Gaining Back What Is Lost: Recovering the Sense of Smell in Mild to Moderate Patients After COVID-19

Abstract

The purpose of our cohort study was to quantify olfactory deficits in Coronavirus disease 2019 (COVID-19) patients using Sniffin' Sticks and a pre-post design to evaluate olfactory recovery. Thirty adult patients with laboratory-confirmed mild to moderate forms of COVID-19 underwent a quantitative olfactory test performed with the Sniffin' Sticks test (SST; Burghardt, Wedel, Germany), considering olfactory threshold (T), odor discrimination (D), and odor identification (I). Results were presented as a composite TDI score (range 1-48) that used to define functional anosmia (TDI ≤ 16.5), hyposmia (16.5 < TDI < 30.5), or functionally normal ability to smell (TDI ≥ 30.5). Patients also self-evaluated their olfactory function by rating their ability to smell on a visual analogue scale (Visual Analog Scale rating) and answering a validated Italian questionnaire (Hyposmia Rating Scale). Patients were tested during hospitalization and about 2 months after symptoms onset. During the hospitalization, the overall TDI score indicated that our cohort had impairments in their olfactory ability (10% was diagnosed with anosmia and more than 50% were hyposmic). Almost all patients showed a significant improvement at around 1 month following the first test and for all the parts of the SST except for odor identification. None of the subjects at 1 month was still diagnosed with anosmia. We also quantified the improvement in the TDI score based on initial diagnosis. Anosmic subjects showed a greater improvement than hyposmic and normosmic subjects. In conclusion, within a month time window and 2 months after symptoms' onset, in our cohort of patients we observed a substantial improvement in the olfactory abilities.

Keywords: COVID-19; Sniffin’ Sticks test—SST; olfactory deficits; olfactory test.

Figure 1.

Plots representing ratings for threshold (A), discrimination (B), identification (C), and combined TDI score (D) during (pink) and after (blue) COVID-19. Within each subplot (from left to right), boxplots show the first to third quartiles, the horizontal line denotes the median, and whiskers denote 1.5 times the interquartile range. The density distribution of the data shows the proportions of given ratings. Black dots and lines in each subplot represent the mean and the 95% confidence interval.

Figure 2.

Plots representing TDI score ratings (A) for each subject during (left) and after (right) COVID-19. (B) Bar plots represent the percentage of individuals classified as anosmic, hyposmic, and normosmic during (left) and after (right) COVID-19. (C) Boxplots show the first to third quartiles, the horizontal line denotes the median, and whiskers denote 1.5 times the interquartile range. Dots represent single subjects in each group. For the differences between the groups: Krustal–Wallis test followed by Wilcox–Mann U test with Bonferroni correction. P values for the pairwise comparisons are P = 0.0085 for anosmia versus hyposmia, P = 0.0126 and P = 0.0015 for hyposmia versus normosmia.

Figure 3.

Plots representing ratings for VAS (A) at the onset of (left), during (middle), and after (right) COVID-19. HRS (B) rating during (left) and after (right) COVID-19. Within each subplot, boxplots show the first to third quartiles, the horizontal line denotes the median, and whiskers denote 1.5 times the interquartile range. The density distribution of the data shows the proportions of given ratings. The pairwise comparisons test used are annotated under the graphs in (A) and (B). (C) Correlation matrices for the different methods we used to quantify olfactory abilities of the patients before and after COVID-19. Crosses indicate nonsignificant correlations.