Dear Editor:
We read with interest the article “Anosmia and Ageusia: Common Findings in COVID‐19 Patients” by Vaira et al. proposing that anosmia and ageusia may be early symptoms in 19.4% of COVID‐19 patients.1 In comparison with China, olfactory and gustatory dysfunction was more prevalent in Europe. Lechien et al. reported that Italy and Germany had shown a higher prevalence of olfactory dysfunction (23.8% and 79.7%, respectively) and gustatory dysfunction (28.9% and 88.8%, respectively),2 whereas Mao et al. reported only 5.1% olfactory dysfunction and 5.6% gustatory dysfunction.3 The differences between populations are quite distinct. Currently, the possible mechanism of olfactory and gustatory dysfunction due to COVID‐19 infection remains to be elucidated, and several possible pathophysiological mechanisms are suggested.
First of all, regarding the neuroinvasive nature of coronavirus, SARS‐CoV2 shares similar structures with other coronaviruses, such as SARS‐CoV and HCoV‐OC43, which are also neuroinvasive and neurotropic in humans. Previous animal models showed that the coronavirus might invade the central nervous system mainly through olfactory and trigeminal nerves. The decreased detection threshold of the trigeminal chemosensory function may follow olfactory dysfunction, which may explain why gustatory dysfunction may coincide with olfactory function. A recent report from the United States showed that acute demyelinating encephalomyelitis developed after COVID‐19 infection. A study from Brann et al. at Harvard University suggested two key genes,4 ACE2 and TMPRSS2, which are required for SARS‐CoV2 entry, are expressed in olfactory epithelium, and the different prevalence rate of olfactory and gustatory dysfunction from COVID‐19 infection in Asian and Europe may be due to different ACE2 or TMPRSS2 gene presentations.5
Second, respiratory viral infection often induces local inflammation of mucous membranes and presents with olfactory and gustatory dysfunction. In addition, allergic rhinitis was also a common symptom of SARS‐CoV2, and subsequent blockage of inspired air could be a possible explanation for olfactory dysfunction. Furthermore, the maintenance of the olfactory neuron declines with inflammation, which results in sensorineural olfactory loss. It has been shown that ACE2‐expressing cells with SARS‐CoV or SARS‐CoV2 infection reveal excessive proinflammatory cytokines, and these cytokines may impair olfactory function.6
BIBLIOGRAPHY
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