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Why did this child appear to have a dirty neck?

The patient

‘I'm so embarrassed, doctor,' said the mum of my next patient, a 12-year-old boy. ‘It looks like he's not washing properly. But I can assure you, he is.'

She was showing me his neck as she talked, to demonstrate a grubby-looking area. There was a similar, though less marked, patch on the other side.

The discolouration had been present for months and seemed to have become more pronounced. Otherwise, there was no significant past medical history, apart from mild obesity. The boy felt perfectly well in himself, and was on no medication.

First instinct

It had to be said that the skin really did look dirty. Closer examination revealed an area of hyper-pigmentation restricted to the skin folds of the neck. The skin in this area had a velvety feel, but there was no sign of any underlying dermatological disorder. Brief questioning confirmed that there were no similar patches elsewhere.

The only time I've previously come across patches like this in children, they were the result of post-inflammatory hyper-pigmentation. So this was my favoured diagnosis, even though the child's mother could not recall any preceding skin problem and there were no dermatological diagnoses detailed in his records.

Differential diagnosis

• Post inflammatory hyper-pigmentation

• Acanthosis nigricans

• Rare cause of hyperpigmentation such as Addison's, haemochromatosis.

Despite the lack of any confirmed preceding skin disease, post-inflammatory hyperpigmentation remained my favoured diagnosis, largely because of the lack of other possibilities. Acanthosis nigricans crossed my mind, as the appearance fitted with this diagnosis – but I had not come across this in children before.

As for rare causes, I discounted these – partly because of them being statistically unlikely but also because the child was well. Besides, disorders of this sort usually produce more widespread pigmentation, and this was definitely very localised.

The hidden clue

In this case, the clue was, in reality, the lack of any further clue. And, in particular, mum's insistence that there had been no preceding skin problem, which rather blew a hole in my post-inflammatory hyper-pigmentation hypothesis. Which left acanthosis nigricans as the only reasonable diagnosis.

Getting on the right track

We're fortunate in our practice to have a partner who is a dermatological expert – so it was time for her opinion. She took one look and immediately confirmed the diagnosis of acanthosis nigricans. This has rare associations with underlying pathology – including diabetes, malignancy and Cushing's – so we thought it reasonable to run a blood screen. Thankfully, this was normal – in this child's case the underlying cause may well have been his obesity. Mum was grateful to, at least, have an explanation for the symptom, even if the treatment simply involved cosmetic advice – and the suggestion that he shed some weight.

Dr Keith Hopcroft is a GP in Laindon, Essex


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