Answer: Allergic fungal sinusitis

Histology: The most characteristic feature of allergic fungal sinusitis is the presence of so-called allergic mucus, which is characterized here as deeply basophilic and amphophilic mucin with a tigroid or striped appearance. This mucin contains degranulated and degenerating eosinophils and numerous Charcot-Leyden crystals. Occasionally, fungal hyphae will be demonstrated but often the use of Gomori Methenamine Silver stain is required to demonstrate the hyphal figures. The hyphal forms may be somewhat distorted and fragmented, and may not in all instances be picture perfect. In some cases, especially those caused by pigmented fungi, the Fontana-Masson stain may be useful in confirming the diagnosis. Tissue cultures are desirable but may not be obtained by the operator in all instances. Aspergillus is a common cause of allergic fungal sinusitis, but other organisms such as curvularia may be the etiology as well.

Discussion: Allergic fungal sinusitis must be distinguished from chronic sinusitis of other infectious etiology because the reaction in allergic fungal sinusitis is hypersensitivity rather than a response to infection. The fungi colonize the sinuses but do not, except in very unusual circumstances, invade the tissue. Antibiotic therapy, therefore, is not usually indicated. Invasive fungal sinusitis is a different disorder and one that usually occurs in immunosuppressed patients. Basically the fungal hyphae may invade vessels or travel along the perineural space. Vascular invasion usually produces infarction. Accordingly, a gross specimen of invasive fungal sinusitis usually has a very dark red or black color. Mucoceles of the paranasal sinuses represent a post-obstructive phenomenon. The mucus content may be clear, yellow or red depending upon whether or not bacterial infection and/or hemorrhage have occurred.