What is the role of CT scanning in the workup of allergic fungal sinusitis (AFS)?

Updated: Mar 16, 2018
  • Author: John E McClay, MD; Chief Editor: Arlen D Meyers, MD, MBA  more...
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Answer

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  • Computed tomography [18]

    • Accumulation of allergic fungal mucin eventually leads to the increasingly well-recognized radiographic findings characteristic of allergic fungal sinusitis (AFS). Heterogeneous areas of signal intensity within paranasal sinuses filled with allergic fungal mucin frequently are identified on CT scans, as depicted in the image below.

      Coronal CT scan showing extensive allergic fungal Coronal CT scan showing extensive allergic fungal sinusitis involving the right side with mucocele above the right orbit and expansion of the sinuses on the right.
      Coronal CT scan showing typical unilateral appeara Coronal CT scan showing typical unilateral appearance of allergic fungal sinusitis with hyperintense areas and inhomogeneity of the sinus opacification; the hyperintense areas appear whitish in the center of the allergic mucin.

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      • Although these findings are not specific for allergic fungal sinusitis (AFS), they remain relatively characteristic of the disease and may provide preoperative information supportive of a diagnosis of allergic fungal sinusitis (AFS). This characteristic, which is best identified using soft-tissue algorithms on CT scan, has been the focus of some interest.

      • An initial theory proposed that the role of hemosiderin occurring within inspissated mucin is responsible for the areas of increased signal intensity. This was disputed by Zinreich et al, who were unable to identify increased hemosiderin within typical allergic fungal mucin. Current evidence points to the presence of accumulations of heavy metals (eg, iron, manganese) and calcium salt precipitation within inspissated allergic fungal mucin as the most likely causes of these radiographic findings. [19]

    • Expansion, remodeling, or thinning of involved sinus walls is common in allergic fungal sinusitis (AFS) and is thought to be caused by the expansile nature of the accumulating mucin. Areas of high attenuation are found within the expanded paranasal sinuses in all patients. Similar radiographic findings can be caused by rare osteoid/chondroid matrix-producing sinonasal sarcomas or meningiomas.

    • Bony erosion of the sinus walls and extension into adjacent cavities have been mentioned in many reports, usually focusing on intracranial extension, as depicted in the 1st image above. [20] In most series, a rate of approximately 20% bony erosion with extension into surrounding vital cavities is reported.

      • A recent review at UT Southwestern focusing on patterns of bony erosion in all the authors' patients found a 20% rate of erosion with extension. Sites of extension included the nasopharynx and pterygomaxillary space and intracranial and intraorbital areas.

      • A statistically significant association was identified between expansion of paranasal sinuses involved with disease and the presence of bone erosion.

      • The ethmoid sinus was the most commonly involved sinus, while the adjacent lamina papyracea was the most common bone to exhibit demineralization, as depicted in the image below. Extension of allergic fungal sinusitis (AFS) beyond the confines of the paranasal sinuses most commonly occurred into the orbit, followed by the anterior, middle, and posterior cranial fossae, respectively.

        Coronal CT scan showing typical unilateral appeara Coronal CT scan showing typical unilateral appearance of allergic fungal sinusitis with hyperintense areas and inhomogeneity of the sinus opacification; the hyperintense areas appear whitish in the center of the allergic mucin.
      • When evaluating children and adults in the authors' population separately, no difference was observed in the amount or location of bony erosion with extension, as depicted in the Table below.

      • Despite the sometimes-remarkable extension into adjacent anatomic spaces, no cases of histologic invasion of fungus into the adjacent barriers of the orbital periosteum or dura of the brain were identified on histologic review.


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