The patient
This 35-year-old woman presented with red lesions on her legs, which she had self-diagnosed as a reaction to insect bites. They had appeared a few days previously, just after a hot weekend, and were sore rather than itchy. She had tried antihistamines but the lesions seemed to be getting worse rather than better.
She had no relevant past medical history other than migraines, for which she took occasional ibuprofen. No other symptoms were volunteered, apart from her feeling a bit below par – which, still in self-diagnosis mode, she attributed to the antihistamines she had been taking.
First instinct
These did look like insect bites – and my view was coloured by the fact that the recent good weather had prompted a mini-epidemic of papular urticaria among the shorts- and skirt-wearing masses. The lesions were oddly symmetrical, though, and the fact that they hurt rather than itched was atypical.
Differential diagnosis
• insect bites
• cellulitis
• drug side-effect
• thrombophlebitis
• erythema nodosum
• diabetic skin complication.
I suspended judgment on the likelihood of insect bites while I considered the other possibilities.
Cellulitis can, of course, cause a painful red rash on the leg – but it is rarely bilateral and would not normally produce multiple, discrete lesions. Thrombophlebitis was a possibility, too, but didn't quite fit, for similar reasons.
A drug reaction is always a possibility when the GP is confronted with an odd skin reaction. And ibuprofen is a common culprit – but she hadn't taken any for some weeks.
I vaguely recalled that diabetes can result in a variety of skin lesions affecting the legs – such as granuloma annulare, necrobiosis lipoidica and pyoderma gangrenosum.
But she was not known to be a diabetic and it would be unusual for the disease to present in this way – and the diagnosis was easy to exclude with a quick finger-prick test.
This left erythema nodosum as a possibility. It is not that common in general practice, and so isn't the first thing you'd think of when confronted with this presentation. Plus there were no obvious triggers to this episode, although many cases have no clear cause.
The hidden clue
A clue was provided by the patient as I was weighing up the possibilities. ‘It seems a bit unfair if it is insect bites, doctor,' she said. ‘Because I had such a bad sore throat that I stayed in bed most of the weekend.'
Getting on the right track
Streptococcal pharyngitis is the most common cause of erythema nodosum – and the timing was perfect.
I arranged a chest X-ray to rule out sarcoidosis and TB, and advised her that she should use her ibuprofen to ease the discomfort. On review a couple of weeks later, her chest X-ray was clear and the lesions were already fading to the characteristic bruised colour of resolving erythema nodosum.
Dr Keith Hopcroft is a GP in Laindon, Essex
What are the sore, red lesions on this woman's leg? What are the sore, red lesions on this woman's leg?
