AKT · Dermatology/Reactive & systemic signs

Erythema nodosum

Septal panniculitis — a reactive inflammation of subcutaneous fat

Overview

A reactive inflammation of subcutaneous fat (septal panniculitis) presenting as crops of tender, red, raised nodules, classically on the SHINS. It is a marker — the task is to find the underlying cause (infection, sarcoidosis, IBD, drugs, pregnancy) because the rash itself is self-limiting.

Recognise

Crops of tender, ill-defined, red/violaceous warm nodules, symmetrically on the SHINS (extensor lower legs)
Lesions do NOT ulcerate; they resolve over weeks fading like a bruise (no scarring)
Often with malaise, fever and arthralgia

Red flags

Associated systemic disease — bilateral hilar lymphadenopathy (sarcoidosis), IBD symptoms, recent streptococcal infection or TB

Differentials & how to tell them apart

Cellulitisunilateral, spreading, infective — erythema nodosum is bilateral discrete tender nodules that do not spread

Nodular vasculitis / erythema induratumcalves rather than shins, may ulcerate (TB association)

Insect bites / thrombophlebitishistory, distribution along a vein

Bruises (non-accidental)consider the pattern/history

Clinical image

We can't host this one — the image is DermNet's and their licence doesn't cover us republishing it. Their page for it is free to view.

Erythema nodosum — clinical imagesDermNet

Investigations

Find the cause: throat swab/ASO titre (streptococcus), chest X-ray (sarcoidosis/TB — bilateral hilar lymphadenopathy), FBC/CRP, and screen for IBD if GI symptoms. Biopsy rarely needed.

Management

Identify and treat the underlying cause + NSAIDs, rest and leg elevation

1Symptomatic relief (NSAIDs, rest, leg elevation, analgesia) — it is self-limiting over weeks.Gate: Erythema nodosum is a SIGN, not a diagnosis — always work up the cause (chest X-ray for sarcoid/TB, strep serology, IBD screen, drug and pregnancy history); the rash settles but the cause may not
2Treat the identified cause; persistent/severe idiopathic disease → potassium iodide or systemic steroids (after excluding infection/TB), specialist input.

Treat the underlying cause — the priority — strep, TB, sarcoid, IBD, drug, pregnancy

NSAIDs + rest/leg elevation + analgesia — symptomatic relief; usually self-limiting

Potassium iodide / systemic steroids — persistent or severe idiopathic disease (specialist, once infection/TB excluded)

Key points

Causes mnemonic — NODOSUM: No cause (idiopathic, commonest), Drugs (sulfonamides, COCP), Sarcoidosis, Other infections (strep, TB), Ulcerative colitis/IBD, Microbiology (streptococcus), pregnancy. The nodules fade like bruises and never ulcerate.

Monitor & prognosis

Resolution; follow up the underlying cause.

Self-limiting over 3–6 weeks; prognosis follows the cause.

Source: NICE CKS; DermNet