What is it?

Erythema nodosum is the most common form of panniculitis – a term referring to inflammatory conditions affecting the subcutaneous fat layer.

What causes it?

Erythema nodosum is a hypersensitivity reaction which can develop due to a number of causes including infections, drugs, autoimmune disorders, pregnancy or malignancy. Streptococcal infections are the most common cause among children. In at least one third of cases, no cause is identified.

What does it look like?

Erythema nodosum presents with bilateral tender red nodules between 2-5cm. These are almost always on the shins but may rarely involve other areas such as the thighs and forearms. Due to their depth, the nodules may be difficult to see and are best appreciated by palpation. There may also be arthritis, fevers and malaise.

How is it diagnosed?

While the diagnosis can be made clinically in straightforward cases, a deep biopsy that includes fat may be necessary when the presentation is less obvious. Investigations to identify any underlying trigger may include: antistreptolysin O titer/throat swab, chest x–ray, CBC, beta-hCG and tuberculosis testing.

How is it managed?

The mainstay of treatment is identifying and treating any underlying cause if possible. Management is supportive and focused on alleviating pain through rest, elevation and compression. NSAIDs are first line treatment, but potassium iodide may be used. The lesions usually improve within 2 weeks but pigment change may last months.

What is the differential diagnosis?

Other forms of panniculitis should be suspected in cases of painful deep nodules which last longer than 6 weeks, are in a location other than shins, ulcerate, and etc. In these cases, the patient should be assessed by a dermatologist and will likely require a biopsy to clarify the diagnosis.