Erythema nodosum

Overview

•Most common form of panniculitis - septal inflammation of subcutaneous fat, no vasculitis

•Young women (20-40 years); 3-6x more common in women; occurs in pregnancy

•~50% idiopathic; always search for reversible cause

Causes

Key associations

Streptococcal pharyngitis - most common infective cause (raised ASO titre)

Sarcoidosis - bilateral hilar lymphadenopathy on CXR

Inflammatory bowel disease - especially Crohn's disease

Pregnancy - hormonal trigger; self-limiting, no fetal harm

Tuberculosis - screen before starting steroids

Behcet's syndrome - alongside oral/genital ulcers, uveitis

OCP - drug cause; cessation may resolve lesions

Idiopathic - ~50%

🎯

Löfgren's syndrome = erythema nodosum + bilateral hilar lymphadenopathy + polyarthralgia - this triad = acute sarcoidosis; good prognosis.

Presentation

•Painful, tender nodules on anterior shins (pretibial) - bilateral, 1-5 cm (up to 10 cm)

•Colour progression - red → purple → yellow-green (bruise-like); no ulceration

•Prodrome - fever, malaise, arthralgia, URTI symptoms 1-3 weeks before rash

•Arthralgia/arthritis - ankles and knees most common

Investigations

•Clinical diagnosis - investigations target underlying cause

🥇 First-line

•pregnancy test (all women of reproductive age), throat swab + ASO titre, CXR (hilar lymphadenopathy/TB), FBC/CRP/ESR, serum ACE

•Mantoux/IGRA - screen for TB in at-risk individuals

•Skin biopsy - rarely needed; only if atypical or diagnosis uncertain

🎯

Young woman with shin nodules + nausea + urinary frequency + non-offensive vaginal discharge - next investigation is a pregnancy test, not biopsy. These are physiological changes of pregnancy.

Management

•Self-limiting - lesions resolve in 3-6 weeks without scarring

🥇 First-line

•treat underlying cause (e.g. streptococcal infection with phenoxymethylpenicillin; stop OCP if implicated)

•Rest and leg elevation - reduces oedema

•NSAIDs (e.g. ibuprofen, naproxen) - symptomatic relief; avoid in pregnancy

🥈 Second-line

•potassium iodide (refractory/recurrent); oral prednisolone (severe/refractory - only after excluding infection, especially TB)

🚨

Do NOT start systemic corticosteroids without excluding tuberculosis first - steroids in TB-associated erythema nodosum risk precipitating disseminated disease.

Prognosis

•Resolves in 3-6 weeks; no scarring or ulceration

•Recurrence in ~30% - usually linked to persistent/recurring underlying trigger

•In pregnancy - self-limiting, no adverse outcomes for mother or fetus

Differential diagnosis - erythemas compared

Key erythema differentials

Feature	Erythema nodosum	Erythema multiforme	Erythema marginatum
Appearance	Tender red/purple nodules on shins	Target (bull's-eye) lesions	Pink rings on extensor surfaces/torso
Key cause	Streptococcus, sarcoidosis, IBD, pregnancy	HSV (most common), drugs	Rheumatic fever (Group A Strep)
Ulceration	No	Possible (mucous membranes in SJS)	No