According to a study, persistent disorders of the sense of smell after a Covid 19 disease are due to immune processes and not to the corona virus itself. Samples from the olfactory mucosa of those affected provide evidence of this. There, US researchers discovered inflammatory processes that continued even in the absence of the Sars-CoV-2 pathogen.
Olfactory dysfunction is one of the most common symptoms of Covid 19 disease, writes Bradley Goldstein’s team at Duke University. This is generally attributed to the fact that the corona virus directly affects the olfactory mucosa. This three to five square centimeter small area lies deep in the nasal cavity in the upper nasal passage on both sides of the nasal septum and contains about ten million olfactory cells as well as supporting and stem cells.
Previous studies had shown that the coronavirus in the mucous membrane does not infect the actual olfactory cells, but the neighboring supporting cells, which supply the olfactory cells with nutrients. “It is believed that in most patients with Covid-related olfactory loss after the virus disappears, normal repair processes replace the population of supporting cells (and accidentally damaged neurons) and restore their function,” Goldstein’s group writes in the journal Science Translational Medicine.
Inflammation of the olfactory mucosa often persists long after the disease has subsided
So far, however, it has been unclear what happens to those people in whom the olfactory disorders last longer – often several months, sometimes even years. It was assumed, for example, that damage to the mucous membrane also affects the population of stem cells that serve to replace the cells in the olfactory mucosa.
To answer the question, the team analyzed mucosal samples from 24 people, nine of them with prolonged loss of smell after Covid disease. During the investigation, the researchers came across T cells that are associated with inflammatory reactions. Such inflammatory processes continued even when the Sars-CoV-2 pathogen was no longer detectable in the tissue. In addition, the tissue contained noticeably few olfactory cells – their number may have fallen as a result of the ongoing inflammatory reaction.
The researchers write that interactions between immune cells and the mucous membrane could possibly lead to functional changes in the supporting and olfactory cells. This reaction differs from the immune process that takes place during an acute corona infection.
“This thorough study provides evidence that persistent olfactory disorders are associated with inflammation of the olfactory mucosa,” says ENT specialist Thomas Hummel from the University Hospital Dresden, who was not involved in the work. “It hasn’t been shown before.” Peter Berlit, Secretary General of the German Society for Neurology (DGN), also speaks of a “very valuable finding”.
However, the researchers admit that their conclusions are based on only a small number of samples of the olfactory mucosa. The Dresden ENT doctor Hummel also criticizes this. “I would have liked a larger number of cases.” This would also have made it possible to check any influences from other factors, such as age. According to Hummel, olfactory disorders tend to last longer in older people.
The authors emphasize that their study provides clues to treatment options. For example, pro-inflammatory immune cells in the area can be specifically inhibited. Since the olfactory mucosa in the nose is easily accessible from the outside, medication can be introduced into this area in a targeted manner.
Hummel notes that the study provides evidence that anti-inflammatory drugs such as steroids or platelet-rich plasma could improve olfactory disorders. So far, the success rate of such therapies is modest, but they may be suitable for certain groups of patients.
Berlit refers to a recent in the journal BMC Medicine published double-blind study from the Netherlands. Accordingly, tablets with the cortisone preparation prednisolone brought no improvement in people who had had olfactory disorders for more than four weeks. In view of the new findings, Berlit thinks it makes sense to now examine the direct application of cortisone to the olfactory mucosa. So far there is no drug therapy with proven effectiveness, but regular olfactory training can help.
In the summer of 2021 had French researchers in the journal Jama Open Network figures publishedhow long olfactory disorders can last. According to this, around 84 percent of the 51 participants affected by smell loss had their sense of smell completely back after four months. By the end of the study, 12 months later, 96 percent of the subjects had regained their sense of smell.