Discussion
In our study, we found that the estimated prevalence of self- reported
OD in our sample was 27.4 % while the prevalence of abnormal olfactory
test was 16.3% which is lower than what was reported by most recent
studies including studies using standard olfactory
tests.5 Furthermore, we found that OD was more
prevalent in females with confirmed COVID-19 infection which is similar
to most published studies.
The low prevalence of OD in our study in comparison with other studies
can be explained by the fact that the majority of our patients were
hospitalized with severe COVID-19 pneumonia. So, the low prevalence
could be explained by the delayed testing which led to the partial
recovery of the olfactory dysfunction. While other studies examined
patients with mild- moderate COVID-19 disease in their early stage.
Moreover, psychophysical tests such as the Q-SIT evaluate one’s sense of
smell at a specific point in time, which could lead to an
underestimation of the prevalence of OD. Jerome R. et al. showed that
about 38.3% of patients with self-reported sudden-onset olfactory
dysfunction found to be normosmic by the psychophysical Sniffin’ Sticks
test. we recorded similar finding of 30.2% having normal Q-SIT while
they subjectively reported OD.17
When we measured the association between the Q-SIT and subjectively
reported OD at the time of test, we found better positive predictive
value and negative predictive value on a cutoff score ≤ 1 (84% and 93%
respectively) in compare to the cutoff score ≤ 2. (Table 5) For that we
have used the former cutoff in all previously mentioned calculations.
This study showed no significant association between olfactory
dysfunction and nasal symptoms. This is supporting the hypothesis of
direct invasion of the olfactory neurons by SARS-CoV-2 as the virus
could be replicated in neural cell line U251 in
vitro.18 That is against other hypothesis of olfactory
cleft blockage due to inflammation or inflammatory cytokines affecting
olfactory neural mucosa.6
It was expected to have significant association between OD and abnormal
tasting (38.4%) as retronasal olfaction is the cause of most gustatory
impairment.19 Moreover, the prevalence of gustatory
impairment (31.8%) in our sample was higher than olfactory impairment.
These data are supported by similar findings of an epidemiological
survey conducted in four European countries.20 while
some studies differentiate between olfactory and gustatory dysfunction,
others just report the prevalence of both anosmia and Ageusia as one
symptom. 7,21,22