Rebecca C. Brady
The term mucormycosis refers to a group of invasive mycoses caused by members of the order Mucorales within the class Zygomycetes.1 The Mucorales are distributed worldwide and commonly grow in decaying organic matter.3Although 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.4 Diabetes mellitus, particularly diabetic ketoacidosis, is the most common predisposing condition.2,5 Additional risk factors include acidosis, neutropenia,8 malignancies,9 burns,10prematurity,11 immunosuppressive conditions, and deferoxamine therapy.15
Infection in humans most commonly occurs following inhalation of the spores of Mucorales into the respiratory tract.4 Spores may also be ingested or introduced directly into abraded skin.16 Germination of spores occurs with hyphal proliferation and invasion of tissue. Infection may spread by direct extension and hematogenous dissemination. The pathologic hallmark of mucormycosis is hyphal invasion of blood vessels with resultant hemorrhage, thrombosis, infarction, and production of black, necrotic debris.6,17
The mechanisms that account for the increased susceptibility to mucormycosis in different patient groups remain incompletely understood. Neutrophils and macrophages are important components of the host response to Mucorales.18,19 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.20 Because these fungi metabolize ke-tones and grow optimally at an acid pH,21 the metabolic conditions encountered in ketoacidotic hosts may enhance their growth.
Clinical manifestations are classified by site of involvement into rhinocerebral, pulmonary, cutaneous, gastrointestinal, disseminated, and miscellaneous infections.1 Rhinocerebral infection occurs most frequently and typically presents as facial pain, nasal congestion, and headache in a poorly controlled diabetic patient.22,23 Infection may spread to the orbit, the cerebral vasculature and brain.24
Most cases of pulmonary mucormycosis have occurred in neutropenic hosts, especially those receiving chemotherapy for leukemia and lymphoma.25 These patients present with unremitting fever and dyspnea. The chest roentgen-ogram may show patchy consolidation and cavity formation. Hemoptysis may be a fatal complication.26
Cutaneous mucormycosis usually occurs at sites of burns, trauma, and invasive procedures in immunosuppressed hosts, including premature infants.11,16,27 The skin lesion may begin as an area of erythema and induration that subsequently develops central necrosis. Skin lesions may also be a manifestation of disseminated infection.28,29
Risk factors for gastrointestinal mucormycosis include malnutrition, prematurity,30,31 kidney transplantation,32 and underlying gastrointestinal disease. The stomach, ileum, and colon are involved most frequently.33 Presenting findings may include nonspecific abdominal pain, hematochezia, or melena. Premature infants may experience necrotizing enterocolitis.28
Disseminated infection most often follows pulmonary invasion and may spread to the brain, liver, spleen, and other tissues.25,34 Clinically, these patients have rapidly progressive multiple organ failure with a high mortality rate. Miscellaneous forms of mucormycosis include endocarditis, osteomyelitis, and pyelonephritis.1
Mucormycosis must be differentiated from other opportunistic infections in immuno-suppressed hosts. Cutaneous mucormycosis may mimic ecthyma gangrenosum, which is commonly due to Pseudomonas aeruginosa.35 Invasion of blood vessels is a major pathologic finding with Aspergillus infection.36 Not surprisingly, the pulmonary, cerebral, and cutaneous manifestations of aspergillosis are clinically indistinguishable from those of mucormycosis. Definitive diagnosis requires demonstration of hyphal elements invading tissue in a biopsy specimen.1,4 Because the Mucorales may colonize body surfaces, swabs of drainage or abnormal tissue are inappropriate. Tissue biopsies, especially of black necrotic lesions, should be sent for histologic examination and for culture. Grinding of tissue should be avoided. Demonstration of irregularly shaped, broad, nonseptate hyphae with right-angle branching by either hematoxylin and eosin or Grocott-Gomori methenamine-silver nitrate staining is the gold standard for diagnosis. The agents of mucormycosis may be difficult to isolate in culture from infected tissues. Cultures of blood, urine, and cerebrospinal fluid are rarely positive.
Successful treatment of mucormycosis requires a coordinated medical and surgical approach.22 If possible, the underlying predisposing condition should be reversed. Metabolic acidosis should be corrected, and the doses of corticosteroids and other immunosuppressive drugs should be lowered if at all possible. All devitalized tissue should be surgically removed. Often, debridement must be repeated daily for several days. Intravenous administration of amphotericin B is the standard therapy.37 Posaconazole, an orally available triazole, has been used as salvage therapy in mucormycosis.37 Duration of therapy for antifungals is often hard to define and should be individualized for the specific location of infection in a particular patient.