This young man was concerned about the slightly itchy lesion that had suddenly appeared on his chest. On examination, it was oval and well defined, with a pink, slightly scaly, raised border and apparent central clearance.
He was otherwise well and had no other signs and no rash elsewhere. He had never previously experienced any skin problems and had no knowledge of recent rashes or illness among his friends or family.
The appearance of the lesion raised the possibility of ringworm, but microscopic findings of skin scrapings from the lesion, mixed with 10% potassium hydroxide solution in the surgery, were negative, with no obvious hyphae or spores. When the sample was sent to the laboratory for further microscopy and culture, no fungal elements were reported.
About a week later, the diagnosis became clear when the patient returned with a crop of similar but smaller, symmetrically distributed lesions mainly on the chest and upper arms; this formed a 'Christmas tree' like pattern that followed the line of the ribs. A diagnosis of pityriasis rosea was made.
The cause of pityriasis rosea is as yet unknown but it is thought to be a reaction to some infectious, possibly viral, agent.
It is most common between the ages of 10 and 35 years and the diagnosis can usually be made on clinical grounds by the appearance of the rash and finding of the initial lesion, known as the 'herald patch', which usually appears up to a week before. More lesions may develop in the ensuing weeks and the rash may persist for up to three months or longer.
Atypical forms of the condition may occur, in which the rash mainly affects the limbs and flexures rather than the trunk. If the diagnosis is in doubt, investigations may be required to exclude other possible causes and a specialist opinion may be advisable.
There is no specific treatment for the condition, which usually clears within three months but sometimes leaves marks of hyperpigmentation that take longer to fade. Symptomatic treatment with emollients, topical corticosteroids and/or sedating antihistamines may help the itching.
There have been some reported problems of miscarriage and premature delivery if pityriasis rosea occurs in pregnancy, but this has not as yet been confirmed. Although there is no effective intervention, the obstetrician and the patient should be made aware of the problem.
- Dr Watkins is a retired GP in Hampshire.