Erythema Nodosum: Etiology and Workup

Overview / Definition Erythema nodosum is a type of panniculitis, specifically an inflammation of the subcutaneous fat. It presents as painful, raised, erythematous nodules, primarily affecting the anterior aspects of the lower legs. Erythema nodosum is often associated with systemic conditions, making it essential to identify the underlying cause for appropriate management. Epidemiology Erythema nodosum is more prevalent in women than men, with a ratio of approximately 3:1. It commonly occurs in young adults, particularly those between the ages of 20 and 40. The incidence may vary depending on geographic location and associated conditions. Pathophysiology / Mechanism The pathophysiology of erythema nodosum involves a hypersensitivity reaction, typically of type IV (delayed-type hypersensitivity). This reaction is triggered by various infectious agents, medications, or systemic diseases, leading to the activation of T-lymphocytes and the release of cytokines, which promotes inflammation in the subcutaneous fat. Clinical Presentation Patients with erythema nodosum typically present with: Symmetrical, painful nodules on the lower extremities, especially the shins. Fever, malaise, and arthralgia may accompany the skin lesions. Lesions are usually tender, firm, and can vary in size from 1 to 10 cm. Over time, the nodules may become purple and resolve with bruising. Systemic symptoms can guide the clinician toward identifying an underlying cause. Diagnosis / Workup The diagnosis of erythema nodosum is primarily clinical, based on the presentation of characteristic nodules. However, a thorough workup is essential to identify potential etiologies: History and Physical Examination: Assess for recent infections, medications, or systemic diseases. Laboratory Tests: Complete blood count (CBC) to check for leukocytosis. Erythrocyte sedimentation rate (ESR) or C-reactive protein (CRP) for inflammation. Serological tests for infectious diseases (e.g., streptococcal infections, tu