A Systematic Review of Imaging Studies in Olfactory Dysfunction Secondary to COVID-19

Abstract

Rationale and objectives: Hyposmia/anosmia is common among patients with coronavirus disease-2019 (COVID-19). Various imaging modalities have been used to assess olfactory dysfunction in COVID-19. In this systematic review, we sought to categorize and summarize the imaging data in COVID-19-induced anosmia.

Material and methods: Eligible articles were included after a comprehensive review using online databases including Google scholar, Scopus, PubMed, Web of science and Elsevier. Duplicate results, conference abstracts, reviews, and studies in languages other than English were excluded.

Results: In total, 305 patients undergoing MRI/functional MRI (177), CT of paranasal sinuses (129), and PET/CT or PET/MRI scans (14) were included. Out of a total of 218 findings reported on MRI, 80 were reported on early (≤ 1 month) and 85 on late (>1 month) imaging in relation to the onset of anosmia. Overall, OB morphology and T2-weighted or FLAIR signal intensity were normal in 68/218 (31.2%), while partial or complete opacification of OC was observed in 60/218 (27.5%). T2 hyperintensity in OB was detected in 11/80 (13.75%) and 18/85 (21.17%) on early and late imaging, respectively. Moreover, OB atrophy was reported in 1/80 (1.25%) on early and in 9/85 (10.58%) on late imaging. Last, among a total of 129 CT scans included, paranasal sinuses were evalualted in 88 (68.21%), which were reported as normal in most cases (77/88, [87.5%]).

Conclusion: In this systematic review, normal morphology and T2/FLAIR signal intensity in OB and OC obstruction were the most common findings in COVID-19-induced anosmia, while paranasal sinuses were normal in most cases. OC obstruction is the likely mechanism for olfactory dysfunction in COVID-19. Abnormalities in OB signal intensity and OB atrophy suggest that central mechanisms may also play a role in late stage in COVID-19-induced anosmia.

Keywords: COVID-19; anosmia; computed tomography; magnetic resonance imaging; olfactory dysfunction; positron emission tomography.

Figure 1

Flow diagram of the study selection process. (Color version of figure is available online.)

Figure 2

A. Coronal T2-weighted magnetic resonance imaging (MRI) of olfactory bulb in a patient with COVID-19-induced anosmia shows rectangular shape of both olfactory bulbs (arrow). Adapted from Kandemirli et al (10) with permission. B. Coronal nonenhanced T2-weighted MRI of a young man with sudden-onset anosmia shows normal olfactory bulb morphology and signal intensity (arrow). Olfactory cleft is also shown (*) with no evidence of mucosal thickening or obliteration. Adapted from Galougahi et al (25) with permission.

Figure 3

(a) Hyperintense foci are noted within both olfactory bulbs of a patient with COVID-19-induced anosmia (arrows). (b) Coronal T2-weighted image at the level of olfactory bulbs revealed bilateral scattered foci of hypointensity (arrows). Both images are adapted from Kandemirli et al (10) with permission.