Answer: Allergic fungal sinusitis

Histology: The respiratory mucosa has a very polypoid appearance, largely as a result of prominent submucosal edema. The submucosa is heavily infiltrated by chronic inflammatory cells including abundant plasma cells. Adjacent to the polyp is an extramucosal mass comprised of mucoid material and eosinophilic cellular debris. This cellular debris tends to be distributed within the mucous in a layered fashion. At higher power, the cellular material is comprised of degranulating eosinophils, “Charcot-Leydin” crystals and sloughed epithelial cells. Branching fungal hyphae are noted on a Gomori Methanamine silver stain stain. These are not present in the tissue. Instead, they are embedded within the so-called “allergic mucin”.

Discussion: Not all fungal infections of the sinonasal tract are the same. The specific type of fungal disease results from a complex interplay between the environmental load, the host immune status, and local tissue conditions. Among the 4 major types, the most significant division is between invasive and non-invasive fungal disease. Invasive disease, particularly the fulminant form, is generally seen in immunocompromised patients. Unlike the present case, it is characterized by direct invasion of fungal forms into the tissue, and this invasion is often associated with ulceration and geographic tissue necrosis.

The non-invasive forms include mycetoma and allergic fungal sinusitis. Mycetomas (fungal balls) are characterized by densely compacted fungal hyphae arranged in concentric growth layers. The fungus tends to elicit little or no inflammatory reaction. Simple curettage is curative.

Allergic fungal sinusitis is a non-invasive form of fungal sinusitis that occurs primarily in young adults with long histories of asthma and nasal polyposis. Most have elevations of their total IgE levels and absolute eosinophil counts. Clinically, the affected sinus contains a firm, rubbery, and tenacious mucoid material. The hallmark histologic feature is the presence of the so-called allergic mucin. An appreciation for allergic mucin should alert the pathologist to look for fungi; however, they may be sparse, and in as many as 40% of cases, they may not be apparent even with special stains. The mainstay of therapy is removal of the tenacious mucus to restore mucociliary function, and systemic steroids. Antifungal therapy has no role in the treatment of allergic fungal sinusitis.