GPs see so many skin rashes and people are understandably concerned about them because they're so visible and can be alarming. They're rarely serious, but often pose a diagnostic challenge for you and your doctor.
So if you develop a rash, how does your doctor decide what it is and what's causing it?
The story of your skin
Even with 21st century medical technology, the modern GP will make most skin diagnoses based on old-fashioned clinical skills – listening to your story and examining you.
Although I have to rely heavily on what I see, there's more to dermatology than simple pattern recognition. With experience it's sometimes possible to make an on-the-spot diagnosis - the typical pearly lumps of molluscum contagiosum spring to mind - but your GP will often need more clues.
It's important to know how long you've had the rash, whether it's there all the time and if it's irritating or itchy. I would want to know about your past medical history, too, and whether any other members of your household are affected.
Several very itchy people in the same house might make me think of scabies. People often gasp with horror when I say the name - I think they imagine a ghastly medieval plague - but infestation with the scabies mite is common, it's no reflection on your standards of hygiene and it's easily treated.
It often takes a while to diagnose though, as it's easy to mistake it for other common itchy rashes such as eczema.
Patients often volunteer information about possible skin irritants they may have encountered, but it's often worth me asking about specific things such as washing powders, gardening (I remember one patient with a nasty itchy rash after clearing some poison ivy in the garden) or bubble baths.
I'll also want to know how you're feeling in general - skin rashes are often an outward sign of a general illness or infection. This is particularly true in children, where a fever and a rash are common bedfellows.
Skin rashes are often an outward sign of a general illness or infection
Sometimes a typical rash points to a diagnosis of one of the common childhood viral infections, such as chickenpox or rubella. The timing of the rash in relation to a fever can be helpful, too. In the viral infection Roseola infantum, for example, the temperature settles and the child starts to feel better after about three to four days, at which point the classic rosy pink rash starts to appear.
Non-specific viral rashes are very common in children and adults. They disappear on their own and usually don't need any specific treatment, but it can feel a bit feeble not to be able to give them a neat label and something to make them go away. It's the dermatological equivalent of 'there's a lot of it about'.
While parents often seem most alarmed by their child's rash, your GP will probably be more interested in how unwell the child is generally.
We're all alert to the rash of meningococcal disease - the classic red or purple spots that don’t disappear when you press a glass tumbler over them.
But in the early stages of the disease the rash is often rather non-specific, and many children don't develop a rash until very late on, if at all. So the rash, although an important clue, is just one part of the meningitis jigsaw.
I also try to remember to ask what you've already used to treat the rash. Topical steroid creams, for example, can alter the appearance of a bacterial or fungal skin infection so it looks nothing like how it should.
Finally, doctors are a common cause of rashes, too, or rather the medicines we prescribe are - name any medicine we prescribe (and many we don't) and I’d bet good money on 'rash' being in the list of its side-effects.
What does it look like?
As I scrutinise your rash, preferably in good light, this is probably what I'm thinking about. What do the lesions look like? There are some jargon words that, in the right combination, can sometimes bring a particular diagnosis miraculously to mind.
They describe whether the lesion is raised (a papule or nodule), flat (a macule), filled with fluid (vesicles or blisters) or pus (pustule).
Then there's scaling, crusting, excoriation (scratch marks), lichenification (thickening) or atrophy (skin thinning). A plaque is a raised uniform thickening of the skin with a well-defined edge: this is typical in psoriasis for instance.
Erythema is a useful word. It simply means redness, and so applies to most rashes, but it's also an impressive sounding label when you’re stuck for ideas. I once saw a doctor tell a frustrated patient with a non-specific viral rash that he had 'idiopathic erythema'. The patient was delighted to have a name for his rash, even though it actually means 'redness of unknown cause'.
All these terms, and others, allow me to describe the rash accurately. Then I must make a note of where the rash is: its distribution.
Where is it?
Certain common skin conditions have a preference for particular sites on the body. Psoriasis, for instance, tends to affect the outer surface of the elbows or front of the knees, whereas eczema tends to affect the inside surfaces of the limbs.
Our old friend the scabies mite typically goes for the wrists, between the fingers and the genitalia. Pityriasis rosea, a dramatic rash thought to be caused by a virus and which goes away on its own, classically makes a Christmas tree pattern on the back or chest.
Looking at the configuration of the rash can also help. Ring-shaped lesions are often due to fungal infections (hence the term 'ringworm', although no worms are involved), but there are other causes, such as granuloma annulare (sometimes associated with diabetes), urticaria (itchy allergic type rash) or resolving psoriasis.
With your story, the distribution pattern of the rash and my jargon words, I can now put them all together and, hopefully, come up with a diagnosis.
So silvery, scaly plaques on the elbows make me think of psoriasis, red nodules on the shins make me think of erythema nodosum and an erythematous maculopapular rash starting at the head and spreading to the trunk and limbs might make me wonder about measles (although thanks to MMR vaccine, I've never seen a case - except myself as a child).
It sounds easy but it's rarely straightforward. If the rash isn’t behaving as I'd expected, or it's taking a while to respond to treatment, then it's usually time to think about further tests - a skin sample for analysis (a biopsy), skin-prick tests for potential allergies, or referral to a dermatologist.
Dr Graham Easton works in a London GP practice with around 10,000 patients. It has three GP partners, three salaried doctors and fully computerised medical records. His medical training was at The Royal London Hospital. He's also an experienced medical journalist who has worked for BBC Radio Science and the British Medical Journal.
All content within BBC Health is provided for general information only, and should not be treated as a substitute for the medical advice of your own doctor or any other health care professional. The BBC is not responsible or liable for any diagnosis made by a user based on the content of the BBC Health website. The BBC is not liable for the contents of any external internet sites listed, nor does it endorse any commercial product or service mentioned or advised on any of the sites. See our Links Policy for more information. Always consult your own GP if you're in any way concerned about your health.
RSS Feed
Comments
The 'Have your say' section for this article is now closed