The loss or reduction of smell, temporarily or chronically, was one of the main consequences of COVID at the beginning of the pandemic.
Currently, there is different research on this phenomenon, so experts are learning more about how SARS-CoV-2 affects smell, and what can be done to recover it.
When talking about smell disorders, it is necessary to clarify the difference between two of them: hyposmia and anosmia. Both are alterations that affect the nasal or cerebral structures that transmit the sense of smell, however, in hyposmia there is a reduction in smell, while in anosmia there is a total loss of the ability to smell.
These disorders are usually due to allergies, viral infections, nasal polyps, head injuries, anemia, or other types of injuries. A new cause was added to this list of possible culprits in 2020: COVID-19.
During the course of infection, cases of anosmia and ageusia (loss of taste) were common, while after infection, cases of hyposmia could occur.
A study published in Otolaryngology-Head and Neck Surgery, surveyed more than 616,000 people in the US who had COVID, and found that, compared to those who had been infected with the original virus, people who had contracted the Alpha variant (the first variant of concern to emerge ) had a 50% chance of suffering alterations in the sense of smell. That chance was reduced to 44% for the Delta (post-Alpha) variant and to 17% for the most recent variant, Omicron.
However, a significant portion of people infected early in the pandemic still experience chemosensory effects (alterations in taste and smell). According to a study published in Rhinologywhich followed 100 people who had had mild cases of COVID-19 and 100 people who had repeatedly tested negative, a year after the infections, 46% of those who had had COVID still had hyposmia.
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In contrast, only 10% of the control group had developed any loss of smell, but for other reasons. Furthermore, 7% of those who had been infected had anosmia.
Why does COVID affect smell?
At the beginning of the pandemic, different studies pointed out that COVID could affect smell because SARS-CoV-2 attacks cells in the nose, called sustainacular cells, which provide nutrients and support to odor-sensitive neurons.
Since then, different investigations have delved into what happens to olfactory neurons after an infection. For example, in a study published in JAMA Neurologya group of researchers analyzed tissues from the olfactory bulb at the base of the brain, a region that transmits nerve impulses that carry information about smells, from 23 people who died from COVID and from a control group of 14 people who died from other causes and who had no detectable coronavirus at the time of their death.
Three of the 23 COVID patients had lost their sense of smell, four had reduced sense of smell, and two had lost both smell and taste. None of the 14 patients in the control group had lost their sense of smell or taste.
After comparing the tissues of patients without COVID-19 with those of people who had been infected with SARS-CoV-2, particularly those whose sense of smell decreased or lost it completely, the experts found that the group with COVID showed more severe vascular injury and far fewer axons (parts of neurons that carry electrical impulses) in the olfactory bulb.
This did not change when other factors, such as the impact of age, were statistically controlled for, suggesting that these effects are not related to aging and are therefore linked to SARS-CoV-2 infection.
“Our findings suggest that SARS-CoV-2 infection in the olfactory epithelium leads to inflammation which, in turn, damages neurons, reduces the number of axons available to send signals to the brain, and results in the olfactory bulb becomes dysfunctional,” said Dr. Chang-Ying Ho, an associate professor of pathology at Johns Hopkins University School of Medicine in Baltimore and lead author of the research.
Other studies, such as the one published in Nature Geneticspoint out that this phenomenon could be due to a genetic mutation in people with COVID, associated with a greater propensity to lose their sense of smell or taste.
Experts also highlight the need to find solutions to this phenomenon, since long-term loss or reduction of smell (or taste) can have serious consequences for quality of life:
- Increase the risk of foodborne illness.
- Appetite problems.
- Involuntary changes in weight.
- Insecurity, self-esteem problems and even difficulty relating to other people.
How to recover the sense of smell?
Many treatments for restoring smell after coronavirus infection are currently being explored, usually in small clinical trials. However, it is still too early to confirm the effectiveness of any of them, so the only thing that most researchers recommend at the moment is that patients train their sense of smell.
To do this, it is necessary to expose yourself to strong-smelling substances, with the aim of promoting the restoration of olfactory signaling. There are investigations that worked with exposure to essential oils of rose, lemon, eucalyptus and clove.
The problem with these types of treatments is that they only seem to work in people who have a partial loss of smell, meaning they might only help a third of people who experienced chemosensory disruption after COVID.
COVID-19 is known to trigger extensive inflammation, which could play a role in impaired smell. For this reason, many researchers are exploring the use of steroids, which reduce inflammation. Another therapeutic possibility is platelet-rich plasma, from the patients’ own blood, which has biochemicals that could induce healing.
Until there are treatments approved by the health authorities, it is recommended to go to a specialist if after a month of the COVID infection the sense of smell does not recover.
There are resources to rehabilitate smell or taste that consist, for example, of smelling a set of common aromas in different concentrations, trying foods with different textures or using colors in food combinations.
Sources consulted: US National Library of Medicine, Mayo Clinic, JAMA Neurology, Nature Genetics, World Health Organization (WHO). Otolaryngology-Head and Neck Surgery, RhinologY.