Erythema nodosum is an inflammation of the sub-cutaneous fat (a panniculitis). It is an immunological reaction, elicited by various bacterial, viral and fungal infections, malignant disorders, drugs and by a vari-ety of other causes (Table 8.7).
The characteristic lesion is a tender red nodule develop-ing alone or in groups on the legs and forearms or, rarely, on other areas such as the thighs, face, breasts or other areas where there is fat (Fig. 8.10). Some patients also have painful joints and fever.
Lesions usually resolve in 6–8 weeks. In the interim, lesions may enlarge and new ones may occur at other sites. Like other reactive erythemas, erythema nodosum may persist if its cause is not removed.
The nodules may be so tender that walking is difficult. Erythema nodosum leprosum occurs when lepromatous leprosy patients establish cell-mediated immunity to Mycobacterium leprae. These patients have severemalaise, arthralgia and fever.
The differential diagnosis of a single tender red nodule is extensive and includes trauma, infection (early cellulitis or abscess) and phlebitis.
When lesions are multiple or bilateral, infection becomes less likely unless the lesions are developing in a sporotrichoid manner. Other causes of a nodular panniculitis, which may appear like erythema nodosum, include panniculitis from pancreatitis, cold, trauma, injection of drugs or other foreign substances, withdrawal from systemic steroids, lupus erythem-atosus, superficial migratory thrombophlebitis, pol-yarteritis nodosa and a deficiency of α1-antitrypsin. Some people use the term nodular vasculitis to describe a condition like erythema nodosum that lasts for more than 6 months.
Erythema nodosum demands a careful history, physical examination, a chest X-ray, throat culture for strepto-coccus, a Mantoux test and an antistreptolysin-O (ASO) titre. If the results are normal, and there are no symptoms or physical findings to suggest other causes, extensive investigations can be deferred because the disease will usually resolve.
The ideal treatment for erythema nodosum is to iden-tify and eliminate its cause if possible. For example, if culture or an ASO test confirms a streptococcal infec-tion, a suitable antibiotic should be recommended. Bed rest is also an important part of treatment. NSAIDs such as aspirin, indomethacin or ibuprofen may be helpful. Systemic steroids are usually not needed. For reasons that are not clear, potassium iodide in a dosage of 400 –900 mg/day can help, but should not be used for longer than 6 months.
Whereas the reactive erythemas are associated with some inflammation around superficial or deep blood vessels, the term vasculitis is reserved for those showing inflammation within the vessel wall, with endothelial cell swelling, necrosis or fibrinoid change. The clinical manifestations depend upon the size of the blood vessel affected.