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Erythema nodosum
Erythema nodosum

Erythema nodosum

Erythema nodosum is a common hypersensitivity reaction that manifests cutaneous nodules that are erythematous and tender. It most commonly occurs over the shins but may happen elsewhere in the body too. It is a form of panniculitis - inflammation of the subcutaneous fat.

Pathophysiology

The pathophysiology behind erythema nodosum is not well understood. It is believed that macrophages and immune complexes are deposited in the subcutaneous fat which leads to an immune reaction that causes the typical appearance.

Most cases are idiopathic (60% of cases). However, it is also associated with a variety of conditions:

  • Autoimmune disease:
    • Inflammatory bowel disease - although it can happen in both subtypes, it is more common in Crohn's disease (for exam purposes).
    • Sarcoidosis
    • Behcet's disease - seen in 40-90% of patients
  • Drugs:
    • Oral contraceptives
    • Sulfonamides
    • Iodide
  • Infections:
    • Streptococcal pharyngitis - streptococcus is currently the most identifiable cause
    • Histoplasmosis
    • Tuberculosis - the most common aetiology in developing countries
    • Leprosy,
  • Pregnancy - 2-6% of pregnancies present with erythema nodosum.
  • Malignancy - leukaemia and lymphoma are the most common malignancies associated with EN lesions

😷 Presentation

  • Painful, subcutaneous nodules on pretibial surfaces. Might be seen on other areas such as the thighs, arms, calves, buttocks, face.

      • The lesions take on a sequential characteristic presentation:

    1. Week 1 - they are initially firm and erythematous.
    2. Week 2 - they then become fluctuant and bluish.
    3. Week 3-6 - they progressively fade to a yellow/brownish hue.

Other features may include:

  • Arthralgia
  • Fever
  • Malaise
  • Hilar lymphadenopathy, uveitis, retinal nodules - in Sarcoidosis.
  • Abdominal pain and changes to bowel habit - in IBD.
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🔍 Investigations

⭐️ The diagnosis of erythema nodosum is a clinical diagnosis.

Investigations can be helpful in assessing the underlying cause:

  • Inflammatory markers (CRP and ESR)
  • Throat swab & antistreptolysin O titre - for streptococcal infection
  • CXR - can help identify mycoplasma pneumonia, tuberculosis, sarcoidosis and lymphoma.
  • Faecal calprotectin - for inflammatory bowel disease.
  • Tuberculin skin test - to screen for TB.

🧰 Management

🧰
Management of erythema nodosum:

Most cases will spontaneously resolve within 6 weeks, however it can last longer.

Management mainly involves investigating and treating the underlying cause.

  • 🥇 It can be managed conservatively with rest and analgesia with NSAIDs. Steroids may be used but should be avoided if there is an infective cause.
  • 🥈 In more severe cases and in those patients who do not respond to initial therapy, oral potassium iodide has been used successfully.