The patient
‘Sorry to bother you with this, doc,' my patient said. ‘It seems so trivial. It's just that I can't get these scratches to heal.'
He rolled up his shirt sleeve to reveal the problem, and continued: ‘We've just got a new kitten at home. I thought some antibiotics might do the trick.'
He told me these claw marks on his arms had been present for weeks and were showing no signs of resolving.
Otherwise, this young man was well and on no medication.
A brief scan through his records revealed no relevant past medical history.
He was also a very rare attender, with minimal entries in his notes despite having been registered with us for years.
First instinct
A cursory examination simply seemed to confirm his story and self-diagnosis.
There were multiple linear lesions on both arms, each compatible with scratches from a cat.
They looked more florid than one might expect after a few weeks, but this I attributed to post-inflammatory hyper-pigmentation or, possibly, mild infection.
Differential diagnosis
• Trauma, with or without infection
• Injection marks
• Dermatological pathology
• Koebner's phenomenon
Strange linear marks on the arms of a young male will always make the GP think of possible drug abuse.
But he denied this possibility and looked far too well to be an injecting addict – besides, the marks were diffuse, well away from veins.
As the lesions were bilateral, I considered a primary dermatological problem, though the picture didn't ring a bell for any of the common skin pathologies we see in general practice – and his notes were devoid of any previous skin trouble.
The only other time I've seen lesions like this is in Koebner's phenomenon – the development of skin lesions at sites of trauma.
These are typically linear, as in this case, and are usually seen in those with skin problems such as psoriasis or lichen planus, though they can be caused by warts and molluscum contagiosum, too.
But I've never seen them look as florid as this and, besides, he had no known underlying skin pathology.
So, simple, slowly resolving cat scratches remained top of the list.
The hidden clue
Something about the atypical nature of the lesions – in particular, their inflamed and oddly scaly appearance – merited a re-think.
Or, more specifically, a re-examination.
A hunt for skin-pathology clues revealed a blank on his arms and nails.
But when, at my request, he rolled up his trouser legs, there it was: a patch of psoriasis on his knee.
Getting on the right track
In his uncomplaining way, he had put up with the psoriasis of his knees for years, assuming it was just dry skin.
But now the lesions on his arm made sense: Koebnerised psoriasis.
Some topical calcipotriol resolved the problem for him.
Dr Keith Hopcroft is a GP in Laindon, Essex
