Paranasal sinus mycetoma

Paranasal sinuses mycetomas, or fungus balls, are indolent and non-invasive fungal colonization of the paranasal sinuses.  

Pathology

Pathogenesis is thought to be a cascade of processes from insufficient mucociliary clearance leading to sinus colonization and chronic inflammatory response. The patient may only have mild symptoms or be asymptomatic. 

Radiographic features

CT

Commonly only a single sinus is affected with a predilection for the maxillary sinus followed by the sphenoid sinus. The frontal and ethmoid sinuses are less often affected. A clue to the diagnosis includes soft tissue density within the sinus with/without foci of calcific deposits. Postobstructive change may be observed if the mycetoma obstructs the sinus drainage pathway leading to partial or complete sinus opacification ². 

Evidence of chronic inflammation can be seen with sclerosis and thickening of the wall of the paranasal sinuses. Careful evaluation of the sinus cavity is prudent to exclude bone erosion that is a feature of acute invasive fungal sinusitis rather than of mycetoma / chronic fungal sinusitis ².

MRI

MRI signal characteristics of mycetomas reflect the internal content of the mycelia, vegetative part of a fungus consisting of a conglomerate of hyphae. This contains primarily carbohydrates with some glycoproteins, macromolecular proteins, and iron and manganese.

Signal characteristics

• T1: low signal
• T2: low signal. The presences of paramagnetic elements further shorten the relaxation times, and this can be a highlight on the susceptibility weighted sequence ⁵
• T1 C+ (Gd): chronic inflammatory change of the sinus mucosa may enhance

Differential diagnosis

• allergic fungal sinusitis
• chronic invasive fungal sinusitis
  • bone erosion
  • soft tissue invasion
  • immunocompromised patients 
• paranasal sinus mucocele
• sinonasal inverted papilloma