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Paranasal sinuses mycetoma

Paranasal sinus mycetoma (fungus ball) is an indolent, non-invasive fungal colonization of the paranasal sinuses, often found incidentally in asymptomatic or mildly symptomatic patients due to impaired mucociliary clearance.

Key Images

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Look For First
  • Single sinus involvement with predilection for maxillary sinus on CT
  • Low T1 and T2 signal on MRI reflecting mycelial content with paramagnetic elements
  • Heterogeneous soft tissue density with or without calcification on CT
  • Susceptibility artifact on T2/SWI from iron and manganese content
Key Image Findings
  • CT demonstrates heterogeneous soft tissue density within the affected sinus with foci of calcification, most commonly in the maxillary sinus followed by sphenoid sinus.
  • Postobstructive changes may occur if the mycetoma obstructs sinus drainage, resulting in partial or complete sinus opacification.
  • Chronic inflammatory changes manifest as sclerosis and thickening of the paranasal sinus wall on CT.
  • MRI T1-weighted imaging shows low signal intensity reflecting the mycelial composition of carbohydrates, glycoproteins, macromolecular proteins, iron and manganese.
  • MRI T2-weighted imaging demonstrates persistently low signal intensity; paramagnetic iron and manganese elements create pronounced susceptibility artifact on susceptibility-weighted sequences.
  • MRI T1-weighted post-gadolinium imaging may show enhancement of the chronic inflammatory change in the sinus mucosa, but the mycetoma core remains non-enhancing.
  • Absence of bone erosion distinguishes mycetoma from acute invasive fungal sinusitis, making careful osseous evaluation essential.
  • The combination of single-sinus involvement, low T1/T2 signal, and calcification on CT is highly characteristic of mycetoma.
Differential Diagnosis
  • Allergic fungal sinusitis: typically shows polysinusitis with high T2 signal from allergic mucin; lacks the profound low T2 signal and calcification seen with mycetoma.
  • Chronic invasive fungal sinusitis: demonstrates bone erosion and soft tissue invasion; mycetoma is indolent and non-invasive without bone destruction.
  • Paranasal sinus mucocele: typically unisinusal but shows high T2 signal from mucoid content; mycetoma shows persistently low T2 signal.
  • Sinonasal inverted papilloma: typically shows mixed signal on MRI without the characteristic low T1/T2 signal and calcification of mycetoma.
  • Immunocompromised-related fungal sinusitis: presents more acutely with potential for invasive features and soft tissue involvement absent in indolent mycetoma.
Discussion

Mycetoma pathogenesis involves impaired mucociliary clearance leading to fungal colonization and chronic inflammatory response, creating an indolent disease course often discovered incidentally.

The low T1 and T2 signal on MRI reflects the high concentration of paramagnetic elements (iron and manganese) within mycelial hyphae, distinguishing it from other common sinus pathology.

Susceptibility-weighted imaging is particularly sensitive for highlighting the paramagnetic mycetoma core, aiding diagnosis.

Postobstructive sinus opacification can occur if the mycetoma obstructs drainage but does not represent aggressive disease.

The absence of bone erosion is a critical distinguishing feature separating indolent mycetoma from aggressive invasive fungal sinusitis, requiring meticulous CT bone evaluation.

Most patients are asymptomatic or have only mild symptoms, so mycetomas are commonly discovered as incidental findings on imaging performed for other reasons.

Reporting Pearls

Describe mycetoma as a non-invasive fungal colonization with characteristic low T1 and T2 signal intensity (especially highlighted on susceptibility-weighted imaging), often with associated calcification on CT, affecting a single sinus most commonly the maxillary sinus, without bone erosion or soft tissue invasion.

Pitfalls
  • Misinterpreting postobstructive sinus opacification as evidence of aggressive disease; mycetoma remains indolent despite secondary sinus filling.
  • Failing to carefully evaluate the sinus wall and adjacent bone for erosion, which would indicate invasive fungal disease rather than benign mycetoma.
  • Confusing mycetoma with allergic fungal sinusitis based on single-sinus presentation; allergic fungal sinusitis typically shows high T2 signal and often involves multiple sinuses.
  • Overlooking susceptibility artifact on T2/SWI sequences that strongly suggests the paramagnetic mycelial composition diagnostic of mycetoma.
Source: Reference source

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