The current study investigated the recovery rates of olfactory and gustatory dysfunction among recovered COVID-19 patients who were not hospitalized while infected.
Smell and gustatory dysfunction are very prevalent in COVID-19 disease. In our previous cohort(1), we reported olfactory and gustatory dysfunction rates as high as 67% and 52%, respectively. Complete recovery of smell function was found in 64.9% of patients, while 34% reported only partial recovery. As reported by Chary et al.(5), we also found no significant difference between the sexes concerning recovery of smell function. However, Paderno et al.(4) found that later recovery of smell was more prevalent among women. This may be because women are more sensitive to minor smell alteration.(6)
The oral cavity strongly expresses angiotensin-converting enzyme 2 (ACE II), particularly in the tongue.(7) A recent study demonstrated that ACE II inhibitors could engender loss of taste perception by alterations in the delicate mechanisms involving G-protein coupled proteins and sodium channels.(8) Moreover, another report suggested that ACE II receptors play a crucial role in cellular entry and are a potential cause of COVID-19 infection.(9) This can explain why dysgeusia is very prevalent in COVID-19 patients. We found a high rate of gustatory recovery (74.2%) among the study group, which can be explained by the regenerative ability of chemosensory receptor cells in the oral cavity.(10)
Olfactory and/or gustatory dysfunction have been widely reported as one of the sole initial manifestations of COVID-19 infection.(1,4,11) In our cohort, we found that patients who presented with OD and/or GD as an initial symptom had significantly lower rates of complete recovery of olfactory and gustatory functions (p=0.000 and p=0.007, respectively).
To the best of our knowledge, no current study in the literature has reported a relation between OD and GD as initial symptoms and different chemosensory recovery rates.
In this analysis, which is a follow-up to the previous study by Biadsee et al.(1), we aimed to further understand the long term impact of the damage to the olfactory neuroepithelium caused by the COVID-19 virus. Chary et al.(5) found that 70% of COVID-19 patients reported OD without nasal obstruction. Furthermore, 64% of their cohort had complete resolution of OD within 15 days after positive PCR diagnosis. D'Ascanio et al.(3) and Paderno et al.(4) found that OD was completely resolved for 84.3% and 88%, respectively, 30 days after diagnosis. Lechien et al.(12) reported complete resolution of OD among 72.6% of patients, within 8 days after the resolution of the disease. Xiong et al.(13) found 11% of their study group had residual OD 2-4 weeks after negative PCR.
None of the abovementioned studies reported on concomitant nasal obstruction rates during follow-up. In our follow-up period, a mean of 59.2 days after the second negative PCR test, we found that OD had resolved in 64.9% of the patients and none reported residual nasal obstruction. Our analysis is characterized by a relatively long follow-up period and high percentage of residual OD compared to reports in the literature (3,4,12,14–16) and provides a complementary data regarding the clinical course of OD and GD.
Post-viral olfactory loss is defined as the persistence or appearance of olfactory dysfunction after recovery from upper respiratory infection.(17) Suggested mechanisms for post-viral olfactory loss include the ability of viruses to penetrate the brain via the fovea ethmoidalis and elicit an immune response that can lead to neuroepithelial damage.(18) The relatively long follow-up period and high percentage of residual OD compared to reports in the literature may suggest that post-viral olfactory loss was more common among our cohort of patients.
Similar to our previous study results, we found that persistent xerostomia was significantly related to hypogeusia. Hypogeusia may cause reduced salivary production since taste is considered the major stimulant of saliva formation.
The main limitations of this study are its relatively small size and the lack of objective measurements for diagnosis of OD and GD. As a follow up study on the same participates, there is a possibility of recall bias. In order to minimize the risk of recall bias we used two strategies; the first, previous scores of each patient were not revealed to him during the present telephone survey. Secondly, the substantial time that elapsed between the two surveys would serve as a “wash-out period”.
Nevertheless, the availability of references in the literature, helped in interpreting the results. Another strength is the long follow-up (a mean of 59.2 days after the second negative PCR test), which enabled good interpretation of patients' outcomes of 2 months after recovery.