Which histologic findings are characteristic of allergic fungal sinusitis (AFS)?

Updated: Apr 13, 2020
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Allergic fungal mucin, as depicted in the images below normally is first encountered at surgery. Therefore, recognition of its presence is the initial step in establishing an accurate diagnosis of allergic fungal sinusitis (AFS). Realizing that mucin, rather than paranasal sinus mucosa, demonstrates the histologic appearance consistent with allergic fungal sinusitis (AFS) is important. Examination of mucosa and polyps obtained from involved paranasal sinuses reveals findings consistent with the inflammation of a chronic inflammatory process and should be performed to exclude fungal invasion. Once mucin is collected, culture and pathologic examination are undertaken.

Left middle meatus with suctioning of thick allerg Left middle meatus with suctioning of thick allergic mucin from the ethmoid bulla in the center of the picture; the end of the suction is in the inferior portion of the picture.
The viscosity of a thick allergic mucin being suct The viscosity of a thick allergic mucin being suctioned from the nasal cavity and vestibule in a patient with allergic fungal sinusitis.

Initially described by Millar and Lamb and Katzenstein et al, histologic examination of allergic mucin reveals a constellation of characteristic findings. [22] Branching noninvasive fungal hyphae are identified within sheets of eosinophils and elongated eosinophilic bodies (Charcot-Leyden crystals), which represent the product of eosinophilic degradation. Use of various histologic staining techniques helps to identify the variety of components within allergic fungal mucin. Hematoxylin and eosin (H&E) staining accentuates the mucin and cellular components of allergic fungal mucin. Using this stain, background mucin often takes on a chondroid appearance, while eosinophils and Charcot-Leyden crystals are heavily stained and become easily detectable.

Fungi fail to stain using this technique and therefore may be difficult to identify. The presence of fungi may be implicated on H&E stain by the resulting negative image against an otherwise stained background. However, fungal hyphae and elements are often rare, scattered, and fragmented within allergic mucin, rendering identification difficult unless specific histologic stains are used. Fungal elements are recognized for a unique ability to absorb silver. This property is the basis for various silver stains, such as the Grocott-Gomori methenamine silver (GMS) stain, which turns fungi black or dark brown. The use of a fungal stain complements the findings of initial H&E stain and is extremely important in the identification of fungi.

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