Erythema nodosum

Meet the patient

A 28-year-old woman arrives with painful red lumps on both shins that appeared over two days, a swollen ankle, and a sore throat she had three weeks ago. She is on the oral contraceptive pill and otherwise well. The nodules are tender, warm, deep, and do not break the surface.^[1]

Two questions decide her work-up and they are the two that decide every EN case: is this truly EN — no ulceration, on the shins, bruise-like? and what fired it? Answer the second and the first becomes irrelevant, because EN is a skin alarm, not a skin disease.^[1]

What EN is — and the three negatives that define it

Erythema nodosum is an acute, reactive septal panniculitis — inflammation in the connective-tissue septa between fat lobules, with the lobules themselves spared. It is the commonest panniculitis by a wide margin. The fat lobules are bystanders, the overlying epidermis is normal, and the blood vessels are intact.^[1]

Three behaviours separate EN from every other tender leg nodule, and they are the behaviours examiners reward: the nodules never ulcerate, they never scar, and they run a bruise-like colour evolution — bright red, then purple, then brownish-yellow — before flattening over two to six weeks. Get those three right and the diagnosis is yours.^[1]

The cardinal insight: EN is a cutaneous alarm signal, a stereotyped response to a remote antigen. Roughly a third to half of cases have no cause found after a complete work-up — these are labelled idiopathic, but "idiopathic" is a conclusion, never a starting point.^[1]

The panniculitis frame — septal, lobular, or mixed

EN does not have named subtypes; it sits inside the panniculitis classification every candidate must draw. The single frame — septal versus lobular, with or without vasculitis — organises the entire differential of "tender leg nodules."^[3]

The classic trap: a "tender leg nodule that ulcerates" is not EN — reclassify it as erythema induratum and investigate tuberculosis. The presence of ulceration, scarring, or vasculitis on biopsy moves you off the EN pathway entirely.^[3]

How common, who, and why it lands on the shins

EN is uncommon but not rare — roughly one to five cases per 100,000 person-years — and overwhelmingly a disease of young adults, peaking between 20 and 40 years with a striking female predominance of about three to five to one.^[1]

The cause profile shifts with age and geography. In children, group A streptococcal pharyngitis dominates. In young adults, sarcoidosis (Lofgren syndrome) and IBD rise. In TB-endemic regions — much of South Asia and sub-Saharan Africa — tuberculosis and the BCG vaccine are prominent precipitants. In the American Southwest, coccidioidomycosis ("valley fever") leads; in the Ohio and Mississippi valleys, histoplasmosis.^[1]

Why it forms — a type IV reaction in the septa

EN is a type IV (cell-mediated, delayed) hypersensitivity reaction unfolding in the subcutaneous septa, with a contributing immune-complex component. A remote antigen — streptococcal M protein, mycobacterial antigen, a drug hapten, a fungal antigen, an uncharacterised sarcoidal antigen — is presented to T cells in the fat. The cytokine cascade recruits neutrophils, then lymphocytes and histiocytes, and finally organises into granulomas, all concentrated in the septa.^[1]

The histological tempo mirrors the reaction. Early lesions (first 24–72 hours) show septal oedema and a neutrophilic infiltrate; established lesions show thickened septa with lymphocytes, histiocytes and giant cells; late lesions may be frankly granulomatous. The hallmark — Miescher radial granulomas, tight knots of histiocytes arranged radially around a central cleft — is characteristic but not pathognomonic, and its absence does not exclude EN.^[1]

The most important negative is that the blood vessels are intact — no vasculitis, no fibrinoid necrosis, no leucocytoclasia. This single fact separates EN from erythema induratum, where a lobular panniculitis with vasculitis produces nodules that ulcerate, scar, and recur. It is also why EN is self-limiting: the tissue architecture is preserved, so once the antigen is cleared the reaction resolves without a trace.^[3]

TNF-alpha is central to the reaction, which is why anti-TNF agents both treat and (paradoxically) occasionally trigger EN. A genetic background modulates susceptibility: the best-characterised link is HLA-DRB1*03, strongly associated with the Lofgren phenotype of sarcoidosis and a favourable prognosis.^[4][8]

The cause hunt — SHINS on the shins

Because EN is a reactive pattern, "the diagnosis of EN" is never complete until the trigger is named or excluded. The mnemonic ties the cause categories to the very site EN favours — the SHINS.^[1]

The table ranks the causes and pairs each with the single highest-yield test:^[1]

Lofgren syndrome — the shortcut examiners love

Lofgren syndrome is the single most rewarding cause-pattern to recognise at the bedside: erythema nodosum plus bilateral hilar lymphadenopathy plus ankle arthritis. It is acute sarcoidosis with an outstanding prognosis — most patients resolve within two years without specific therapy — and it is strongly linked to HLA-DRB1*03.^[4][8]

When the full triad is present, the diagnosis is clinical and no biopsy is required. The combination is specific enough that tissue confirmation adds risk without benefit. This is a favourite viva point because it is one of the few situations in dermatology where you deliberately do not biopsy.^[4]

