and underlying varicose veins because the patient may have rubbed and scratched the psoriasis. This should not put you off making the diagnosis.
Eczema Red and scaly but with
small erosions or breaks in the skin surface, sometimes oozing and often secodarily infected with Staph aureus.
For more information on Eczema
View this reference
Additional Diagnostic features Eczema is usually itchy. The small erosions or breaks in the skin surface are the crucial diagnostic feature as is the distribution on the front of the elbow and behind the knee. Contact eczema remains localised at unusual areas depending on what and where you have been applying something you are allergic to.
Tinea Red and scaly but with a
spreading edge. Scrape this for microscopy and culture. There may be small pustules in the edge in rare cases. Treatment with topical steroids will reduce the redness and scale making diagnosis more difficult.
For more information on Tinea
View this reference
Additional Diagnostic Features The slowly spreading scaly edge is a hallmark of the condition. Seldom itchy. Usually not symmetrical like psoriasis or eczema. Fungal infections from animals are usually more inflammed.
Pityriasis rosea Red and scaly but has the
herald patch appearing first with it's
peripheral scale , followed within the week by the other oval shaped, scaly lesions running along the rib lines. May involve axillae and groin but seldom further down the limbs. Trunkal rash predominantly. Guttate psoriasis is your main differential diagnosis.
For more information on Pityriasis rosea
View this reference
Additional Diagnostic Features The peripheral scale is not found on every lesion. It is seldom seen on the face. The sun helps it to go more quickly. No other systemic symptoms despite it probably being a viral infection. Usually not itchy.
Pityriasis versicolor This red scaly disease has
fine bran like scale elicited by scratching the surface. It is usually seen on the trunk or under the breasts. It presents as white spot disease in tanned individuals and a red scaly disease on white skin.
Additional Diagnostic Features Often a very light pink colour and easy to miss. Look especially on the upper back, axillae and under breasts.
For further information on Pityriasis versicolor
See this reference or view additional images in
Globalskinatlas.
The M in PMs PET is for
Mycosis fungoides. This is an old name for T cell lymphoma of the skin. It is quite rare but important to diagnose because an early lesion can look like psoriasis or low grade eczema. This can be seen in the composite image below.
Additional Diagnostic Features This is a clinical diagnosis early because the initial biopsy may not be diagnostic. Fixed or slow growing plaque, sharp edges sometimes in bizarre shapes. Itchy later when they thicken.
For further information
View this reference or see other images in
Globalskin Atlas
We have mentioned that the PMs PET is called PETAL. The
AL is to remind you of some other red scaly diseases including the
Annular erythema called EAC or erythema annulare centrifugum and the L to remind you of Lupus erythematosus and Lichen planus.
EAC will remind you of a fungal tinea infection but it has a trailing or inward looking scale rather than at the edge as in a fungus and the lesions grow quickly and often merge together.
See this example from
Globalskin Atlas
The
L for lupus erythematosus refers to the discoid type which is usually seen on the face or back. On the face you will mistake it for sun induced skin cancer but if you freeze it then it will just come back around the frozen area! The scale is throughout the lesion, and is very adherent sometimes going into the hair follicles and giving a
carpet tack appearance underneath when you try to lift it off!
See these examples from
Globalskin Atlas or for more information on Discoid Lupus
View this reference.
Other P diagnoses There are several other rarer skin diagnoses that are red and scaly and begin with Pityriasis. These include pityriasis rubra pilaris, pityriasis lichenoides and pityriasis alba. A composite of these conditions is shown below and there are links to global skin atlas and dermnet for further information.
