Quantitative Analysis of the Olfactory System in COVID-19: An MR Imaging Study

Abstract

Background and purpose: Anosmia or hyposmia, often accompanied by changes in taste, is recognized as a common symptom that can assist in the diagnosis of coronavirus disease 2019 (COVID-19). The pathogenesis of olfactory dysfunction in COVID-19 is not yet fully understood. MR imaging represents a useful anatomic imaging method for the evaluation of olfactory dysfunction associated with varying etiologies, including viral infection, trauma, and neurodegenerative processes. This case-control study was conducted to compare quantitative measurements of olfactory anatomic structures between patients diagnosed with COVID-19 associated with persistent olfactory dysfunction and healthy controls.

Materials and methods: This study has a retrospective design. Cranial MR imaging was performed on all participants in both the patient and control groups. The bilateral olfactory bulb volume, olfactory tract length, and olfactory sulcus depth were measured in all patients.

Results: A total of 116 people aged 18-60 years, including 36 patients diagnosed with COVID-19 and 80 controls, were included in the study. All measured values were compared between the patient and control groups. The right, left, and total olfactory bulb volume values were significantly lower in the patient group than in the control group. The patient group also had significantly lower right and left olfactory sulcus depth and olfactory tract length values compared with those in the control group.

Conclusions: MR imaging findings can be used to demonstrate olfactory injury in patients with COVID-19. The olfactory pathway may represent an alternative route for virus entry into the central nervous system.

FIG 1.

CONSORT flow diagram. HT indicates hypertension; DM, diabetes mellitus; TD, thyroid dysfunction; PD, Parkinson disease.

FIG 2.

Illustration of the olfactory system on coronal plane MR imaging. The medial orbital gyrus (red area), gyrus rectus (green area), olfactory sulcus (blue area), and olfactory bulb (brown area) are shown.

FIG 3.

Coronal 3D-FIESTA-C MR image of a 41-year-old man showing the right and left olfactory bulbs as hypointense ovoid structures (arrow). The olfactory sulcus (green dashed lines) is seen as a hyperintense line between the medial orbital gyrus (blue star) and gyrus rectus (yellow star). Note the hyperintense CSF surrounding the OBs.

FIG 4.

Sagittal multiplanar reconstruction of 3D-FIESTA-C MR imaging of a 27-year-old man showing the left olfactory tract (crosshairs).

FIG 5.

Coronal plane 3D-FIESTA-C MR images of a 33-year-old female patient with loss of smell who was proved to have COVID-19. The olfactory bulbs are seen as atrophic (A). The olfactory bulbs are still atrophic, though the loss of smell has improved, on MR image of the same patient 3 months later (B).

FIG 6.

Boxplots of the right OBV, left OBV, and total OBV values in both patient and control groups.

FIG 7.

Boxplots of the right OTL, left OTL, right OSD, and left OSD values in both patient and control groups.

FIG 8.

Coronal 3D FIESTA-C MR images of a 26-year-old male from control group (A) and a 25-year-old female patient with COVID-19 anosmia (B). Normal and increased signal intensity in bilateral olfactory bulbs (red and green arrows, respectively).

FIG 9.

Coronal 3D-FIESTA-C MR images of a 28-year-old man from the control group (A) and a 36-year-old female patient (B). Note normal and abnormal olfactory bulbs (red and green arrows, respectively) and olfactory sulci (red and green dashed lines, respectively).

FIG 10.

Sagittal multiplanar reconstruction of 3D-FIESTA-C MR images of a 42-year-old woman from the control group (A) and a 36-year-old female patient (B) showing normal and abnormal olfactory tracts (red and green arrows, respectively).