Computed tomography (CT) of the paranasal sinuses may show hyperdensities with small calcifications that represent allergic mucin concretions. There may be adjacent hypodense opacification of the involved sinus with air-fluid levels representing retained mucus. More advanced disease can lead to loss of adjacent bony sinus margins and even erosion of the anterior skull base due to expansile pressure. There may be extensive polyps that accompany mucosal thickening. Magnetic resonance imaging (MRI) reveals low signal intensity on T1-weighted images and areas of signal void on T2-weighted images, possibly due to the presence of paramagnetic metals within the allergic mucin concretions (3).

While AFS may be suspected based on the clinical signs and symptoms, and positive allergy testing, definitive diagnosis requires histopathologic confirmation. The identification of allergic mucin at the time of surgery with the absence of histologic evidence of fungal invasion confirms the diagnosis. The presence of mucosal necrosis, granulomas, or giant cells is indicative of a more serious, chronic invasive fungal sinusitis and must be distinguished from AFS (8). Fungal stains of the allergic mucin reveal hyphae of a variety of fungal species. Bipolaris is most common, but Alternaria, Exserohilum, Curvularia, and Aspergillus have been identified. Patients have elevated specific and total IgE levels and positive skin testing. Total IgE levels are often high at the time of presentation, but decrease as therapy is instituted (8).

As AFS is an immune-mediated condition, an alternative diagnosis must be sought in patients who are immunocompromised. They should undergo biopsy of diseased mucosa and bone to rule out invasive fungal infection. It is also important to consider a broad differential diagnosis in patients with extensive nasal polyps. The presence of asthma and aspirin sensitivity should alert the clinician to Samter's syndrome. Other causes of CRS and nasal polyps include Churg-Strauss syndrome, inhalant allergies, and cystic fibrosis.

Ancillary studies that can assist with treatment planning and counseling include smell testing, acoustic rhinometry, skin testing for inhalant allergies, and endoscopically guided biopsy and/or cultures from the middle meatus (5).

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