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What's causing the rash in this young girl's axilla?

Was this girls underarm rash fungal, bacterial, voral or even a chemical burn? Dr Oliver Starr explains his diagnostic process.


The patient

This well-behaved young girl attended with her mother and baby brother. Out of nowhere, overnight, she had discovered this sore rash in her left axilla. She swore she had no idea how it had come about, and found it painful to lift her arm above her head. Other than this she was her usual smiley self.

Her history included congential pulmonary stenosis but her most recent echocardiogram showed good function and the only sequela was the need for prophylactic antibiotics prior to dental surgery.

She had been brought in for the usual bout of mild childhood illnesses like sore throats, ear aches and impetigo but was generally healthy.

I knew the mother from other appointments and she was sensible.

First instinct

My simplistic approach to rashes extends to thinking: is it bacterial/fungal/viral or inflammatory? These weren't crusted over like bacterial folliculitis usually is and it didn't look like any fungal skin conditions I had seen, which tend to be flatter. My hunch was that this was viral and I thought some of the bumps looked vesicular.

Differential diagnoses

• Folliculitis.

• Chemical burn.

• Herpes simplex.

• Varicella zoster (shingles).

Folliculitis doesn't tend to be painful, but is usually itchy. But the lesions didn't have a crusted, bacterial appearance and I would have expected it to take longer to get established.

A chemical burn from sneakily using Mum's deodorant was a distinct possibility at her age and, although she denied it, she certainly blushed when I asked her.

Herpes simplex was quite likely and the lesions certainly resembled typical herpes collections that I have seen in textbooks. But I wasn't certain enough to administer antiviral medication, either topical or oral.

Shingles was a differential that was raised when we discussed the photo at our daily practice meeting. But as with the herpes simplex differential, I didn't want to prescribe acyclovir to a 7 year old without good reason, particularly as she was systemically well.

I was reminded of an old medical school adage: don't just do something, stand there. So we left the lesions alone and I booked her in to see me in two days.

Getting on the right track

It all became clear later that week. On her return, further crops of the lesions had appeared in a linear distribution following the T2 dermatome, but not crossing the midline. It fitted exactly with shingles.

When I saw her for a sore throat three weeks later she was entirely well.

Dr Oliver Starr is a sessional GP in Hertfordshire

Rash in axilla Rash in axilla

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