A Comparative Olfactory MRI, DTI and fMRI Study of COVID-19 Related Anosmia and Post Viral Olfactory Dysfunction

To evaluate how COVID-19 anosmia imaging findings resembled and differed from postinfectious olfactory dysfunction (OD).

A total of 31 patients presenting with persistent COVID-19 related OD and 97 patients with post-infectious OD were included. Olfactory bulb MRI, DTI and olfactory fMRI findings in both groups were retrospectively assessed.

All COVID-19 related OD cases were anosmic, 18.6% of post-infectious OD patients were hyposmic and remaining 81.4% were anosmic. Mean interval between onset of OD and imaging was 1.5 months for COVID-19 related OD and 6 months for post-infectious OD. Olfactory bulb volumes were significantly higher in COVID-19 related OD than post-infectious OD. Deformed bulb morphology and increased olfactory bulb signal intensity was seen in 58.1% and 51.6% with COVID-19 related OD; and 63.9% - 46.4% with post-infectious OD; without significant difference. Significantly higher rate of olfactory nerve clumping and higher QA values at orbitofrontal and entorhinal regions were observed in COVID-19 related OD than post-infectious OD. Absence of orbitofrontal and entorhinal activity showed no statistically significant difference between COVID-19 related OD and post-infectious OD, however trigeminosensory activity was more robust in COVID-19 related OD cases.

Olfactory bulb damage may play a central role in persistent COVID-19 related anosmia. Though there is decreased olfactory bulb volume and decreased white matter tract integrity of olfactory regions in COVID-19 related anosmia, this is not as pronounced as in other post-infectious OD. Trigeminosensory activity was more robust in COVID-19 related OD. These findings may reflect better preserved central olfactory system in COVID-19 related OD compared to COVID-19 related OD.