Had Sherlock Holmes gone into medicine, he’d have been a dermatologist

There are hundreds of different rashes, each with tell-tale signs that, with careful observation, can lead clinicians to a diagnosis.

By Phil Whitaker

Robbie had had a sore throat and fluey symptoms, and the large patch of inflamed skin that appeared on one side of his abdomen hadn’t particularly bothered him, especially as it started to fade when he began to feel better. That morning, though, he’d woken to find himself covered in dozens of similar patches scattered all over his body.

He looked worried. Dermatological diseases aren’t often serious, but they can cause distressing symptoms such as itch, and they almost invariably have an impact on our psychology. Our skin is our interface with the world. When something goes wrong with it, it can create embarrassment and even shame – ancient notions of uncleanliness, sinfulness and contagion persist to this day. Robbie was in his early twenties; he wanted to be out playing the mating game. Were he to disrobe in front of a partner looking like this, they’d run a mile.

I asked a few questions as he unbuttoned his shirt. Two of the commonest dermatological conditions we see – eczema and psoriasis – have variants that might have fitted the bill, but they tend to run in families and Robbie had no affected relatives. One of my other hunches was confirmed as soon as I saw the rash. Lines of oval pinky-red patches swept symmetrically across his trunk in gentle curves, recalling the branches of a Christmas tree.

“It’s something called pityriasis rosea,’ I explained. The virus that had made him feel poorly also affects the skin. It starts with a single “herald patch”, just as Robbie had noticed, followed a week or two later by a dramatic outbreak of secondary lesions. These tend to be flat and salmon pink, and often have a ruff of fine scales around their edges. Robbie’s rash lacked this last feature – but it’s rare to see textbook cases of anything.

Dermatology is the branch of medicine Sherlock Holmes would have most relished were he, like Watson, to have become a doctor. There are hundreds of different rashes, each with tell-tale signs that, with careful observation, can lead clinicians to a diagnosis.

One of the most intriguing features of rashes is that they tend to have characteristic distributions. Coxsackievirus typically affects the extremities and the oral area, hence its colloquial name, “hand, foot and mouth disease”. Parvovirus homes in on the sides of the face; it’s known as “slapped-cheek syndrome”. The many different patterns of eczema also affect particular areas: atopic eczema inflames the insides of the elbows and behind the knees; seborrhoeic eczema presents along the sides of the nose, the eyebrows, and the front of the chin. An intensely itchy condition, lichen planus, has a predilection for the wrists, where it has a violet hue, and the insides of the cheeks, where it forms lacy white webs.

In a few instances, we understand why these characteristic distributions occur. Shingles, caused by reactivation of the chickenpox virus, reaches the skin by travelling along a single nerve: this results in a one-sided rash confined to the area – known as a dermatome – supplied by the affected nerve. The virus that caused Robbie’s pityriasis probably migrates in a similar way, although its “Christmas tree” distribution maps multiple dermatomes on both sides of the body.

For the most part, we have no idea why different rashes affect certain areas of skin and not others. With enough research we could develop some fascinating insights. However, it is unlikely this would enhance treatment, so such research will probably never be funded. I rather like the mystery of it all – it’s as though nature is playing us, dropping us little clues to help us make our diagnoses.

Many rashes we can do nothing about, but at least with the right diagnosis I could tell Robbie what to expect. He was pleased to hear his pityriasis would eventually get better, but dismayed to learn that it could take a few months and there was nothing we could do to hurry the process along. As he got up to leave, I sensed he’d resigned himself to an enforced pause in the mating game.