As soon as the first reports from Wuhan, to Iran and later to Italy, we knew that the loss of sense of smell (anosmia) was a symptom significant of infection by the coronavirus SARS-CoV-2 responsible for the epidemic of Covid-19. Today, after months of results rigorous clinical or sometimes more anecdotal, an explanation on the reasons why this virus sometimes causes a loss of sense of smell in people it infects.
Most often, the loss of sense of smell is the result of an infection by a virus, which attacks the upper respiratory tract or sinus, such as viruses causing colds. Some of them may belong to the coronavirus family that are not fatal.
If they are known to result in the loss of odorats, these viruses are not fatal, contrary to the virus of SARS, of MERS or Covid-19. In most of these cases, the sense of smell comes back when the symptoms disappear. In fact, its loss is simply the result of a stuffy nose : the molecules of aroma cannot reach the olfactory receptors, and the odors are not perceived. It happens sometimes however that the loss of smell persists for months, or even years.
In the case of the novel coronavirus SARS-CoV-2, however, the pattern of the loss of sense of smell differs. Many patients affected by the Covid-19 have been described to have suffered a sudden loss of sense of smell, followed a week or two later by a return just as suddenly this meaning.
It is interesting to note that many of these people have indicated that their nose was not clogged when they have lost the sense of smell. In some, this loss is extended beyond the end of their recovery, and several weeks after that they had still not recovered their sense of smell. Any theory trying to explain the reasons for this anosmia must therefore take into account these two situations.
The sudden return of the smell described by the patients indicates that the inability to perceive odors could have been home obstructive : after infection, the aromatic molecules would no longer have been able to reach the olfactory receptors. It is the same type of loss of sense of smell than the one that suffers when one uses a nose clip.
Scanners of the nose and sinuses performed on patients Covid-19 with loss of sense of smell revealed that the part of the nose responsible for the perception of odors, the slot olfactory, finds himself blocked by swelling of the soft tissues as well as mucus. This situation is known under the name of ” syndrome of the slot olfactory “. The rest of the body as well as their sinuses have a normal appearance, and the people involved have no problem breathing through the nose.
It is now known that SARS-CoV-2 infects our body by binding to the receptor ACE2 is found on the surface of cells of our upper airways. A protein called TMPRSS2 and helps the virus to invade these cells. Once inside, they replicate, triggering, in turn, an inflammatory reaction of the immune system. It is at this time to begin the havoc that will wreak havoc on the body in some patients.
We originally thought that the virus was able to infect and destroy the olfactory neurons, the cells that transmit the signals generated by the attachment of aromatic molecules on receptors located in the nose and the region of the brain where these signals are interpreted as ” smells “.
However, the work carried out by an international team have recently demonstrated that the receptor ACE2 required for the virus to enter cells were not present on the surface of olfactory neurons.
Instead, they were detected at the cell surface ” sustentaculaires “, which provide structural support to the neurons.
These cells support are likely to be those that are damaged by the virus during infection. The immune response would then be a swelling of this area, while leaving intact the neurons involved in olfaction. Once the virus is eliminated, the swelling regress. The aromatic molecules could then again reach their receptors, and the smell would be thus restored.
But then, how to explain that in some cases, the sense of smell does not return ? The explanation remains for the moment theoretical but is drawn from what we know of the inflammation in other systems. Inflammation is the body’s response to damage, and results in the production and distribution of chemical substances that destroy the tissues affected by these damages.
When the inflammation is severe, the cells nearby are also damaged, or even destroyed, the victims of collateral damage. We believe that this is the reason why some people do not regain the sense of smell before long : their olfactory neurons have been victims of this type of damage.
The return of the sense of smell is so much slower, because it is necessary that the olfactory neurons regenerate from the stock of stem cells present in the nose. The initial fetch is often associated with a distortion of odor perception called “parosmie” : the people who are affected have the impression that things do not feel as they felt before the infection. The aroma of coffee, for example, can evoke the burning, or a chemical smell, even a smell of dirt or sewage.
Olfaction is sometimes referred to as the ” Cinderella of the 5 senses “, because it is often neglected by scientific research. It is, however, found on the front of the stage during the pandemic Covid-19. The positive point is that we in it will allow us to learn a lot about the way in which viruses are implicated in the loss of sense of smell. But what can we expect from this knowledge people who have currently lost the sense of smell ?
The good news is of course that the olfactory neurons can regenerate. They grow at almost everyone, all the time. This regeneration can be exploited to our benefit, and guided by putting in place a “physiotherapy nasal” : it is to train them to perceive odours.
It has been proven that the fact of exposing themselves on a daily basis, repeatedly and consciously has a number of fragrance substances is in fact beneficial in many situations of loss of sense of smell. There is no reason to think that it will not be the same when the loss of odour perception is due to the Covid-19.