What Causes Red, Tender Nodules on the Lower Legs?

Lower extremity erythema nodosum is an inflammatory condition characterized by tender, red-to-violet subcutaneous nodules—typically 1 to 5 cm in diameter—that most commonly appear on the anterior shins. These lesions are not ulcerative and do not drain; they evolve over days to weeks, fading from red to purple, then yellowish-brown as they resolve, often without scarring.

The underlying pathophysiology involves septal panniculitis—an immune-mediated inflammation centered in the fat lobule septa—triggered by a delayed-type hypersensitivity reaction. While the exact immunologic cascade varies by etiology, it frequently features neutrophil infiltration early on, followed by lymphocytes and histiocytes, with preserved fat cell architecture (distinguishing it from other forms of panniculitis).

Etiologically, erythema nodosum is rarely idiopathic. In adults, common triggers include streptococcal upper respiratory infections, sarcoidosis, inflammatory bowel disease (particularly Crohn’s disease), and medications such as oral contraceptives or sulfonamides. In endemic regions, tuberculosis and fungal infections like coccidioidomycosis must be considered. Less frequent but important associations include malignancy (especially lymphoproliferative disorders), pregnancy, and autoimmune conditions such as Behçet disease or systemic lupus erythematosus.

Diagnosis rests on clinical assessment supported by targeted investigations: throat culture or anti-streptolysin O titers for recent streptococcal infection; chest imaging and angiotensin-converting enzyme (ACE) level for sarcoidosis; stool studies and serologic markers (e.g., ASCA, anti-CBir1) for IBD; and purified protein derivative (PPD) or interferon-gamma release assay (IGRA) when tuberculosis is suspected. Skin biopsy—though not always necessary—is definitive when performed early and shows characteristic septal inflammation without vasculitis or necrosis.

Management focuses on identifying and addressing the underlying cause. Symptomatic relief includes leg elevation, NSAIDs (e.g., naproxen), and potassium iodide in refractory cases. Corticosteroids are reserved for severe, persistent disease unresponsive to first-line therapy. Prognosis is generally favorable, with spontaneous resolution within 3–6 weeks—though recurrence occurs in up to 20% of patients, particularly when associated with chronic conditions like sarcoidosis or IBD.