Author: Dr Amanda Oakley, Dermatologist, Hamilton, New Zealand,1997.
Erythema nodosum is a skin condition where red lumps form on the shins, and less commonly the thighs and forearms. It is a type of panniculitis, an inflammatory disorder affecting subcutaneous fat.
Most cases of erythema nodosum occur between the ages of 20 and 45, with a peak from 20 to 30. It occurs less often in the elderly and in children. It is 3 to 6 times more common in adult women than in adult men. However, the sex incidence before puberty is about equal.
Most people with erythema nodosum are otherwise in good health but it is often associated with recent infection or illness.
Erythema nodosum appears to be a hypersensitivity reaction with a number of different causes.
Common causes of erythema nodosum in New Zealand are:
Symptoms of the underlying disease may be present in some patients with erythema nodosum, for example, a sore throat in those with streptococcal infection.
Red lumps appear on the shins or about the knees and ankle. They vary in size between a cherry and a grapefruit and in number from 2 to 50 or more. Usually, there are 6–12 lumps on the front and sides of the legs and knees. The thighs, outer aspects of the arms, face and neck are less frequently involved and at these other sites, the lesions are smaller and more superficial. The nodules are slightly raised above the surrounding skin; they are hot and painful, bright red when they first appear, later becoming purple then fading through the colour changes of a bruise.
Erythema nodosum nodules continue to erupt for about 10 days. The 'bruising' colour-change starts in the second week, becomes most marked in the third week, then subsides at any time from the end of the third week to the sixth week. Aching of the legs and swelling of the ankles may persist for some weeks, especially if the patient does not rest up. New crops of erythema nodosum may occur over a number of weeks. Rarely, 2 or 3 large lesions merge to form a crescentic ring, which spreads for some days before fading.
Other symptoms may include:
The skin disorder is diagnosed clinically and a skin biopsy is often performed. The pathology of erythema nodosum shows inflammation around the septum between lobules of fat in subcutaneous tissue, without vasculitis.
Tests done in patients with erythema nodosum include:
Mild cases of erythema nodosum subside in 3–6 weeks. Cropping of new lesions may occur within this time, especially if the patient is not resting. Sometimes, erythema nodosum may become a chronic or persistent disorder lasting for 6 months and occasionally for years.
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