Since the outbreak of COVID-19, perhaps no symptom has been more dramatic than loss of smell. While estimates vary, some studies have shown that 96% of COVID-19 patients experience partial or total loss of smell. But this is usually temporary, and most people will recover within a few weeks.
The COVID-19 pandemic has been going on for more than two years, and scientists still don't fully understand how the coronavirus is causing patients to lose their sense of smell, but now there are some new clues.
First, scientists have found that COVID-19 causes two types of loss of smell: acute loss of smell and chronic loss of smell.
Acute loss of smell refers to the appearance of other symptoms, such as runny nose or nasal congestion, when a person is infected with COVID-19, resulting in inflammation of the nasal cavity.
Dr Raj Sindwani, an otolaryngologist from the Cleveland Clinic in the USA, explains: "The odor particles must pass through the nasal cavity to the top of the cavity, where nerve endings are. If the inner wall of the nasal cavity is swollen and produces too much mucus, it blocks the odor particles from reaching the nerve endings, so you can't smell the smell. ”
Chronic loss of smell may also occur after contracting COVID-19, but weeks or months later, patients still do not recover.
It has been suggested that because of local inflammation of the patient's nerves, which leads to nerve damage, no matter what the virus does while it is active, it will destroy the nerve endings and lead to the inability to perceive odors.
There is also the view that genetic factors play a role.
Recently, a study published in Nature Genetics showed that researchers from 23andMe in the United States revealed genetic risk factors for whether individuals are susceptible to olfactory or taste loss symptoms after contracting new crown pneumonia. The study found that loss of smell or taste is an indicator that can better predict whether an individual is infected with the new crown virus compared to other symptoms such as cough or fever.
In the study, the team found that 47,298 people (68 percent) experienced loss of smell or taste in 69,841 self-reported data from participants who tested positive for the coronavirus, with a higher rate of symptoms in women than in men (72 percent in women and 61 percent in men), and younger people who were more likely to experience loss of smell or taste. In addition, East Asians and African Americans are at significantly lower risk of loss of smell or taste after COVID-19 infection than Europeans and Americans.
Through genome-wide association study (GWAS) analysis, the researchers found that a gene loci near the UGT2A1 and UGT2A2 genes on chromosome 4 was associated with loss of smell or taste. However, this genetic risk factor only increases the odds of losing their sense of smell by 11 percent, meaning that some people who carry this gene don't lose their sense of smell.
Dr Justin Turner, associate professor of otolaryngology head and neck surgery at Vanderbilt University Medical Center, said: "This suggests that genetic factors contribute to the risk of loss of smell or taste, but does not mean that it is the cause of loss of smell; and, if there is an effect, it is only a small factor. Because in this study, there were many people with this genetic variability, but they did not lose their sense of smell. ”
Two previous mainstream views have been that the coronavirus directly affects the sense of smell by infecting olfactory neurons in the olfactory bulb, or indirectly by destroying support cells.
Previously, in a study published in Science Advances, a team of researchers from Harvard Medical School showed that the new coronavirus destroys support cells involved in recognizing odors, thus causing a loss of smell.
The researchers say that when support cells are damaged, they can regenerate and function again. However, this can take weeks or months. This may explain why many people are unable to regain their sense of smell within a few months.
This is also the theory with the most evidence at present.
In addition, in a study published in Nature Neuroscience, a team of researchers from the National Institutes of Health (NIH) suggested that loss of smell may be due to the coronavirus causing inflammation and bleeding in the parts of the brain that control the olfactory bulb.
Dr Richard Doty, professor of otolaryngology head and neck surgery at the University of Pennsylvania, said: "It is unclear whether the virus enters the olfactory bulb, which is still a problem, but many other viruses do enter the olfactory bulb. Even herpes viruses can enter the brain through the olfactory pathway, so this is not impossible. But whether the olfactory bulb plays a role in olfactory loss is still inconclusive. ”
Sindwani said: "It is likely that all these theories are correct. Perhaps it is not a single factor that works, but multiple factors. Perhaps it is genetic factors that work in one patient and support cells that work in another patient. Maybe the support cells may be the reason why genes can't restore the odor. ”
He said the coronavirus is a brand new virus, so scientists haven't found all the answers yet. Maybe in the future we can know.
As for how patients with olfactory loss can regain their sense of smell, the researchers said that it is rare for patients with COVID-19 to be unable to regain their sense of smell. Studies have shown that only about 5% of patients still have symptoms of loss of smell after six months.
To regain a sense of smell, the researchers suggest that patients can use medications that include methods of flushing their noses with saline to drain mucus, as well as the use of prescription steroid sprays to reduce inflammation.
In addition to this, patients can also undergo olfactory training, that is, training the nose to recognize odors again.
The researchers explain that just as there are base colors such as red, yellow, and blue, the smell can also be a mixture of floral, fruity, aromatic, and resinous flavors.
For example, put a branch under your nose and inhale twice a day for 15 to 20 seconds, then think about what roses taste like and try to remember it. Combining visual images with the smell of these odor particles can quickly get the sense of smell to work.
Some clinical trials have used flavors to help determine a patient's sense of smell after contracting COVID-19.
The researchers believe that stories about loss of smell will give the general public a new understanding of smell.
They say people may not realize that their ability to smell is reflected in every aspect of their daily work and life, and they won't realize its importance until they lose it.
Thesis Link:
https://www.science.org/doi/10.1126/sciadv.abc5801