Introduction
Sinusitis has traditionally been a clinical diagnosis. However, incidental paranasal sinus abnormalities are increasingly detected and reported in MRI studies for patients referred for neuroradiological evaluation. It remains unclear the true prevalence of paranasal pathology in the Irish population. This study aimed to assess the prevalence of incidental morphologic paranasal sinus changes on MRI brain studies, the pattern of sinus involvement and correlation with patient demographics and seasonality.

Methods
Over a 6-month period in winter and summer (January to June 2011), patients in an Irish tertiary referral university teaching hospital undergoing MRI of the brain, were retrospectively analysed for paranasal inflammatory changes. Imaging was performed on a 1.5T MR scanner using a dedicated head coil. Axial T1-weighted, T2-weighted, fluid attenuation inversion recovery and gradient echo sequences were obtained. The scan field of view extended from the base of skull to the vertex, with full visualisation of the frontal, mastoid air cells and partial visualisation of the anterior ethmoid and maxillary sinuses. Axial T2-weighted sequences were reviewed for paranasal changes which included mucosal thickening ≥5 mm, air-fluid levels with fluid depth ≥5 mm, polyps and complete sinus opacification. While less stringent criteria for paranasal changes have been used in other studies, stricter parameters were used in this study to limit non-clinically paranasal inflammatory changes. Inclusion criteria included patients aged ≥20 years with imaging of the sinuses available for review. MRI studies specifically investigating sinus disease such as acute and chronic sinusitis or malignant disease were excluded. All studies were reviewed by two trained readers in consensus.

Results
Two-hundred and twenty-one patients met the inclusion criteria for the study. The overall prevalence of paranasal sinus inflammatory changes was 45% (100/221). Patients aged 40-69 years were more likely to be affected (53%, 59/112) than those younger or older (38%, 41/109). Prevalence of these changes peaked in middle age and declined steadily after patients reached their 7^th decade (Figure 1).

Fig 1. Prevalence of paranasal sinus inflammatory changes detected incidentally on MRI brain studies by age groups in decades.

The majority (73%, 73/100) of patients with MRI showing paranasal inflammatory changes were in a single sinus group. The majority of changes were found in the maxillary sinuses. As measured over the months of January and June, the overall prevalence of paranasal sinus inflammatory changes was not overly affected by the winter (47%, 47/100) or summer (43%, 43/100) seasons. However, certain subgroups showed marked variation: in males younger than 50 years, 63% (12/19) had changes in winter, but 0% (0/11) in summer. For females in their 20s there was a threefold greater prevalence in winter (70%, 7/10 versus 20%, 1/5), compared to those in their 30s and 40s (33%, 7/21 versus 64%, 9/14).

Discussion
There is well documented literature of incidental paranasal sinus pathology detected in MRI studies with a prevalence ranging from 24.7% to as high as 59%^1-6. The reported prevalence of 45% (110/221) in our series compares favourably with data from other authors internationally^2,7. However, analysis of the methods used in these studies demonstrate more inclusive cut-off values of what characterises abnormal mucosal thickening; >2 mm (Patel et al., 1996) and >3 mm (Gordts et al., 1996)^1,7. By comparison, Tarp et al used a 5 mm cut-off for mucosal thickening in their series and reported a prevalence of 31.7%⁵. It has also been shown that mucosal thickening less than four mm is not normally of clinical importance⁸. Therefore, the prevalence of paranasal pathology within our study may actually be higher due to the use of a threshold value of 5mm and may also indicate a larger proportion of clinically relevant findings.

Similar to other studies, the maxillary sinus group predominates as the most frequently affected sinus in our study^2,5. The reason may be anatomical as the mucus drainage pathway of the maxillary sinus relies on active ciliary clearance against gravity and around the uncinate processes. This factor likely predisposes the maxillary sinus to thickened mucosa comparatively. Interestingly, certain younger sub-groups in our series had a drastically increased prevalence of morphological change in the sinuses during the winter months of January compared to the summer months of June. Puhakka et al previously described increased radiographic findings in the paranasal sinuses after upper respiratory tract viral illness.⁹ It is likely that the seasonal variation observed is secondary to the seasonality of upper respiratory tract viral illness. It is also reasonable to infer, that there will be individuals within the study population with symptoms of sinusitis that were either unrecognised or not the presenting complaint under investigation.

In conclusion, our study shows paranasal sinus inflammatory changes in almost half of all patients undergoing MRI brain. The maxillary sinus was frequently the most affected, and changes were more prevalent in younger patients in the winter-spring season. Although we did not examine the correlation of these incidental imaging findings with clinical symptoms, the prevalence of mild paranasal sinus changes is so common, that it would be reasonable to consider these findings as normal radiological variants on MRI. More pronounced changes, however, may be of clinical significance. Correlation with clinical symptoms and signs remains crucial to determine the clinical significance of these imaging findings.

Conflict of interest
None declared.

Correspondence
Dr R Wilson. Sunshine Coast University Hospital, Queensland, Australia.
Email: [email protected]
Tel:+61406982975

References
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