A nose knows why some people still can’t smell long after recovering from Covid-19.

The allergic reaction in the sense of smell has been found to be the reason why some people are unable to smell long after the symptoms have subsided. A small, peer-reviewed study It was published Wednesday in the journal Science Translational Medicine. In some cases, an immune or inflammatory response was found in patients with loss of smell after 16 months Recovering from covid-19.

Compared to people who could smell normally, patients with chronic olfactory dysfunction had fewer olfactory sensory neurons, the cells in the nose responsible for detecting odors and sending that information to the brain. According to Brad Goldstein, co-author of the study and a sinus surgeon at Duke University, compared to healthy people, patients with olfactory loss had, on average, 75% fewer neurons.

“We think that sensory loss is almost entirely disease-related,” Dr. Goldstein said.

Loss of smell is common A symptom of covid-19Although the prevalence varies widely, including what causes the infection, head and neck specialists said.

Most covid-19 patients who experience loss of smell recover within weeks of illness. But the symptom can last for a year or more Up to 7% of patientsThere is an analysis of February.

Dr. Goldstein and his colleagues wanted to identify what was damaged or changed in people with long-term loss of smell. “If we don’t know what’s broken, it’s hard to say how to fix it,” he said.

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They took nasal tissue samples from nine patients who had been unable to smell for a long time after Covid-19 infections and compared them with cells from healthy people. Patients with persistent loss of smell He had more T-cells.The white blood cells in their noses, which play an important role in the immune response, are found in their noses, the study says. The T-cells were making interferon-gamma, a disease-related substance, Dr. Goldstein said, and the support cells seemed to be responding to it.

The supporting cells protect and nourish the sense organs. Without them, the sense of smell cannot survive. Research shows that the virus that causes covid-19 does not directly attack the sense of smell, but can attack supporting cells.

A study of 24 patients found that patients with loss of smell had lower levels of certain types of anti-inflammatory cells and more types of cells than healthy people. The healthy group included two people who had recovered from Covid-19 but did not have long-term loss of smell.

Covid-19 researchers said the study has strengthened evidence that inflammation may be the cause Long covid symptoms. An April study in the journal JAMA Neurology A tumor was found among the dead covid-19 Patients in the olfactory bulb, the part of the brain responsible for receiving and processing information from the olfactory neurons in the nose.

Neuroinflammation may contribute to loss of smell and other neurological disorders Long-term covid-related symptoms Such as brain fog, said Cheng-Ying Ho, co-author of the April study and associate professor of pathology at Johns Hopkins University School of Medicine.

Dr. Ho, who was not involved in the new study, said inflammation that starts in the nasal cavity can spread to the brain. She said the new study is promising, but with a small sample size, more work is needed in more patients. Because the vaccination status of the participants was not collected, it is unclear whether the presence of the vaccines played a role in the response in the olfactory system, she said.

In a study conducted last year, more than 400 patients with smell problems More than 40% reported symptoms of depression And nearly 90% reported enjoying food less.

“People think that loss of smell is not a very important symptom of Covid compared to more serious symptoms like pneumonia, but it can bother some patients,” said Dr. Ho.

Researchers have found that the brain regions associated with the sense of smell are closely related to the brain regions that control memory and emotion.

Sandeep Robert Datta, a co-author of the new study and professor of neurobiology at Harvard Medical School, said he and others are conducting further research into the causes of smell loss following the loss of smell caused by the Covid-19 infection. The research can be conducted for medical purposes. There are no effective treatments for long-term loss of smell, Dr. Datta said.

“Smell gives you a sense of place. Without it, it can be very disorienting,” says Dr. Datta.

Write to Dominique Mosbergen at dominique.mosbergen@wsj.com

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