How COVID-19 May Cause Smell Loss in Some People? Scientists Discover

Sunday, June 21, 2020 - 09:51

Scientists have recently discovered a model for how COVID-19 may cause smell loss after months of reports.

Many people with COVID-19 reported a sudden loss of sense of smell and then a sudden and full return to a normal sense of smell in a week or two, the National Interest reports.

Interestingly, many of these people said their nose was clear, so smell loss cannot be attributed to a blocked nose. For others, smell loss was prolonged and several weeks later they still had no sense of smell. Any theory of anosmia in COVID-19 has to account for both of these patterns.

This sudden return of a normal sense of smell suggests an obstructive smell loss in which the aroma molecules cannot reach the receptors in the nose (the same type of loss one gets with a clothes peg on the nose).

Now that we have CT scans of the noses and sinuses of people with COVID-19 smell loss, we can see that the part of the nose that does the smelling, the olfactory cleft, is blocked with swollen soft tissue and mucus – known as a cleft syndrome. The rest of the nose and sinuses look normal and patients have no problem breathing through their nose.

We know that the way SARS-CoV-2 infects the body is by attaching to ACE2 receptors on the surface of cells that line the upper respiratory tract. A protein called TMPRSS2 then helps the virus invade the cell. Once inside the cell, the virus can replicate, triggering the immune system’s inflammatory response. This is the starting point for the havoc and destruction that this virus causes once in the body.

Initially, we thought that the virus might be infecting and destroying the olfactory neurons. These are the cells that transmit the signal from the aroma molecule in your nose to the area in the brain where these signals get interpreted as “smell”.

However, an international collaboration showed recently that the ACE2 proteins the virus needs to invade the cells were not found on the olfactory neurons. But they were found on cells called “sustentacular cells”, which support the olfactory neurons.

So why does smell not return in some cases? This is more theoretical but follows from what we know about inflammation in other systems. Inflammation is the body’s response to damage and results in the release of chemicals that destroy the tissues involved.

When this inflammation is severe, other nearby cells start to be damaged or destroyed by this “splash damage”. We believe that accounts for the second stage, where the olfactory neurons are damaged.

Recovery of smell is much slower because the olfactory neurons need time to regenerate from the supply of stem cells within the lining of the nose. Initial recovery is often associated with distortion of the sense of smell known as parosmia, where things don’t smell like they used to. For many parosmics, for instance, the smell of coffee is often described as burnt, chemical, dirty and reminiscent of sewage.

This article is republished from The Conversation under a Creative Commons license.

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