What is erythema nodosum?
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, i.e. an inflammatory disorder affecting subcutaneous fat.
Who gets erythema nodosum?
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.
What are the causes of erythema nodosum?
Erythema nodosum appears to be a hypersensitivity reaction with a number of different causes.
Common causes of erythema nodosum in New Zealand are:
- Throat infections; these may be due to streptococccus, or viral in origin.
- Other bacterial infections, such as Mycoplasma pneumoniae.
- Sarcoidosis; erythema nodosum is often associated with enlargement of the lymph nodes (bihilar lymphadenopathy) in the lungs in sarcoidosis. This is known as Löfgren syndrome. It may result in a dry cough or some shortness of breath.
- Tuberculosis (TB); erythema nodosum occurs with the primary infection with TB. TB in New Zealand is currently uncommon.
- Pregnancy or the oral contraceptive pill; erythema nodosum may occur after the first 2 or 3 cycles on the pill. EN may occur in pregnancy, clear after delivery, then recur in subsequent pregnancies.
- Other drugs; other drugs which have been reported to cause erythema nodosum include: sulphonamides, saliclyates, other nonsteroidal anti-inflammatory drugs (NSAIDs), bromides, iodides and gold salts.
- Inflammatory bowel disease (ulcerative colitis or Crohn disease)
- Other causes; there are many other causes of erythema nodosum but these are uncommon in New Zealand.
Erythema nodosum leprosum is a particular variant of erythema nodosum that affects some people being treated for leprosy.
Clinical presentation of erythema nodosum
Symptoms of underlying disease may be present in some patients with erythema nodosum, e.g. 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:
- Fever, general aching and feeling unwell (malaise) especially when the nodules first occur.
- Joint aches or arthralgias occur in over half of those presenting with erythema nodosum, regardless of cause. The knee jonts are almost always affected, the other large joints less commonly. Joint symptoms may persist for months afterwards but always resolve completely.
- Conjunctivitis is less frequent.
How is erythema nodosum diagnosed?
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:
- Throat swab
- Sputum or gastric washing if TB is suspected
- Complete blood count and C-reactive protein (CRP) and/or erythrocyte sedimentation rate (ESR)
- ASO titre (a test for streptococcal infection)
- Chest X-ray (looking for TB and sarcoidosis)
- Virus studies
- Yersinia titres
- Mantoux test or QuantiFERON gold (tests for TB)
Treatment of erythema nodosum
- If an underlying infection is found, this should be treated.
- Bed rest is advised if pain and/or swelling is severe.
- Firm supportive bandages or light compression stockings should be worn.
- Anti-inflammatory medications reduce discomfort
- Potassium iodide has been reported to be effective but is not easy to obtain.
- Oral tetracyclines have anti-inflammatory properties and may reduce discomfort and duration of disease
Mild cases of erythema nodosum subside in 3 to 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.