Olfactory hallucinations: a link to Covid-19

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Following coronavirus infection, some people complain of having lost their sense of smell. Others say they smell odors that don't exist.

These olfactory hallucinations, called phantosmia, are usually a good sign. They show that the olfactory system, damaged by the virus, is recovering.

Since the beginning of the Covid-19 epidemic, many patients have complained of losing their sense of smell. This anosmia is a very common symptom that usually appears within the first three days of infection.

What is anosmia? What is the ghost?

Among these smell disorders, the most common is anosmia, that is, the total loss of smell. A disability that, in most cases, disappears in a few weeks. "We conducted an online survey and collected several thousand responses. It shows us that two-thirds of patients regain their sense of smell on average within two weeks. Only 5-10% of patients still have persistent smell disorders after a year", specifies Camille Ferdenzi, a CNRS researcher at the Neuroscience Research Center of Lyon.

It is during this recovery phase, which is more or less long depending on the case, that olfactory hallucinations can occur. Some people say they smell an odor that doesn't really exist. Women and the elderly seem to be particularly affected.

Why does the coronavirus affect the sense of smell?

Upon entering the respiratory tract, the coronavirus triggers local inflammation. Edema or swelling of the nasal mucosa causes obstruction. Odorous molecules can no longer pass through.

Another mode of action may explain the olfactory disorders. "Since the virus is very present in the respiratory tract, it will attack the support cells of the olfactory epithelium. It also attacks, more deeply, the stem cells that, by differentiating, allow the regeneration of olfactory neurons", explains Camille Ferdenzi. Even if the nose is not stuffed up, these cells are so weakened that they can no longer carry out their functions: the entire system for capturing, recognizing and memorizing odors is disrupted.

Why smell a scent that doesn't exist?

The cells of smell have a certain capacity for recovery. Similarly, the olfactory neurons involved in odor recognition will gradually regenerate after the viral attack. But during this recovery phase, all this beautiful mechanics don't work normally. And this is where hallucinations can occur. "Often, phantosmias are linked to the regeneration of the olfactory system. When this system repairs itself, anarchic phenomena can occur and neurons do not reconnect normally. It is very worrying, but it is actually a very good signal. This means that the olfactory system is in the process of regeneration. Often, everything returns to normal, but it can take time, "says the researcher.

What types of smells are these?

"In our study, patients who complain of phantosmia often report unpleasant odours. Burning odours, such as grill or cigarette smoke, are the most common ones", emphasizes Camille Ferdenzi.

But it also happens that patients complain of bad odors, such as fermentation, garbage or sewage; or metallic or chemical odors.

How to treat loss of smell and phantosmia?

In a patient complaining of olfactory disorders, and in particular of olfactory hallucinations, local treatment is recommended first. "It is about decongesting the nasal mucosa using corticosteroids, antihistamines or local anti-inflammatories".

If phantosmia persists, in some cases it is possible to try a neuroleptic drug to somehow calm the abnormally stimulated olfactory cells.

The re-education of smell is effective

When this pharmacological treatment is not enough to solve the problem, an olfactory re-education should be considered. "It is an easy solution to implement and whose effectiveness we know", says Camille Ferdenzi.

Other pathologies cause olfactory hallucinations

In addition to the coronavirus, all diseases that damage the nasal mucosa and olfactory neurons can lead to olfactory disorders. This is the case, for example, of infectious sinusitis. The sense of smell will recover thanks to the local anti-inflammatories.

More seriously, a brain injury involving the olfactory bulb can be the cause of olfactory disorders. This can occur, for example, after a head injury, during epilepsy, or even Parkinson's disease. The chances of recovery, thanks to the re-education of smell, vary depending on the extent of the injuries.

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