The clinical story — onset, evolution, the bruise

The onset is acute. Over hours to a few days, crops of bilateral, symmetric, tender, erythematous nodules appear on the anterior shins — the dependent, relatively immobile subcutis over the tibia. Individual nodules measure 1 to 5 cm, sit deep in the fat (so borders are poorly demarcated), and are warm, firm, and exquisitely tender. They never break the surface and never suppurate.^[1]

Prodromal symptoms — fever, malaise, and an arthralgia of the ankles and knees — precede the nodules by one to three weeks in roughly half of patients. Each lesion then runs a stereotyped colour evolution over one to two weeks: bright red, deepening to purple, fading through brown and yellow-green — exactly like a resolving bru. The nodules flatten and disappear over two to six weeks, leaving no ulceration and no scar. New crops may appear for several weeks if the trigger persists.^[1]

A strictly unilateral distribution should make you reconsider. Bilateral shin involvement is so characteristic that one-sided nodules point instead to trauma, insect bite, superficial thrombophlebitis, or localised infection. Less commonly EN appears on the thighs, forearms, or rarely the trunk or face.^[1]

The differential — EN versus erythema induratum

The single most examined discriminator is EN versus erythema induratum (nodular vasculitis). Get this comparison right and the rest of the "tender leg nodule" differential falls into place.^[3]

The discriminator line: shins, no ulceration, no vasculitis = EN; calves, ulceration, vasculitis = erythema induratum. Look specifically for the "vasculitic companions" — livedo reticularis, a mononeuritis, ulceration, necrosis — because their presence immediately reclassifies the eruption away from EN.^[3]

The bedside round — examine the skin, then hunt the trigger

The skin tells most of the story; the rest of the examination hunts the cause. Examine the lesions in good light, palpate them (deep, tender, non-fluctuant, non-ulcerated), and document the colour evolution of each crop — lesions at different stages simultaneously (red, purple, brown) is itself diagnostic. Then move systematically away from the skin.^[1]

A focused cause hunt: throat (exudate, cervical lymphadenopathy for strep), chest (auscultation and a chest X-ray for sarcoidosis, TB, fungal), abdomen (tenderness, perianal disease for Crohn), joints (ankles, knees for Lofgren and IBD arthritis), eyes (uveitis for sarcoid, IBD, Behcet), and mucosa (recurrent aphthae and genital ulcers for Behcet). Confirm or refute the Lofgren triad at the bedside — when all three are present, the diagnosis is made and no biopsy is required.^[1][6]

Investigations — find the trigger, not the skin diagnosis

In a classic case the clinical diagnosis is secure and skin biopsy is not mandatory; the effort goes into the cause. Biopsy is reserved for atypical, ulcerating, persistent, recurrent, or immunosuppressed presentations. The trap that wastes tissue: a punch biopsy that does not reach the subcutaneous fat is useless — it shows normal epidermis and dermis and misses the panniculitis. Always specify that fat is required, and favour an incisional or wedge biopsy of a deep nodule.^[1]

The core panel for every patient: a chest X-ray (sarcoidosis, TB, fungal, bacterial — one of the highest-yield single tests), an ASO titre and throat swab (strep), a careful drug history including the oral contraceptive pill, and a pregnancy test in any woman of reproductive age. Add CBC, ESR and CRP (expected to be raised, non-specific) and direct further tests by the history.^[1]

A normal chest X-ray, normal ASO, negative pregnancy test, no drug culprit and no GI symptoms after this panel places a patient in the idiopathic group (30–50 percent). This is legitimate only after the panel is complete. A chest X-ray showing bilateral hilar lymphadenopathy with ankle arthritis completes Lofgren syndrome and obviates biopsy.^[1][4]

Management — treat the cause, rest the legs

EN itself is self-limiting and benign; the goal is symptom relief while the cause is found. There is no role for antibiotics directed at the skin lesions, and no role for empirical corticosteroids until infection — particularly tuberculosis — has been excluded.^[1]

Layer 1 — treat or remove the trigger

This is the most effective single intervention — treating the underlying cause rather than the lesions. For microbiologically confirmed group A streptococcal pharyngitis, penicillin remains the treatment of choice, given orally as a complete course; the IDSA guideline sets out the diagnosis and dosing. Discontinue the oral contraceptive pill or any other implicated drug. Treat active tuberculosis before any immunosuppression. Optimise IBD therapy — EN usually parallels gut activity and settles as the bowel is controlled. Lofgren sarcoidosis generally needs no specific therapy beyond observation and NSAIDs for arthralgia.^[9][2][5]

Layer 2 — symptomatic anti-inflammatory therapy

Treatment directed at the lesions themselves is symptomatic — bed rest, leg elevation, cool compresses, compression stockings, and an NSAID such as ibuprofen relieve the pain while the cause is treated; none of these options treats the triggering cause. In a reported case, saturated solution of potassium iodide combined with an NSAID produced resolution within about three weeks.^[1][10]

