Approach Considerations
Antemortem diagnosis of mucormycosis has improved, leading to improving patient survival.
Once mucormycosis is suspected, obtaining tissue for culture is vital (see Biopsy and Histologic Features). However, isolating fungus from infected tissue is usually difficult, and analysis of nasal and sputum swabs is rarely helpful. [9]
Specimens should be obtained from areas such as suggestive skin lesions, black eschars found in the nasopharynx, and nasal discharge that may appear like clotted blood. These specimens should be immediately sent to the microbiology laboratory for culturing.
Because fungi of the order Mucorales contaminate laboratory specimens, use caution when these organisms are isolated from bronchial washings, sinus samples, and sputum obtained from immunosuppressed hosts.
Although Rhizopus or Mucor species can be contaminants, the laboratory finding of these organisms in specimens from patients who are immunosuppressed or from patients with certain risk factors for mucormycosis should not be ignored.
CT Scanning and MRI
Computed tomography (CT) scanning and magnetic resonance imaging (MRI) are valuable in delineating extent of disease for most forms of mucormycosis. [10] These 2 imaging modalities are also helpful in planning surgical debridement when needed.
In rhinocerebral mucormycosis, opacification of the sinuses, bone destruction, and osteomyelitis may be noted. In some patients, images may reveal minimal changes even when extensive tissue destruction is present.
Biopsy and Histologic Features
Biopsy of necrotic lesions from pulmonary, rhinocerebral, and mucocutaneous sites is appropriate for obtaining specimens for microscopy and cultures. Sensitivity is better with tissue staining than with culturing, but collecting tissue for studies is critical for diagnosis.
Routine stains, such as hematoxylin and eosin (H&E) stains, help in visualizing Mucor hyphae, whereas Grocott methenamine silver (GMS) stain and periodic acid-Schiff (PAS) stains help to demarcate fungal elements in tissue. However, GMS stains may not reveal chlamydospores of Mucor fungi.
Histologic findings
Direct microscopic findings of hyphal elements in tissue biopsy specimens are important for the diagnosis of mucormycosis.
Biopsy material can be examined with potassium hydroxide (KOH), H&E, and GMS stains. Another useful stain is cresyl violet, which colors Mucor fungi walls brick red while coloring other fungi purple or blue.
Upon microscopic examination, fungi of the Mucorales order are characterized by aseptate hyphae, which vary in width up to 50 µm. These hyphae are broad and branch from the main hyphal trunk; they are often angled 90°. [5] Identification of most of the Zygomycetes is accomplished by observing the morphology of the sporangia, such as presence or absence of the columellae and apophyses, arrangement and number of sporangiospores, and absence or presence of rhizoids.
Fungal hyphae of Mucor species can often be differentiated from other fungi, such as Aspergillus and Fusarium species. Hyphae of Mucor species are aseptate or pauciseptate, they are broad and thick (6-25 mm wide), they have nonparallel edges, and they possess irregularly shaped fungal elements with relatively infrequent acute-angle and nonrandom branching (see the following image).
Mucormycosis with broad, aseptate hyphae (hematoxylin and eosin, original magnification ×40).
Characteristic but not pathognomonic histologic findings include angioinvasion, with the Mucor fungi invading the walls of arteries, resulting in the necrosis and thrombosis of surrounding tissue, as can be seen in the first image below. Immunohistochemical methods of staining biopsy material are available in specialized laboratories. Perineuronal invasion can also occur (see the second image below).
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Black eschar on the skin of an immunocompromised patient.
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Mucormycosis with broad, aseptate hyphae (hematoxylin and eosin, original magnification ×40).
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Angioinvasion (hematoxylin and eosin, original magnification ×10).
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Perineural invasion (hematoxylin and eosin, original magnification ×20).
