Lucio phenomenon is a rare reaction seen in patients with multibacillary leprosy due to endothelial invasion of Mycobacterium leprae. It is most commonly seen in Mexico and Central America, although cases have been reported in South Asia, Southeast Asia, Africa, and the Middle East.

The pathogenesis is thought to be mediated by complement activation of immune-complex deposition. Leprosy bacilli invade blood vessel endothelium, causing endothelial proliferation and narrowing of the vascular lumen, progressing to a cutaneous hemorrhagic infarction. Reports have suggested that certain stressors, such as pregnancy, concurrent infection, and cold weather, may precipitate Lucio phenomenon. The disease commonly occurs in untreated lepromatous leprosy, but it also occurs in patients with partially treated lepromatous leprosy.

Manifestations of Lucio phenomenon typically occur 1-3 years after the onset of leprosy. Patients present with sudden onset of painful, violaceous infiltrative patches or plaques that may become hemorrhagic, bullous, and then ulcerated. Subcutaneous nodules may be a feature. The limbs are mainly affected, but in severe cases, truncal and facial involvement are also reported. Ulcers heal with atrophic scars after 2-4 weeks. Although constitutional symptoms are rare, patients may have fever, myalgia, or arthralgia. Lymphadenopathy, hepatosplenomegaly, and leukocytosis with neutrophilia may be seen.

Lucio phenomenon may be complicated by sepsis from superadded infection.