Erythema Nodosum

Erythema nodosum is a form of panniculitis, inflammation of the subcutaneous fat. It is a delayed hypersensitivity reaction to a wide range of underlying triggers rather than a specific disease in itself.

It is characterised by tender, erythematous nodules typically affecting the anterior shins. Every patient requires a systematic search for an underlying cause.

Causes

Infection is a common trigger, including streptococcal infection (particularly in children), tuberculosis, and a range of other bacterial, viral and fungal infections depending on geographical exposure.

Sarcoidosis is a major cause in adults, with erythema nodosum occurring as part of Löfgren syndrome together with bilateral hilar lymphadenopathy, arthralgia and fever, a combination that carries a favourable prognosis and does not necessarily require tissue biopsy for diagnosis.

Inflammatory bowel disease, particularly Crohn disease, is a recognised association, with erythema nodosum activity sometimes paralleling intestinal disease activity.

Medications, particularly certain antibiotics, oral contraceptives and other drugs, and pregnancy, are recognised precipitants.

A substantial proportion of cases remain idiopathic despite thorough evaluation, particularly in younger patients without other systemic features.

Tender, erythematous, warm subcutaneous nodules, typically several centimetres in diameter, appear symmetrically over the anterior shins, the classical and most common site, though the extensor surfaces of the forearms and thighs can also be affected.

The nodules characteristically evolve in colour over one to two weeks, progressing through a bruise like sequence from red to violaceous to yellow green, reflecting the resolving inflammatory and haemorrhagic process.

They resolve without ulceration or scarring, an important distinguishing feature from other forms of panniculitis or vasculitis that may ulcerate.

Accompanying constitutional symptoms (fever, malaise) and arthralgia, particularly affecting the ankles and knees, are common and can precede the skin lesions.

Symptoms and signs pointing toward the underlying cause (preceding sore throat suggesting streptococcal infection, respiratory symptoms and hilar lymphadenopathy suggesting sarcoidosis, gastrointestinal symptoms suggesting inflammatory bowel disease) should be actively sought in the history and examination.

Diagnosis is predominantly clinical, based on the characteristic appearance, distribution and evolution of the lesions, with biopsy generally reserved for atypical presentations or where the diagnosis is uncertain, since the clinical picture alone is frequently sufficiently distinctive.

Investigation is directed at identifying an underlying cause.

• Throat swab and antistreptolysin O titre for suspected streptococcal infection.
• Chest radiograph to assess for hilar lymphadenopathy suggesting sarcoidosis (with further sarcoidosis specific investigation where this is identified).
• Tuberculosis screening according to geographical risk and exposure history.
• Inflammatory markers and, where gastrointestinal symptoms are present, further evaluation for inflammatory bowel disease.

A thorough medication history is essential given the range of drugs recognised to trigger this reaction pattern.

Skin biopsy, where performed, shows septal panniculitis without vasculitis, a pattern that helps distinguish erythema nodosum from other causes of leg nodules, including nodular vasculitis and other forms of panniculitis with a lobular rather than septal distribution of inflammation.

Treatment of any identified underlying cause is the primary management approach, with the skin lesions themselves generally resolving as the underlying trigger is addressed or, in idiopathic or post infectious cases, spontaneously over three to six weeks.

Rest, leg elevation and compression stockings provide symptomatic relief given the dependent, often painful nature of the lesions.

Nonsteroidal anti inflammatory drugs are first line for symptomatic relief of pain and inflammation in the absence of a contraindication.

Potassium iodide, given orally, is an established and effective treatment for more persistent or troublesome disease, though its mechanism of action in this condition remains incompletely understood.

Corticosteroids may be used for severe, persistent or refractory disease once significant infection (particularly tuberculosis) has been reasonably excluded, given the risk of exacerbating an unrecognised underlying infective process.

Reassurance regarding the generally self limiting, benign course of erythema nodosum itself, once serious underlying causes have been excluded or appropriately addressed, supports patient understanding.

The overall prognosis and any need for further management are determined predominantly by the identified underlying cause rather than by the skin manifestation itself.