Board Review: Targetoid Lesions Differential Diagnosis

Overview / Definition Targetoid lesions are characterized by a concentric ring pattern resembling a target. These lesions can vary in color, size, and morphology, often presenting as erythematous macules or papules with a darker central area. The differential diagnosis encompasses a wide range of conditions, including infectious, autoimmune, and neoplastic disorders. Epidemiology Targetoid lesions can occur in individuals of any age, though certain conditions tend to present more frequently in specific demographics. For instance, erythema multiforme is more common in young adults, while targetoid lesions due to Lyme disease may be more prevalent in endemic areas during summer months. Understanding the epidemiology of these lesions assists in recognizing potential etiologies. Pathophysiology / Mechanism The pathophysiology of targetoid lesions varies by underlying cause. In conditions like erythema multiforme, target lesions result from a hypersensitivity reaction, often triggered by infections (e.g., herpes simplex virus) or medications. In contrast, the characteristic target lesions of Lyme disease arise from a localized inflammatory response to the Borrelia burgdorferi spirochete. Understanding these mechanisms can guide diagnostic and therapeutic approaches. Clinical Presentation Targetoid lesions typically present as: Coloration: Erythematous with central necrosis or vesiculation. Shape: Concentric rings, often described as “bull's-eye.” Location: Can occur anywhere on the body, but often found on the extremities. Associated Symptoms: May be accompanied by pruritus, pain, or systemic symptoms depending on the underlying cause. Diagnosis / Workup The evaluation of targetoid lesions involves a careful history and physical examination. Key steps include: History: Assess for recent infections, medication use, travel history, and exposure to ticks. Physical Examination: Document the number, size, and distribution of lesions. Laboratory Tests: May include serologic te