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Allergic fungal rhinosinusitis

Allergic fungal rhinosinusitis (AFRS) typically presents in young immunocompetent patients (mean age 22–31 years) with chronic headaches, nasal congestion, and history of chronic sinusitis. It is the most common form of fungal rhinosinusitis, particularly prevalent in warm, humid climates, and accounts for 5–10% of chronic hypertrophic sinus disease requiring surgery.

Key Images

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Look For First
  • Near-complete multi-sinus opacification with central hyperdense material on CT and marked T2 hypointensity centrally on MRI
  • Serpiginous or layered hyperdense central material surrounded by peripheral hypodense inflamed mucosa and nasal polyposis
  • Characteristic rim enhancement pattern: T2 hyperintense peripheral inflamed mucosa with central non-enhancing material on post-gadolinium T1
  • Sinus expansion with bone remodeling and erosion (more frequent than in other chronic rhinosinusitis forms)
Key Image Findings
  • CT shows near-complete sinus opacification with centrally located, often serpiginous hyperdense material (fungal concretions and heavy metals) surrounded by a peripheral rim of hypodense inflamed mucosa.
  • Expansion of involved sinuses with remodeling and thinning of bony walls is seen in almost all patients with complete opacification; bone erosion occurs in 33–>90% of cases, affecting the lamina papyracea and skull base.
  • MRI demonstrates marked T2 hypointensity in the central sinus corresponding to inspissated allergic mucin with high concentrations of metals (iron, manganese, calcium), high protein content, and low free water.
  • T1-weighted signal of central allergic mucin is variable (most often hypointense to isointense due to high protein content; occasionally hyperintense); peripheral inflamed mucosa appears intermediate-to-low signal.
  • T1 post-gadolinium shows peripheral mucosal enhancement with no enhancement of the central sinus contents, a key distinguishing feature from neoplasms.
  • The peripheral inflamed mucosal thickening is typically T2 hyperintense and enhances, which can create the paradoxical appearance of an aerated sinus on T2WI when surrounded by hypointense central mucin.
  • Multi-sinus involvement (ethmoid > maxillary > frontal > sphenoid) is the rule, with frequent nasal polyposis and unilateral or bilateral presentation, though bilateral involvement is more common.
  • Potential intraorbital and intracranial extension may occur due to pressure remodeling by expanding mucin rather than direct fungal invasion.
Differential Diagnosis
  • Sinus fungal mycetoma: represents fungal colonization without hyperimmune response, typically involves only the maxillary sinus, lacks the multi-sinus opacification and bone remodeling characteristic of AFRS.
  • Sinonasal mucocele: shows similar chronic expansive features with sinus enlargement but lacks bone erosions, distinguishing it from AFRS.
  • Non-Hodgkin lymphoma: presents as a sinonasal mass that is homogeneously hyperdense on non-contrast CT, lacking the central-peripheral signal dichotomic pattern and peripheral mucosal rim enhancement of AFRS.
  • Sinonasal polyposis: may present with nasal polyposis and sinus opacification but lacks the characteristic central hyperdensity on CT and T2 hypointensity on MRI unique to AFRS.
  • Eosinophilic mucin rhinosinusitis (non-fungal variant): histologically identical to AFRS but lacks demonstrable fungi; often bilateral, associated with aspirin-exacerbated respiratory disease, and not reliably separable on imaging alone.
Discussion

AFRS is defined by a type I (IgE-mediated) hypersensitivity response to aerosolized environmental fungi, most commonly dematiaceous fungi (Bipolaris, Curvularia, Alternaria) and hyaline molds (Aspergillus, Fusarium), and is associated with asthma in 40–50% of cases.

Bent and Kuhn criteria require all five major features for diagnosis: documented type I hypersensitivity to fungi, nasal polyposis, characteristic CT findings, eosinophilic allergic mucin without fungal tissue invasion, and positive fungal stain from surgical specimens.

The central T2 hypointensity in AFRS results from high concentrations of metals (especially iron and manganese), high protein content, and low free water in densely packed allergic mucin, reflecting the pathophysiology of fungal bioaccumulation.

Bone erosion and sinus wall remodeling in AFRS result from chronic pressure exerted by expanding mucin rather than direct fungal invasion of bone, explaining why resorption is more extensive than in other forms of chronic rhinosinusitis.

The rim enhancement pattern (peripheral enhancement without central enhancement) is a critical distinguishing feature from sinonasal malignancies, which typically show more homogeneous or central enhancement.

Recurrence after surgical debridement is common but significantly reduced by adjunctive systemic or topical steroid therapy; antifungal therapy has equivocal efficacy.

Reporting Pearls

Describe AFRS as a multi-sinus expansile lesion with characteristic dual-signal appearance: T2 hypointense central fungal mucin (due to high metal and protein content) surrounded by a T2 hyperintense, gadolinium-enhancing rim of inflamed mucosa, with sinus wall remodeling and variable bone erosion, best distinguishing it from mimics like mucoceles or malignancy.

Pitfalls
  • The paradoxical appearance of a hypointense central T2 signal rimmed by hyperintense peripheral mucosa can mimic an aerated sinus on T2WI; correlation with T1, contrast-enhanced images, and CT is essential to avoid misinterpreting opacified sinus for normal anatomy.
  • Absence of central enhancement on post-gadolinium imaging may be misinterpreted as necrosis of a neoplasm rather than the characteristic non-enhancing allergic mucin of AFRS; comparison with non-contrast T1 and T2 signal helps clarify the diagnosis.
  • Bone erosion frequency is highly variable (33–>90%) across reported series and should not be used as a definitive diagnostic criterion; presence of erosion is more common in AFRS than other chronic sinusitis but is not pathognomonic.
  • Non-fungal eosinophilic mucin rhinosinusitis cannot be reliably distinguished from fungal AFRS on imaging alone; definitive diagnosis requires histopathological demonstration of fungal elements and positive fungal cultures from surgical specimens.
Source: Reference source

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