Olfactory system measurements in COVID-19: a systematic review and meta-analysis

Abstract

Purpose The neurotropism of SARS-CoV-2 and the consequential damage to the olfactory system have been proposed as one of the possible underlying causes of olfactory dysfunction in COVID-19. We aimed to aggregate the results of the studies which reported imaging of the olfactory system of patients with COVID-19 versus controls. Methods PubMed and EMBASE were searched to identify relevant literature reporting the structural imaging characteristics of the olfactory bulb (OB), olfactory cleft, olfactory sulcus (OS), or olfactory tract in COVID-19 patients. Hedge’s g and weighted mean difference were used as a measure of effect size. Quality assessment, subgroup analyses, meta-regression, and sensitivity analysis were also conducted. Results Ten studies were included in the qualitative synthesis, out of which seven studies with 183 cases with COVID-19 and 308 controls without COVID-19 were enrolled in the quantitative synthesis. No significant differences were detected in analyses of right OB volume and left OB volume. Likewise, right OS depth and left OS depth were also not significantly different in COVID-19 cases compared to non-COVID-19 controls. Also, we performed subgroup analysis, meta-regression, and sensitivity analysis to investigate the potential effect of confounding moderators. Conclusion The findings of this review did not confirm alterations in structural imaging of the olfactory system, including OB volume and OS depth by Covid-19 which is consistent with the results of recent histopathological evaluations. Supplementary Information The online version contains supplementary material available at 10.1007/s00234-022-03014-8.

Olfactory Dysfunction in the COVID-19 Era: An Umbrella Review Focused on Neuroimaging, Management, and Follow-up

“…Despite the abovementioned facts, Mohammadi et al [40] conducted a meta-analysis on 180 COVID-19 patients and 308 healthy subjects. They found no statistically significant difference between the COVID-19 patients and non-COVID-19 controls regarding the anatomic imaging of the olfactory system, including OB volume and OS depth.…”

“…Eight articles assessed the neuroimaging findings of COVID-19 patients with OD. Several neuroimaging modalities were applied to delineate the impression of COVID-19 on olfactory systems, including computed tomography (CT) scan, positron emission tomography (PET), and magnetic resonance imaging (MRI) [13][14][15][16][17][18][19]. Moreover, different Structures were evaluated to highlight the impact of COVID-19 on the OD, such as Olfactory cleft, olfactory bulb, olfactory tract, olfactory sulcus, olfactory gyrus, and brain cortex related to olfactory sensation.…”

“…The imaging findings of olfactory cleft (OC) in COVID-19 patients were reported in seven reviews [13][14][15][16][17][18][19] . In the visual inspection of the MRI images, inflammation and obliteration of the OC were observed, suggesting that the virus has reached the brain through the olfactory nerves [14].…”

“…Six reviews described the imaging findings of olfactory bulbs (OB) in COVID-19 patients [13,[15][16][17][18][19]. It was demonstrated that no significant differences were observed between COVID-19 patients with OD and non-COVID-19 individuals in terms of mean volume of OB [18]. However, the altered dimension of OB is a common finding in other studies.…”

“…Brain imaging findings of the olfactory sulcus, gyrus, and cortex in individuals with COVID-19 and OD were described in five studies [13,14,[17][18][19]. Cortical areas responsible for processing olfactory stimuli were shown to be altered.…”