EN is associated with a wide variety of conditions, including:
In about 30–50% of cases, the cause of EN is unknown.
- Streptococcal infection which, in children, is by far the most common precipitant,
- Primary infection of Tuberculosis
- Mycoplasma pneumoniae
- Histoplasma capsulatum
- Yersinia
- Epstein-Barr virus
- Coccidioides immitis (Valley fever)
- Cat scratch diseas.
- Inflammatory bowel disease (IBD)
- Behçet's disease
- Sarcoidosi.
- Sulfonamides
- Penicillins
- Oral contraceptives
- Bromides
- Hepatitis B vaccination.
- Non-Hodgkins lymphoma (NHL)
- Carcinoid tumours
- Pancreatic cance.
EN may also be due to excessive antibody production in lepromatous leprosy leading to deposition of immune complexes.
There is an association with the HLA-B27 histocompatibility antigen, which is present in 65% of patients with erythema nodosum.
A useful mnemonic for causes is SORE SHINS (Streptococci, OCP, Rickettsia, Eponymous (Behçet), Sulfonamides, Hansen's Disease (Leprosy), IBD, NHL, Sarcoidosis.
Erythema nodosum is diagnosed clinically. A biopsy can be taken and examined microscopically to confirm an uncertain diagnosis. Microscopic examination usually reveals a neutrophilic infiltrate surrounding capillaries that results in septal thickening, with fibrotic changes in the fat around blood vessels. A characteristic microscopic finding is radial granulomas, well-defined nodular aggregates of histiocytes surrounding a stellate cleft.
Additional evaluation should be performed to determine the underlying cause of erythema nodosum. This may include a full blood count, erythrocyte sedimentation rate (ESR), antistreptolysin-O (ASO) titer and throat culture, urinalysis, intradermal tuberculin test, and a chest x-ray. The ESR is typically high, the C-reactive protein elevated, and the blood showing an increase in white blood cells.
The ESR is initially very high, and falls as the nodules of erythema nodosum. The ASO titer is high in cases associated with a streptococcal throat infection. A chest X-ray should be performed to rule out pulmonary diseases, in particular sarcoidosis and Löfgren syndrome.
Pre-eruptive phase => The first signs of erythema nodosum are often flu-like symptoms such as a fever, cough, malaise, and aching joints. Some people also experience stiffness or swelling in the joints and weight loss.
Eruptive stage => Erythema nodosum is characterised by 1–2-inch (25–51 mm) nodules (rounded lumps) below the skin surface, usually on the shins. These subcutaneous nodules can appear anywhere on the body, but the most common sites are the shins, arms, thighs, and torso. Each nodule typically disappears after around two weeks, though new one may continue to form for up to six or eight weeks. A new nodule usually appears red and is hot and firm to touch. The redness starts to fade and it gradually becomes softer and smaller until it disappears. Each nodule usually heals completely without scarring over the course of about two weeks. Joint pain and inflammation sometimes continues for several weeks or months after the nodules appear.
Less common variants of erythema nodosum include:
- Ulcerating forms, seen in Crohn's disease
- Erythema contusiforme, when a subcutaneous hemorrhage (bleeding under the skin) occurs with a erythema nodosum lesion, causing the lesion to look like a contusion (bruise)
- Erythema nodosum migrans (also known as subacute nodular migratory panniculitis), a rare form of chronic erythema nodosum characterized by asymmetrical nodules that are mildly tender and migrate over time.