Answer: Allergic fungal sinusitis

Histology: The histology is compatible with the sinonasal tract. The lining epithelium is a pseudostratified ciliated columnar epithelium (i.e. respiratory epithelium). The respiratory mucosa has a very polypoid appearance, largely as a result of prominent submucosal edema. Inflammatory cells – mostly eosinophils – infiltrate the submucosa. Adjacent to the polyp is an extra-mucosal material comprised of mucous and cellular debris. This material has a distinctly laminar appearance at low power. At higher power, the extra-mucosal material is comprised of a mucous with embedded eosinophils, sloughed epithelial cells, degenerating cellular debris, and abundant “Charcot-Leydin” crystals. A Gomori Methanamine Silver stain (GMS) demonstrates fungal hyphae. Morphologically, these forms are consistent with aspergillus: They demonstrate uniform width, septations, and acute 45-degree angle branching. They are confined to the mucous material and are not noted within the respiratory mucosa. Notably, there is no evidence of granulomatous inflammation or necrosis.

Discussion: 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 mucous material. The histologic appearance of this so-called “allergic mucin” is distinctive. At low power it has a “tigroid” appearance. This is due to the laminated deposition of degenerating cellular debris within fibrin and mucous. 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.

Due to vast differences in treatment and prognosis, allergic fungal sinusitis must be distinguished from other forms of fungal sinusitis. In mycetomas, the fungal hyphae are typically arranged in dense concentric growth layers and are not associated with allergic mucin. Chronic indolent sinusitis is an invasive form of fungal sinusitis in that the organisms penetrate the mucosa. Disease progression is indolent. The invasive fungi typically induce a chronic granulomatous host reaction. Fulminant sinusitis is usually seen in the immunocompromised host (profoundly neutropenic). The fungi show a destructive and fulminant pattern of invasive growth. Angiocentric invasion is common, and there may be prominent tissue necrosis with little associated host inflammatory reaction.