Erythema Nodosum (EN)

Definition

• Erythema nodosum (EN) is a hypersensitivity reaction characterised by painful, red, tender subcutaneous nodules, typically on the shins, though the thighs and forearms can also be affected. It is considered a type of panniculitis(inflammation of subcutaneous fat). EN most commonly presents in young adults (peak age 20–30), with a 3–6× higher incidence in females.

Aetiology & Causes

• Sarcoidosis: Most common identifiable cause

• Infections:

  • Streptococcal throat infection (check for recent sore throat)

  • Viral (e.g. EBV, CMV)

  • Mycobacterium tuberculosis (TB)

  • Chlamydia

• Drugs: Oral contraceptives (OCPs), NSAIDs, iodides

• Inflammatory Diseases: Crohn’s disease, ulcerative colitis

• Pregnancy: EN may appear during pregnancy but often resolves postpartum

• Idiopathic: Up to 50% of cases have no identifiable cause

Clinical Features

• Lesions:

  • Tender, hot nodules often grouped in clusters (6–12 lesions)

  • Size can range from small “cherry” nodules to larger “grapefruit-sized” lumps

  • Commonly located over shins, can also appear on thighs and forearms

  • Duration of each lesion is around 10 days, then they may undergo colour changes (bluish, bruise-like) in the second week

  • Typically resolve spontaneously in 3–6 weeks, but can persist or recur

• Systemic Symptoms:

  • Arthralgia, particularly affecting knees and ankles

  • Fever, malaise, and generalised “flu-like” symptoms

Diagnosis

• Primarily clinical based on characteristic nodules on the lower limbs.

• Skin Biopsy (deep incisional or excisional) if the diagnosis is uncertain, showing a septal panniculitis without true vasculitis.

• Investigations to identify underlying cause:

  • Throat swab, antistreptolysin O titre (ASOT) if strep infection suspected

  • Chest X-ray (CXR) to rule out sarcoidosis, TB

  • Basic blood tests: FBC, U&E, ESR, CRP

  • Mantoux test (or IGRA) if TB suspected

  • Stool culture, serology for infections based on clinical suspicion (e.g. COVID-19, Mycoplasma)

  • Screening for inflammatory bowel disease if GI symptoms

Management

1. Supportive Care

   • Rest, limb elevation to reduce swelling/pain

   • Compression stockings if there is significant oedema

   • NSAIDs (e.g. ibuprofen) for pain and inflammation, provided no contraindications

   • Potassium iodide (specialist consideration) and oral tetracyclines have been used anecdotally to reduce inflammation

2. Address Underlying Cause

   • Streptococcal infection → appropriate antibiotics (e.g. penicillin)

   • Sarcoidosis → consider referral to respiratory specialist and possible corticosteroid therapy

   • IBD → treat flares, refer to gastroenterology if needed

   • Drug-induced → discontinue the offending medication if feasible

3. Pharmacological Therapy

   • Systemic corticosteroids in severe or persistent cases (e.g. prednisolone)

   • Use cautiously given side-effect profile; weigh risks and benefits

4. Lifestyle & Follow-up

   • Light exercise or activity within comfort limits

   • Adequate hydration and rest

   • Monitor nodules for resolution or progression

   • Investigate if recurrent or persistent (re-check for chronic infections, autoimmune conditions)

Prognosis

• Self-limiting: Most lesions resolve within 3–6 weeks without scarring, though bruising and colour changes may persist briefly.

• Chronic or Recurrent: Can last months or years if the underlying cause is not addressed or if there is ongoing inflammation (e.g. in sarcoidosis or IBD).

• Monitoring: Follow up to ensure resolution and to manage any systemic cause.

Notes

• If EN is recurring or persistent, always reassess for undiagnosed causes such as sarcoidosis, TB, IBD, or underlying malignancy (rare).

• Consider pregnancy implications—EN may spontaneously resolve postpartum.

• Some patients experience significant pain and functional limitations—supportive measures and analgesia are key.