Coffee tastes like coffee because we perceive a variety of aromas in the nose.
But after a Covid 19 illness, this ability is sometimes limited.
Photo: Grace Cary / Getty Images
Imagine the scent of a rose in bloom.
The smell of freshly baked bread.
A bowl of strawberries.
And now imagine that instead of these fragrances you perceive acrid stench: of faeces, burnt rubber, of gasoline and decay.
This is how many people describe how they smell after a Covid 19 disease.
The actual illness has long been over, but then weeks, sometimes even months later, disorders of the sense of smell appear.
This phenomenon is called parosmia in medicine.
Affected people smell something different than they should smell - and usually they perceive unpleasant and bad rumors.
Some patients cannot smell for months after the illness
It is well known that an infection with the Sars-CoV-2 virus can affect the sense of taste and smell: According to initial evaluations, 70 to 80 percent of those affected experience temporary restrictions, some also notice a total loss of smell.
In such cases one speaks of anosmia, a loss of smell.
And smell and taste do not always return when the disease is over.
The results of an online
in the journal
earlier this year
suggest that around two-fifths of patients still suffer from an impaired sense of smell two to three months after the disease.
As with other Long Covid symptoms, it is not yet clear why the odor disorder sometimes occurs much later.
What happens when the body loses odor?
Humans perceive smells through sensory cells in the olfactory mucous membrane.
This tissue is located at the top of the nasal cavity and contains millions of olfactory cells.
With a common cold you can no longer smell anything because the nose is swollen and mucous and the access to the olfactory mucous membrane is blocked.
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With a Covid 19 infection, however, you usually do not have a runny nose.
Instead, researchers have assumed so far, that the nerves themselves are attacked indirectly or directly: the olfactory cells in the nasal mucous membrane are surrounded by a framework of supporting cells.
The supporting cells contain a protein called ACE2 - the angiotensin converting enzyme 2 - and they contain hundreds of times more of it than other tissue cells.
These ACE2 proteins make it easier for the Sars-CoV-2 viruses to penetrate the cells.
This is because the so-called spike proteins on the virus envelope can dock onto the ACE2 proteins particularly well, as various studies have shown.
This leads to inflammation and can damage the olfactory cells.
The result is a sudden and sometimes long-term loss of smell.
Why do those affected smell the "wrong" fragrances?
“This is a qualitative disruption.
The scents are perceived in a twisted way, ”explains Thomas Hummel.
He is professor at the Clinic and Polyclinic for Otorhinolaryngology at the University Hospital Dresden and head of the Interdisciplinary Center for Smell and Taste there.
“Smell is the coding of scents. You can imagine it like an alphabet - the word rose in this model would consist of four components, the letters R, O, S and E. The scent molecules activate different types of receptors, which in a sense represent the letters. ”The receptors then redeem in the brain Pattern off. But: "If certain receptors are not there, there are gaps in the pattern," says Hummel. Then a wrong signal arrives in the brain.
The psychologist Kathrin Ohla, who teaches at the Helmut Schmidt University of the German Armed Forces in Hamburg and co-heads the Global Consortium for Chemosensory Research (GCCR), describes parosmia as a kind of interconnection problem: “With Covid-19 something similar happens as in one Fuse box if the wires blow.
If the cables are connected incorrectly during the repair, then the correct information will not arrive.
And this leads to a wrong interpretation. "
Is an odor disorder dangerous?
Parosmia can be "catastrophic" for those affected, says Ohla, and psychologically extremely stressful.
The complete inability to perceive smells can even be life-threatening in individual cases: According to media reports, a Texas family barely escaped from their burning house in January 2021.
The family members were sick with Covid-19 and did not smell anything, neither the smoke nor the smell of burned plastic and the like.
Hummel and Ohla agree: the sense of smell provides security in everyday life - if it falls away, it can have consequences.
“Think of food poisoning if you can't smell spoiled food.
Or a cable that is simmering.
Smell conveys a number of signals.
Those who cannot smell anything live more uncomfortably and insecure, «says Thomas Hummel.
And Kathrin Ohla also says: »A saucepan on the stove or a cake in the oven - if something burns, we smell it. And we can run off and switch off the stove.
If you can no longer do that, it means a great effort. «Life becomes more difficult.
What are the consequences for the sense of taste?
Tasting can also be restricted by a Covid 19 disease, says Hummel, but a real taste disorder is rare: in the medical sense, taste refers to the flavors sweet, sour, salty, bitter and umami.
"What is colloquially known as tasting is essentially the fine taste, the perception of aromas," says Hummel.
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Although we also taste the senses on the tongue via sensory cells, the main work is done by the nerves in the olfactory mucous membrane.
We would recognize a tangerine as a tangerine because we smell its aromas.
It only tastes sweet and sour on the tongue.
Nevertheless, the impression that one perceives tastes worse should not be deceptive: "If the olfactory enhancement is missing, the gustatory perception can also be reduced," explains the doctor.
Kathrin Ohla, on the other hand, is convinced that a corona infection can also disturb the real sense of taste.
"Our data show that both senses can clearly be affected." There are reports from those affected who can no longer differentiate between sugar and salt.
This cannot be due to a disturbed sense of smell.
What helps with an odor disorder?
All those who notice a restriction of their senses after a Covid 19 illness, who feel insecure and impaired - how quickly can they hope for improvement?
What helps with an odor disorder?
Those affected should first seek a medical diagnosis, advises Thomas Hummel: »Take a smell test at the doctor.
Something else can be behind an odor disorder. "
Parosmia cannot only be triggered by Sars-CoV-2 viruses. A skull injury can also lead to an olfactory disorder, as can polyps, i.e. tissue protrusions in the nasal mucosa, or the use of certain medications. An impaired sense of smell can also be an early sign of neurological diseases such as Alzheimer's or Parkinson's. Anyone who finds that they no longer smell the way they used to should therefore seek medical advice as soon as possible.
"If it is likely that the odor disorder was triggered by a viral infection, there are two options," says Thomas Hummel: "The first is to wait and see.
Smelling cells regenerate on their own. ”Second, to support regeneration, one can also train smelling.
Hummel suggests placing four bottles with different scented oils in the bathroom and sniffing each scent for half a minute in the morning and in the evening, over a period of three to nine months.
Several studies have also shown that olfactory training can have a positive effect on the recovery of the olfactory system.
The sense of smell does not always come back
Kathrin Ohla, however, also knows of cases in which the sense of smell has not returned after a corona disease.
“Ideally, taste cells will grow back after two weeks; for the olfactory nerve cells, this can take several months.
But there are also patients in whom the sense of smell has still not recovered 400 days after infection.
Then we have to assume that he won't come back. "
Even then, there is still a small chance of spontaneous recovery, says the scientist, but only a small one.
In order to better understand the consequences - and the extent - in the future, more data and long-term studies are needed, says Ohla.
Together with the GCCR, she has therefore developed a smell and taste test for at home, with which you can regularly check how well the sense works or how quickly the perception improves.
“This problem is likely to affect a few million people worldwide.
These people need care and support, ”she says.
And expresses a modest consolation: Not everyone is associated with a permanent odor disorder and a permanently reduced quality of life.
"Some can live very well with the memory of taste."