Erythema Nodosum
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Direct answer
Erythema nodosum is the commonest form of septal panniculitis, presenting as tender, erythematous nodules on the anterior shins that evolve through colour changes over weeks. It is a reactive hypersensitivity response rather than a primary skin disease, most often linked to streptococcal infection, sarcoidosis, tuberculosis, drugs or inflammatory bowel disease. Management means finding and treating the cause, with analgesics and rest for symptoms.
What you must remember
- Lesions: tender, warm, erythematous nodules on the anterior shins, usually bilateral, which later bruise and fade.
- Histology: septal panniculitis, with inflammation in the fibrous septa of subcutaneous fat while the lobules are spared.
- Sarcoidosis: Löfgren syndrome combines erythema nodosum, bilateral hilar lymphadenopathy and arthralgia, with a favourable course.
- Streptococcal link: erythema nodosum typically follows a throat infection, supported by a raised antistreptolysin O titre.
- Tuberculosis: an important cause in endemic regions, so tuberculosis should be excluded.
- Other causes: oral contraceptives, sulphonamides, inflammatory bowel disease, pregnancy and Behçet disease.
- Leprosy reaction: erythema nodosum leprosum is a type 2 reaction in lepromatous or borderline leprosy, needing leprosy-specific treatment.
- Work-up: chest radiograph, ASO titre, ESR or CRP, and tuberculosis screening guided by risk.
A targeted search for the cause
A young woman with painful red shin nodules and fever needs a targeted search for a cause rather than a single test. The steps below move from confirming the lesion to treating the trigger.
- Confirm the lesion. Tender, warm, erythematous shin nodules point to septal panniculitis, and a biopsy is needed only when the picture is atypical.
- Take a focused history. Ask about sore throat, cough, bowel symptoms, new drugs, contraception, pregnancy and contact with tuberculosis.
- Examine the chest. Bilateral hilar lymphadenopathy on chest radiograph points to sarcoidosis, and Löfgren syndrome carries a favourable course.
- Send targeted tests. An ASO titre for recent streptococcal infection, ESR or CRP, and tuberculosis screening guided by risk.
- Exclude leprosy and Behçet. In endemic areas, check for leprosy features, since erythema nodosum leprosum needs its own treatment, and look for oral or genital ulcers suggesting Behçet disease.
- Treat the cause and the pain. Treat the trigger, use analgesics and rest, and expect self-limiting resolution over weeks in idiopathic cases.
Where candidates lose marks
Trap questions here mostly test whether candidates see erythema nodosum as a sign rather than a diagnosis.
- Trap: treating erythema nodosum leprosum as ordinary erythema nodosum Correct: it is a type 2 leprosy reaction needing leprosy-specific therapy and specialist review.
- Trap: accepting erythema nodosum as a primary skin disease Correct: it marks an underlying systemic cause, so the search must continue beyond the skin.
- Trap: assuming a negative ASO excludes a streptococcal cause Correct: the titre may be normal or delayed, so the clinical history and other markers still matter.
Frequently asked questions
Which systemic disease causes erythema nodosum with bilateral hilar lymphadenopathy?
Sarcoidosis is the classic cause of erythema nodosum with bilateral hilar lymphadenopathy, a combination known as Löfgren syndrome. Arthralgia often accompanies it, and the presentation usually follows a favourable course.
How does erythema nodosum leprosum differ from erythema nodosum?
Erythema nodosum leprosum is a type 2 reaction in lepromatous or borderline leprosy, whereas idiopathic erythema nodosum is a hypersensitivity response without leprosy. Erythema nodosum leprosum needs leprosy-specific treatment and specialist review, not just analgesia.
Which histology pattern defines erythema nodosum?
Erythema nodosum shows septal panniculitis, with inflammation in the fibrous septa of subcutaneous fat while the lobules are spared. This pattern separates it from lobular panniculitis, which affects the fat lobules themselves.
Why is tuberculosis considered in erythema nodosum?
Tuberculosis is an important cause of erythema nodosum in endemic regions, and it may accompany primary infection with hilar lymphadenopathy. Screening with a chest radiograph and tuberculosis testing is therefore part of the work-up.
Which test helps confirm recent streptococcal infection in erythema nodosum?
The antistreptolysin O titre helps confirm recent streptococcal infection in erythema nodosum, although it can be normal or delayed. The clinical history of throat infection should be considered alongside the result.