Layer 3 — second-line agents for refractory, recurrent, or severe disease

When lesions persist beyond six weeks, recur repeatedly, or are unusually severe, escalate:^[1]

In Behçet disease — whose nodules can be clinically and histologically EN-like — colchicine is a mainstay of the mucocutaneous disease and is combined with other agents when mucocutaneous involvement is refractory.^[11] Systemic corticosteroids are held in reserve for severe, refractory disease and are never started until infection — particularly tuberculosis — has been excluded.^[1]

Potassium iodide is the most-studied second-line agent for refractory EN — a long track record, a clear if incompletely understood mechanism (suppression of neutrophil chemotaxis and the delayed hypersensitivity reaction), and a hard contraindication in pregnancy with mandatory TSH monitoring.^[7]

The subtypes and scenarios that bite

EN in inflammatory bowel disease is the commonest cutaneous manifestation of IBD, and Crohn disease outranks ulcerative colitis. The well-tested feature is that EN activity parallels intestinal disease activity — it flares before or during a bowel relapse and settles as the gut is controlled. Investigate with stool calprotectin and colonoscopy; treat the IBD. Paradoxically, EN may also appear as a reaction to the anti-TNF agents used for IBD — a careful drug history resolves it.^[2][5]

EN of pregnancy may occur in any trimester or the puerperium and is benign and self-limiting. Manage with bed rest, leg elevation and cool compresses; avoid NSAIDs after 20 weeks (oligohydramnios, premature ductus arteriosus closure), and avoid potassium iodide (fetal goitre) and colchicine (teratogenic) throughout. The prognosis is excellent for mother and baby.^[1]

Childhood EN: streptococcal infection is among the commonest identifiable causes, and children are no exception — confirm group A streptococcal pharyngitis microbiologically (throat swab, ASO titre) and treat with oral penicillin, the treatment of choice, completing the full course even as the nodules settle.^[1][9]

Behcet disease produces nodules that are clinically and histologically EN-like — a septal panniculitis, sometimes with a vasculitic component. The company they keep gives them away: recurrent oral aphthae (obligatory), genital ulceration, uveitis, skin pathergy. Colchicine is first-line for the mucocutaneous disease.^[6]

How patients come to harm — the preventable list

EN itself is benign and does not scar, so the serious harms are missed causes and iatrogenic injury:^[1]

• Giving corticosteroids before excluding tuberculosis in a TB-endemic region — activating or disseminating latent TB is the preventable disaster.
• Mislabelling erythema induratum as EN and sending a TB-associated lobular vasculitis home as "self-limiting."
• Stopping the work-up at "idiopathic" before completing the core panel — the commonest reason a "recurrence" later turns out to be missed IBD.
• Missing IBD when EN with GI symptoms is the first clue — EN can predate a bowel diagnosis by months.
• A biopsy that misses the fat — a superficial punch is non-diagnostic and wastes the patient's tissue.^[1]

Prognosis and disposition

The prognosis is excellent. Individual lesions resolve over two to six weeks without ulceration or scarring, though new crops may continue for several weeks if the trigger persists. Most patients are managed entirely as outpatients; admission is almost never required for the skin itself. The long-term outcome is governed by the cause: streptococcal EN resolves completely after antibiotics; IBD-associated EN follows the bowel; Lofgren syndrome resolves within two years in the great majority, with HLA-DRB1*03 portending the best outcome.^[1][4][8]

Recurrence is the main reason patients re-present, most often when the trigger persists or recurs — continued OCP use, repeated streptococcal infections, uncontrolled IBD. It should prompt a re-review of the original work-up, not reflexive escalation of immunosuppression. Give a clear safety-net to return urgently if nodules ulcerate, spread beyond the shins, persist beyond six weeks, or if new systemic symptoms (fever, weight loss, cough, diarrhoea) appear.^[2]

Evidence and regional deltas

There are no large randomised trials in EN; management rests on pathophysiological rationale, retrospective series and expert consensus, remarkably uniform across regions. The Perez-Garza 2021 practical algorithm underpins most of the work-up; Wick 2017 summarises the septal–lobular classification; the Rogler 2021 and Antonelli 2021 papers ground the IBD–EN relationship; Abdelghaffar 2024 and the Sikorova 2023 HLA study define Lofgren syndrome and its favourable HLA-DRB1*03-linked prognosis.^{[1][2][3][4][5][8]}

Regional deltas: the framework is globally consistent, but the cause profile and pre-steroid work-up differ by geography. In TB-endemic regions, tuberculosis and the BCG vaccine are leading precipitants and an IGRA plus chest X-ray must precede any corticosteroid. In endemic-fungal belts, fungal serology and chest imaging are core, not optional. Lofgren syndrome carries the same excellent prognosis worldwide, but the threshold to biopsy is lower wherever TB or fungal disease is prevalent, because the hilar lymphadenopathy of sarcoidosis and of infective granulomatous disease can look identical.^[1]

The mantra, and the mnemonic

The mantra: shins, tender, no ulcer, no scar — now find the cause, and never steroid until TB is gone.^[1]

Ward-round test — three stems, thirty seconds each