Evaluation of the olfactory bulb volume and morphology in patients with coronavirus disease 2019: can differences create predisposition to anosmia?

Abstract

OBJECTIVE: This study aimed to investigate whether the volume and morphology of the olfactory bulb are effective in the occurrence of anosmia in patients after COVID-19 infection. METHODS:The olfactory bulbus volume was calculated by examining the brain magnetic resonance imaging of cases with positive (+) COVID-19 polymerase chain reaction test with and without anosmia. Evaluated magnetic resonance imaging images were the scans of patients before they were infected with COVID-19. The olfactory bulbus and olfactory nerve morphology of these patients were examined.The brain magnetic resonance imaging of 59 patients with anosmia and 64 controls without anosmia was evaluated. The olfactory bulb volumes of both groups were calculated. The olfactory bulb morphology and olfactory nerve types were examined and compared between the two groups. RESULTS:The left and right olfactory bulb volumes were calculated for the anosmia group and control group as 47.8±15/49.3±14.3 and 50.5±9.9/50.9±9.6, respectively. There was no statistically significant difference between the two groups. When the olfactory bulb morphology was compared between the two groups, it was observed that types D and R were dominant in the anosmia group (p<0.05).Concerning olfactory nerve morphology, type N was significantly more common in the control group (p<0.05).CONCLUSIONS: According to our results, the olfactory bulb volume does not affect the development of anosmia after COVID-19. However, it is striking that the bulb morphology significantly differs between the patients with and without anosmia. It is clear that the evaluation of COVID-19-associated smell disorders requires studies with a larger number of patients and a clinicoradiological approach.

MRI evidence of olfactory system alterations in patients with COVID-19 and neurological symptoms

“…Moreover, since the olfactory loss was self-reported and not assessed by any objective test, OD prevalence might have been underestimated, as reported in previous studies [ 9 , 36 ]. Our results, along with a previous study finding no difference in OB volume between COVID-19 patients with and without self-reported olfactory loss [ 37 ], suggest that OB atrophy may be associated with the neurological sequelae of COVID-19, even without leading to (self-perceivable) olfactory loss, supporting the hypothesis of SARS-CoV-2 using the olfactory pathway as a gateway to the brain. This is further supported by the findings of Gu et al [ 38 ] and Buzhdygan et al [ 39 ], but true evidence of the neuroinvasive potential of the Coronavirus is still missing.…”

Olfactory Bulb Volume and Morphology Changes in COVID-19 Patients With Olfactory Disorders Using Magnetic Resonance Imaging

“…12 Besides, although virus entry to host cells other than CNS is known to be mediated by angiotensin-converting enzyme 2 receptors, which are relatively prominent in human airway epithelia, lung parenchyma, vascular endothelia, kidney, and small intestine, it does not play a major role in transmission to CNS because it is reported that angiotensin-converting enzyme 2 expression is very low in CNS, which supports the hypothesis on direct transmission of the virus through olfactory pathway. 1,2,12 Moreover, in the past, anatomical and autopsy studies revealed that SARS-CoV-2 RNA and protein have been demonstrated to be abundant in the neurinal tissues of olfactory system, as well as in the brain and brain stem in COVID-19 patients, which shows neurotropism of the SARS-CoV-2. 13,14 Building on these reasons, there is a hypothesis that virus might cause morphological changes in olfactory structures.…”

“…According to the past studies, OBV greater than 45 mL and OSD greater than 7 mm were considered as normal. 1,11,17 However, in literature studies, there is variability in timing of imaging. 9,13,18 Some of the past studies revealed that OBV decreases in early chronic period after COVID-19 infection in patients who had COVID-19 infection with associated smell disorders during their disease while it turns to normal in late chronic period after the resolution of the disease.…”

“…One of the nonrespiratory involvements is central nervous system (CNS) with presenting symptoms including ischemic infarction, meningitis, encephalitis, bleeding, acute hemorrhagic necrotizing encephalopathy, and venous thrombosis. 1,2 Smell and taste disorders have been reported as the common symptoms. 3 In vast majority of infected cases, smell disorders including anosmia and hyposmia start suddenly shortly after the upper respiratory tract infection and the recovery time ranges from 1 to 3 weeks.…”

“…It is attached to the cerebral hemisphere by a long stalk so called the olfactory stalk or olfactory peduncle. 1 In 1998, Yousem et al 6 developed the measurement standards for OB and olfactory tract using MRI images. There are many studies in the literature exploring the associations of quantitative measurements of OB and olfactory tract with the ODs due to many reasons such as viral infections, trauma, and neurodegenerative disorders.…”