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What is causing the irritating rash on this woman's buttocks?

Dr Oliver Starr describes how he found the cause of a long-lasting rash

This woman had put up with an irritating rash on her lower back and buttocks for more than a year. She had given up telling doctors about it and was seeing me about something else. She was overweight, with a BMI of 40. I could see from her notes how many creams and pills we'd tried for this rash and asked if I could have a look at it again. I was astonished to see a massive whirling discolouration. It was well demarcated and showed central clearing.

The initial treatments had been clotrimazole cream, flucloxacillin and a fucidin/bethametasone combination cream. Those hadn't worked, so someone else had tried a miconazole/hydrocortisone combination cream.

With no relief, she had seen me early this year and out of desperation I had given her oral itraconazole for 10 days. I had written ‘fungal' in her notes and I was a little disappointed when there was no response. She saw another doctor the next month and I felt slighted when he wrote ‘not fungal' and gave her mometasone ointment. She was told it was a sweat rash and she needed to lose weight.

Differential diagnoses

• intertrigo

• fungal skin infection

• dermatitis herpetiformis.

The layman might refer to a ‘sweat rash' when they really mean intertrigo – an infection in the skin creases. It could be fungal, as in tinea pedis caused by Trichophyton rubrum, or bacterial, as in erythrasma caused by Corynebacterium minutissimum. Interestingly, both normally respond to azole antifungals. For others, a ‘sweat rash' is actually miliaria – occlusion of the sweat ducts that results in intensely itchy, tiny papules.

The main fungi that affect the skin are of three groups: the dermatophytes, the Candida, and the Malassezia species. Candida and Malassezia are commensals, with disease being triggered by an internal or external event such as immunosuppression or antibiotic use.

Dermatitis herpetiformis is a blistering condition involving an abnormal IgA immune response to the gliadin component of gluten. Some 80% of people with this intensely itchy rash, often found on the buttocks, have coeliac disease too.

As a result of scratching the rash and causing erosions and crusts, it can resemble other conditions such as eczema and scabies.

Getting on the right track

I tried to take scrapings from her buttocks, but nothing would really come off. Despite the lack of response to itraconazole I still felt confident this was fungal, and didn't want to ascribe it to her weight. As a last resort, I gave her oral terbinafine daily for a month.

I was delighted when the rash disappeared within a week. Her skin returned to normal and had stayed that way at four months' follow-up. In retrospect, given the sensitivity to terbinafine, it was probably a dermatophyte species.

Dr Oliver Starr is a GP in Stevenage, Hertfordshire

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