You wake up one morning with a cough and chest pain. You feel nauseous and sense the onset of a fever. You are still unsure of the cause of these symptoms, until you notice that you cannot taste or smell. At this point you are confident that you have been infected with COVID-19. You are now one of many who have been afflicted with dysosmia and dysgeusia, symptoms burdening many COVID-19 patients.
Dysosmia and dysgeusia are terms used to describe a variety of taste and smell disorders, ranging from impaired sensory function to complete loss of smell and taste [1]. Research has found that many COVID-19 patients experience these symptoms. A study by Coelho and others found that over 98% of subjects surveyed rated their overall sense of smell as “good” or “very good” prior to contracting COVID-19, whereas only 10% were able to make that same report after infection with the virus. Up to 66% of patients reported a complete loss of smell [2]. Another study by Estomba and others found that 81% of participants completely lost and 17% partially lost their sense of smell with the contraction of COVID-19 [3]. Dysgeusia presence varies more widely, with two studies finding 28.6% and 65% of patients, respectively, to suffer from this symptom [4],[5].
Although researchers have sought to explain the development of these symptoms with the contraction of COVID-19, the exact pathogenesis of dysgeusia and dysosmia remains unclear. Some studies have hypothesized that SARS-CoV-2 — the virus that causes COVID-19 symptoms — infects the human central nervous system (CNS) and may have neuroinvasive effects. Coronaviruses enter the CNS through the olfactory or peripheral trigeminal nerves [1]. The olfactory nerve is a cranial nerve responsible for the sense of smell [6]. Part of the function of the trigeminal nerve involves carrying pain, temperature, and touch modalities from the mucosa of sinuses, the nose, and the mouth [7]. Through this pathway to the CNS, the virus may damage these nerves, causing dysosmia and dysgeusia [1].
In addition to these sensory nerves, SARS-CoV-2 may also cause damage to the olfactory epithelium and bulb [8]. The olfactory epithelium is the layer of neurons and cells lining the nasal cavity, and the olfactory bulb is the part of the brain responsible for receiving sensory information from the olfactory receptor neurons [9],[10]. This damage may lead to dysosmia. The viral infection and inflammatory response caused by SARS-CoV-2 may cause disruption of saliva composition and taste transduction, and they may also impair the continuous renewal of taste buds. All of these physiological changes likely lead to dysgeusia [8].
A few researchers have also suggested that the consumption of chemicals and disinfectants — which many COVID-19 patients have resorted to in attempts to reduce their symptoms — could contribute to dysosmia and dysgeusia. However, given that many patients do seek proper medical treatment rather than using dangerous household remedies, this hypothesis does not explain all instances of dysosmia and dysgeusia in COVID-19 patients. In addition, further investigation is required to definitively conclude that these chemicals do, in fact, lead to the development of taste and smell disorders [1].
Another notable feature of the dysgeusia and dysosmia of COVID-19 patients is their persistence even in post-recovery. Some research has found that 11% of COVID-19 patients with dysosmia continued to experience a loss of smell for over four weeks beyond its onset [8]. Another study found that COVID-19 patients who developed dysosmia and fully recovered had experienced this symptom for an average of 10 days, while patients who only partially recovered had lost their sense of smell for an average of 12 days [3]. A study by Sheng et. al. found that the average time it took patients to recover from dysgeusia was 12 days, with 10.2% of patients still experiencing this symptom before being discharged from the hospital [4].
Although dysosmia and dysgeusia are unpleasant symptoms of COVID-19, most patients do eventually regain their senses of taste and smell within several weeks. Even after the global pandemic, COVID-19 will continue to be a study topic of interest for years to come. Further research will likely provide more insight into the pathways through which the virus causes dysosmia and dysgeusia and how to best mitigate these symptoms.
References:
Mehraeen, E., Behnezhad, F., Salehi, M.A. et al. (2021). Olfactory and gustatory dysfunctions due to the coronavirus disease (COVID-19): a review of current evidence. Eur Arch Otorhinolaryngol 278, 307–312. https://doi.org/10.1007/s00405-020-06120-6.
Coelho, D.H., Reiter, E.R., Budd, S.G., Shin, Y., Kons, Z.A., Costanzo, R.M. (2021). Quality of life and safety impact of COVID-19 associated smell and taste disturbances. American Journal of Otolaryngology. Volume 42, Issue 4, 103001, ISSN 0196-0709, https://doi.org/10.1016/j.amjoto.2021.103001.
Chiesa-Estomba, C.M., Lechien, J.R., Radulesco, T., Michel, J., Sowerby L.J., Hopkins, C., Saussez, S. (2020) Patterns of smell recovery in 751 patients affected by the COVID-19 outbreak. European Journal of Neurology 27(11): 2318-2321. https://doi.org/10.1111/ene.14440.
Sheng W.H., Liu W.D., Wang J.T., Chang S.Y., Chang S.C. (2021) Dysosmia and dysgeusia in patients with COVID-19 in northern Taiwan. Journal of the Formosan Medical Association. 120(1):311-317. https://doi.org/10.1016/j.jfma.2020.10.003.
Zayet S., Klopfenstein T., Mercier J., Kadiane-Oussou N.J., et al. (2020). Contribution of anosmia and dysgeusia for diagnostic of COVID-19 in outpatients. Infection 49(5): 361–365. https://doi.org/10.1007/s15010-020-01442-3.
Helwany, M., Bordoni, B. (2021) Neuroanatomy, Cranial Nerve 1 (Olfactory). In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK556051/.
Walker, H.K. (1990) Cranial Nerve V: The Trigeminal Nerve. In: Walker, H.K., Hall, W.D., Hurst, J.W., (Eds.). Clinical Methods: The History, Physical, and Laboratory Examinations (3rd ed). Boston: Butterworths. https://www.ncbi.nlm.nih.gov/books/NBK384/.
Meunier, N., Briand, L., Jacquin-Piques, A., Brondel, L., Pénicaud, L. (2021). COVID 19-Induced Smell and Taste Impairments: Putative Impact on Physiology. Frontiers in Physiology, vol. 11. https://www.frontiersin.org/article/10.3389/fphys.2020.625110.
Purves D, Augustine GJ, Fitzpatrick D, et al., editors. (2001). The Olfactory Epithelium and Olfactory Receptor Neurons. Neuroscience. 2nd ed. Sunderland (MA): Sinauer Associates. https://www.ncbi.nlm.nih.gov/books/NBK10896/.
Purves, D., Augustine, G.J., Fitzpatrick, D., et al., editors. (2001). The Olfactory Bulb. Neuroscience. 2nd ed. Sunderland (MA): Sinauer Associates. https://www.ncbi.nlm.nih.gov/books/NBK11158/.