Lepra reactions are acute inflammatory episodes that may be seen in patients with leprosy (infectious agent Mycobacterium leprae). These reactions may occur at any time before, during, or after treatment for the infection. However, they occur most commonly within one year of starting multidrug therapy. Two types of lepra reactions, type 1 and type 2 reactions, affect 30%-50% of patients with leprosy. Patients with lepromatous and borderline leprosy are most commonly affected by type 2 reactions. The type 2 reaction is also known as erythema nodosum leprosum (ENL).

Patients present with sudden onset crops of painful, tender, and erythematous subcutaneous nodules that may subsequently ulcerate. The lesions are common on the face and extensor surfaces of the upper and lower extremities. In addition, there may be systemic signs such as fever, malaise, arthralgias, and myalgias. Other affected organs may include the eyes, kidneys, testes, and lymph nodes. Type 2 reactions are caused by immune complex formation and deposition. They typically are seen in patients with borderline lepromatous or polar lepromatous disease. Other risk factors for type 2 reactions include hormonal changes in women that come with puberty, pregnancy, and lactation. Reactions may last from days to weeks; they may recur or persist over months.

Prompt diagnosis and treatment of lepra reactions is necessary to prevent significant neurologic morbidity and disability.