Discussion

In this study, we describe the prevalence of olfactory and gustatory disturbance during and following COVD-19 infection in a Victorian population during the first wave between February and April 2020. Olfactory disturbance is an often under-reported symptom in the general population and has a paucity of community testing when compared with assessment of hearing and vision loss [10]. Despite being less recognised, it can portend a potentially significant deleterious functional and psychological impact [11]. However, Landis et al. demonstrated functional anosmia in 5% and hyposmia in 15% in patients attending a German otolaryngology clinic after formal assessment with a validated olfactory assessment tool [12]. Studies which analysed older cohorts, such as the Blue Mountains Eye Study in New South Wales, Australia looking at those aged 60 years and over [13] and the Memory Aging project (MAP) [14] assessing a cohort in the North Eastern Illinois, U.S.A. with a mean age of 81 years, found the incidence of olfactory disturbance to be 27% and 55% respectively. These rates demonstrate the large number of individuals within the general population with olfactory disturbance as well as the marked association with increasing age.

In our study, the most frequently reported symptoms during the acute COVID-19 illness were taste and smell disturbance. Similarly, we report that hypogeusia and ageusia are also important components of COVID-19. Although, it has to be kept in mind that many people confuse loss of flavour perception with taste loss which qualifies the present reports [15]. Further, both smell and taste disturbance were noted by 74% of individuals. This is a markedly greater frequency of presentation when compared with other viral illnesses but also when compared with other studies assessing olfactory loss in COVID-19 cases which reported a prevalence of 54% and 44% for olfactory and gustatory dysfunction respectively [16]. Our findings of 65% and 63% for olfactory and gustatory disturbance respectively are greater than a recent meta-analysis which showed an olfactory dysfunction prevalence of 53% among COVID-19 patients and 44% gustatory dysfunction [16]. While this may be a consequence of the relatively small sample size in our study, all patients represented were confirmed positive cases through laboratory testing which may not have been the case with all participants that were included within the meta-analysis. Our results are consistent with other observations which have demonstrated acute olfactory loss in association with COVID-19 infection [17]. The seemingly innocuous symptom of olfactory disturbance can result in an under-reporting of true incidence as suggested by public health reporting figures [8]. Other studies have used formal smell assessment tools such as Sniffin’ Sticks in their assessment of smell disturbance [18]. The incorporation of a direct question asking about subjective perception of olfactory loss coupled with a numerical scale allows for potential pre-existing olfactory disturbance and provides an opportunity for assessment of relative changes in olfactory loss. Further, as olfactory loss can occur in the context of nasal obstruction preventing odorants reaching the region of olfactory mucosa, we assessed the proportion of individuals who experienced olfactory disturbance in the absence of nasal obstruction which we found to be 67% participants.

A variety of infective causes may elicit olfactory disturbance, it has been proposed that the majority were due to viral particles which elicit an immune response that in turn causes both peripheral and central olfactory damage. Experimental virus inoculation has been noted to result in direct insult to olfactory epithelium causing apoptosis and fibrosis [19]. Whilst this has been proposed to have a protective effect by preventing more proximal viral infiltration, reduction in olfactory bulb volume has been noted with more severe cases of olfactory loss [20]. This parameter has been proposed as a prognosticator for olfactory recovery with individuals whose olfactory bulb volume being preserved to have higher chances of achieving improvement in olfactory function [20]. Following on from this finding, olfactory retraining has been shown to increase rates of improvement in olfactory loss, but needs to be applied in early stage for better outcome [21]. This therapy involves repeated presentations of odours representing four smell groups and has been hypothesised to achieve its efficacy through central pathway regeneration or through plasticity effecting novel olfactory pathways. Whilst a number of medical treatments have been proposed to aid olfactory recovery, these have not been found to be efficacious. More recently, vitamin A drops applied in the Kaiteki position to aid delivery to the olfactory niche in conjunction with olfactory retraining therapy has shown promise and future studies are awaited to further support this finding [22]. The incorporation of a validated olfactory assessment tool in our protocol, such as Sniffin Sticks, would have afforded interparticipant comparisons [18]. Additionally, we would have been able to determine functional anosmia levels and exclude those individuals that continued to feel that their olfactory sense was intact but were in fact noting trigeminal sensation. This would have resulted in potentially greater rates of anosmia than we noted. This is supported by our finding that no participants reported being anosmic prior to their illness which is in contrast to the population incidence described by Landis et al. eluded to earlier [12]. The accuracy of our results may have been impacted by recall bias owing to the retrospective nature of the study design and also by the range in intervals between initial diagnosis and data collection. Future studies may be able to reduce these confounding variables through prospective data collection. Further, our finding of olfactory disturbance concomitant with nasal obstruction may have been due to the nasal obstruction alone. Mitigating this factor was not practical in our cohort as it would have entailed using nasal decongestants prior to formal psychosocial olfactory assessment using validated tools that were limited by quarantine restrictions in place to minimize infectious transmission. In this study, olfactory and gustatory disturbances appear to be a frequent presenting symptom in COVID-19 infection, often presenting in the early stages of the illness. Our results suggest these symptoms occur more frequently than other studies have reported.

In conclusion, we have presented high rates of acute olfactory and gustatory disturbance in association with COVID-19. However, our results also suggest high rates of improvement of these disturbances in a relatively short period. The absence of early improvement should prompt the treating physician or the patient to commence olfactory retraining. This provides valuable prognostic information to those individuals affected by symptoms. Future studies may wish to include objective olfactory testing and also assess recovery in those with ongoing olfactory and gustatory disturbance.