Erythema Nodosum

Also known as: Septal Panniculitis

A skin condition causing tender, red bumps under the skin, usually on the shins, representing an allergic reaction to infections, medications, or other diseases.

Source: American Academy of Dermatology: Erythema Nodosum

Updated: Aug 07, 2026

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Red Flag Warning & Emergency Situations

Nodules that ulcerate or drain pus (suggests infection or erythema induratum, NOT EN).
Hemoptysis, night sweats, or severe weight loss (suggests TB or malignancy).

Emergency Management: None directly related to the rash itself, but underlying causes (like acute severe inflammatory bowel disease or systemic infections) may require urgent intervention.

Core Definition:

Erythema nodosum (EN) is the most common form of panniculitis, presenting as an acute, self-limiting nodular eruption characterized by tender, erythematous, inflammatory nodules. The lesions are typically distributed symmetrically on the extensor surfaces of the lower extremities (pretibial areas).

Detailed Overview

EN is not a primary disease but a delayed-type hypersensitivity reaction (Type IV) to various antigenic stimuli. The sudden appearance of these painful nodules is often accompanied by systemic symptoms such as fever, malaise, and arthralgia. Identifying the underlying trigger is the most critical aspect of managing the patient, as EN can be the first manifestation of serious systemic diseases like sarcoidosis, tuberculosis, or inflammatory bowel disease.

Epidemiology & Demographics

Peak incidence is between 20 and 30 years of age. It is 3 to 6 times more common in women than in men.

Etiological Mechanism

Idiopathic in up to 50% of cases. The most common identifiable causes include Streptococcal pharyngitis (especially in children), oral contraceptives, sarcoidosis (Löfgren syndrome), and inflammatory bowel disease.

Primary Causes

Streptococcal Infections (Group A beta-hemolytic strep)

Sarcoidosis

Inflammatory Bowel Disease (Crohn's, Ulcerative Colitis)

Medications (Oral Contraceptives, Sulfonamides, Penicillins)

Tuberculosis and Endemic Fungi (Coccidioidomycosis)

Recent Upper Respiratory Infection: Specifically a sore throat due to Strep pyogenes within the last 1-3 weeks.
Female Gender: Hormonal factors likely play a role, given the association with OCPs and pregnancy.
Underlying Autoimmune Disease: Presence of IBD or sarcoidosis strongly predisposes individuals to EN.

EN is an immunologic response characterized by immune complex deposition and neutrophil infiltration in the septa of the subcutaneous fat. This delayed hypersensitivity reaction leads to septal panniculitis. Initially, the septa are widened by edema and a neutrophilic infiltrate. Over time, the infiltrate becomes predominantly mononuclear, with histiocytes and multinucleated giant cells forming Miescher radial granulomas (the histopathologic hallmark). There is no primary vasculitis.

Characteristic Clinical Presentation

Tender Skin Nodules: Deep-seated, intensely painful, red nodules measuring 1 to 5 cm.
Arthralgias: Joint pain, stiffness, and occasionally frank arthritis, usually of the ankles and knees, preceding or accompanying the rash.
Fever and Malaise: Low-grade fever and fatigue are common prodromal symptoms.
Color Evolution: Nodules initially are bright red, then evolve to a purplish color, and finally turn yellowish-green resembling a deep bruise (erythema contusiforme).

Physical Examination Signs

Bilateral, symmetrical, poorly circumscribed erythematous nodules on the anterior shins.
Nodules are warm, firm, and exquisite to palpation.
Absence of ulceration or scarring (lesions resolve completely).

Clinical Risk: Uncontrolled or untreated conditions may progress to the following complications:

Chronic/Recurrent EN: Occurs in a minority of patients, usually linked to persistent underlying triggers like IBD.
Missed Systemic Disease: The primary complication is failing to diagnose the serious underlying cause (e.g., untreated TB).

Diagnostic Criteria & Guidelines

Diagnosis is mostly clinical based on the characteristic appearance and location of the nodules. A deep incisional biopsy (reaching the subcutaneous fat) is definitive, showing septal panniculitis with Miescher radial granulomas, but is rarely needed unless presentation is atypical. Extensive workup is required to find the underlying cause.

Differential Diagnosis

Erythema Induratum (Nodular Vasculitis) - involves posterior calves, tends to ulcerate.
Cellulitis
Superficial Thrombophlebitis
Alpha-1 Antitrypsin Deficiency Panniculitis

Laboratory Tests & Biomarkers

Throat Culture and Antistreptolysin O (ASO) Titer: Elevated ASO titer confirms recent streptococcal infection.
Erythrocyte Sedimentation Rate (ESR) & CRP: Significantly elevated during the acute phase.
Tuberculin Skin Test (PPD) or IGRA: May be positive if underlying TB.

Imaging Modalities & Findings

Chest X-Ray:

Acute Phase
Bright red, extremely tender nodules with prominent systemic symptoms.
Resolving Phase
Lesions flatten and undergo bruise-like color changes (purple to yellow). Pain subsides.

First-Line Treatment:

Management is primarily symptomatic as the condition is self-limiting (resolves in 3-6 weeks). Bed rest and leg elevation are crucial. NSAIDs: Indomethacin 25-50 mg TID or Naproxen 500 mg BID for analgesia and anti-inflammatory effect. Treat the underlying cause (e.g., Penicillin V 500 mg BID for 10 days for Strep pharyngitis).

Second-Line & Adjunctive Therapy

Potassium Iodide (SSKI) 300-900 mg daily in divided doses (causes rapid resolution but mechanism is unclear). Systemic corticosteroids (e.g., Prednisone 40 mg daily tapering over 2 weeks) can be used for severe, refractory cases ONLY AFTER infectious causes (TB, deep fungi) have been definitively ruled out.

Surgical & Procedural Management

None indicated, other than deep incisional skin biopsy for diagnosis in atypical or refractory cases.

Recommended Lifestyle Changes

Strict rest and elevation of the legs, as standing drastically increases pain and prolongs the disease course.
Use of supportive compression stockings once the nodules are less exquisitely tender.

Patient Counseling & Advice

Reassure the patient that while the nodules are extremely painful, they will not scar or ulcerate, and typically resolve completely within 4 to 6 weeks. Emphasize that the bruise-like color change is a normal part of the healing process.

Follow-Up & Monitoring Schedule

Follow up in 2-4 weeks to ensure resolution and to review the results of the diagnostic workup (chest x-ray, ASO titers) to rule out serious systemic diseases.

Preventive Strategies

Prompt treatment of streptococcal pharyngitis. Avoidance of known trigger medications if a prior episode was drug-induced.

Excellent. The nodules invariably resolve without atrophy or scarring within 3 to 6 weeks. Prognosis depends entirely on the underlying associated condition.

Frequently Asked Questions

No, a hallmark of erythema nodosum is that it heals completely without ulcerating or leaving permanent scars, though a temporary dark spot may linger.

Erythema nodosum is often an 'alarm bell' for conditions inside the body, such as sarcoidosis or tuberculosis, which often show up first on a chest X-ray.

Authoritative Sources & Evidence References

American Academy of Dermatology: Erythema Nodosum:
View Official Guideline

Key Literature & References:

Evidence Erythema nodosum: an update on diagnosis and management