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What is the cause of the painful nodules on this man's lower legs?

GP Dr Hercules Robinson describes how lesions a patient assumed were midge bites had a more complex origin

 

The patient

This 25-year-old man presented with tender nodules on both lower legs, which he had first noticed about three weeks previously following a walking holiday. He had been extensively bitten by midges, which he thought were the cause of the lesions. But the other signs of his midge bites had all resolved, leaving these lesions. He seldom attended the surgery and was on no medication.

First instinct

These lesions looked like erythema nodosum and I explained to him that they were not related to the midge attack.

Differential diagnosis

Erythema nodosum can be caused by a variety of conditions.

It may be associated with a variety of infections: streptococcus, TB, primary coccidiosis, other deep fungal infections, yersinia and syphilis.

It may accompany sarcoidosis, Behcet's disease and inflammatory bowel disease.

It can occur in pregnancy, with oral contraceptives and with sulphonamides. Some 20% of cases may be idiopathic.

Other lesions to consider are erythema induratum from TB, but these occur on the posterior surfaces of the legs and may ulcerate. Lupus panniculitis causes tender nodules on the buttocks and posterior arms.

He didn't have a history of a recent sore throat suggesting a streptococcal infection. There was no history of foreign travel to South or Central America or parts of the US, which might have suggested coccidiomycosis – a common opportunistic infection with HIV in patients in these endemic areas. There was no history of night sweats or weight loss that could have pointed to TB. He was on no medication.

There was no history of exposure to an STI. I discounted Behcet's on the grounds that there was no history of genital ulceration but there was a history of intermittent oral ulceration.

The hidden clue

He did mention he intermittently had attacks of abdominal pain with some associated diarrhoea, but no history of rectal bleeding.

Outcome

I arranged some investigations. His FBC revealed a very mild normocytic normochromic anaemia. His ESR and CRP were slightly elevated. A throat swab and ASO titre were negative.

A chest X-ray was normal with no changes suggestive of sarcoidosis. His liver and renal function were normal.

I was suspicious that he might have an inflammatory bowel disorder. Proctoscopy was normal, which suggested ulcerative colitis was unlikely.

I referred him to our local surgeon and he underwent colonoscopy. A number of areas of inflammation were seen and biopsies suggested the diagnosis of Crohn's disease.

The leg lesions resolved after about six weeks.

Dr Hercules Robinson is a GP in Caithness

Erythemanodosum Erythemanodosum

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