Zygomycosis is an umbrella term for all diseases caused by fungi of the class Zygomycetes. The more common term mucormycosis refers to a group of invasive mycoses caused by members of the order Mucorales, within the class Zygomycetes. Rhizopus species are the most commonly isolated agents of mucormycosis.
PATHOGENESIS AND EPIDEMIOLOGY
The Mucorales are distributed worldwide and commonly grow in decaying organic matter. Although exposure to the airborne spores of these thermotolerant, rapidly growing fungi is universal, human disease is infrequent and is indicative of a serious underlying predisposing condition. Diabetes mellitus, particularly diabetic ketoacidosis, is the most common predisposing condition in patients with mucormycosis. Underlying disease accompanied by acidosis such as uremia, malnutrition, and congenital metabolic aciduria may also predispose to mucormycosis. Additional risk factors include neutropenia, hematologic malignancies, burns, prematurity, corticosteroid therapy, solid organ transplantation, bone marrow transplantation, and deferoxamine therapy for management of iron overload states.
Infection in humans most commonly occurs following inhalation of the spores of Mucorales into the respiratory tract. Spores may also be ingested or introduced directly into abraded skin. Germination of spores occurs with hyphal proliferation and invasion of tissue. Infection may spread by direct extension and hematogenous dissemination. Regardless of the tissue involved, the pathologic hallmark of mucormycosis is hyphal invasion of blood vessels with resultant hemorrhage, thrombosis, infarction, and production of black, necrotic debris. The reasons these fungi target the vasculature are not well understood.
Neutrophils and macrophages are important components of the host response to Mucorales. Thus, defects in their function likely contribute to the pathogenesis of mucormycosis. Iron is an important growth factor for these fungi; hence, interactions between iron molecules and transferrin have been postulated to play a role in predisposing deferoxamine-treated patients to the development of mucormycosis. Because these fungi metabolize ketones and grow optimally at an acid pH, the metabolic conditions encountered in ketoacidotic hosts may enhance their growth.
The clinical manifestations of mucormycosis are classified by site of involvement into rhinocerebral, pulmonary, cutaneous, gastrointestinal, disseminated, and miscellaneous infections. Rhinocerebral infection occurs most frequently and typically presents as facial pain, nasal congestion, and headache in a poorly controlled diabetic patient. From the nasal mucosa and paranasal sinuses, infection may spread to the orbit, resulting in orbital cellulitis, paresis of extraocular muscles, and proptosis. Further extension into the cerebral vasculature and brain can lead to cavernous sinus thrombosis, brain infarcts, and focal neurologic deficits.
Most cases of pulmonary mucormycosis have occurred in neutropenic hosts, especially those receiving chemotherapy for leukemia and lymphoma. Clinically, these patients present with unremitting fever and dyspnea. The chest roentgenogram may show patchy consolidation and cavity formation. Infection progresses rapidly, and hemoptysis may be a fatal complication.
Cutaneous mucormycosis usually occurs at sites of burns, trauma, and invasive procedures in immunosuppressed hosts, including premature infants. The ...