Loss of odor retains post COVID

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More than 50% of healthcare workers infected with SARS-CoV-2 report that their sense of smell has not returned to normal on average 5 months after infection, a new study shows.

The findings illustrate that odor problems are common not only during the acute phase of COVID but also “long-term” and that these problems should be “considered” when monitoring these patients, study researcher, et al. Johannes Frasnelli, Professor, Department of Anatomy, University of Quebec at Trois-Rivieres, Quebec, Canada, told Medscape Medical News.

Loss of sense of smell can affect quality of life because it affects food and drink, and can be dangerous, Frasnelli said. “If your sense of smell is weakened, you may unknowingly eat spoiled food or you may not smell smoke or gas in your house,” he said.

In addition, Frasnelli noted that impaired sense of smell is associated with higher rates of depression.

The findings will be presented at the annual meeting of the American Academy of Neurology (AAN) in 2021 in April.

A “striking” discovery

Studies show that about 60% of patients with COVID lose their sense of smell to some degree during the acute phase of the disease. “But we wanted to go further and look at the long-term consequences of losing smell and taste,” Frasnelli said.

The analysis included 813 health workers in the province of Quebec. For all patients, SARS-CoV-2 infection was confirmed by nasopharyngeal viral smear testing.

Participants completed a 64-item online questionnaire that asked questions about three senses: sense of smell; delicious, which includes flavors such as sweet, sour, bitter, salty, salty, and umami; and trigeminal, which includes sensations like the hotness of hot peppers and the “freshness” of mint.

They were asked to rate them on a scale from 0 (no perception) to 10 (very strong perception) before infection, during infection, and currently. Other symptoms, including fatigue, were also asked.

Most of the respondents were infected with the first wave of the virus in March and April 2020, and they answered the questionnaire on average five months later.

The vast majority of respondents (84.1%) were women, which Frasnelli says is not surprising because women predominate in health care.

The analysis showed that the average odor scores were 8.98 before infection, 2.85 during the acute phase, and 7.41 when respondents responded to the questionnaire. The sense of taste was less affected and recovered faster than the sense of smell. Taste scores were 9.20 before infection, 3.59 during the acute phase, and 8.05 after COVID.

Among the 580 subjects who indicated impaired sense of smell during the acute phase, the average odor rating when answering the questionnaire was 6.89, compared to 9.03 before infection. More than half (51.2%) reported that they did not regain full olfactory function.

The fact that the sense of smell did not return to normal for half of the participants so long after the infection “is new and quite astonishing,” Frasnelli said.

However, he noted, this does not necessarily mean that all those with compromised sense of smell “have huge problems”. In some cases, he said, the problem is “more subtle.”

Not a CNS problem?

Subjects also performed a home test for chemosensory dysfunction (CD-HT). They are asked to prepare common household items in a special way, such as peanut butter, sugar, salt and vinegar – for example, to add sugar or salt to water – and to give feedback on how these smells and tastes they have.

For this CD-HT analysis, 18.4% of respondents reported having permanent odor loss. This, Frasnelli said, builds on the evidence from the responses they reported themselves and suggests that in some cases the problem is more than that the senses are not returning to normal.

“Approximately 50% of the questionnaire said their sense of smell had not yet returned to normal, and when we look at the CD home test, we see that almost 20% of respondents do have quite severe sense of smell damage,” he said.

The results showed no differences in gender, although Frasnelli noted that most of the sample were women. “It’s embarrassing to look at sex data because it’s a bit distorted,” he said.

Male subjects were older than female participants, but there was no difference in impairment between age groups. Frasnelli said this is “quite interesting,” as older people usually lose their sense of smell.

Researchers have not yet examined whether the results differ according to the type of health worker.

They also did not examine in detail whether the severity of the infection affects the risk of prolonged odor damage. Although some research suggests that odor problems are more common in less severe cases, Frasnelli noted that this could be because odor loss is not a big problem for patients struggling with serious health problems.

As for other symptoms, many subjects reported prolonged fatigue; some reported exhausting fatigue, Frasnelli said. However, he warned that this is difficult to interpret because the participants were health workers, many of whom returned to work during the pandemic and may not have fully rested.

He also noted that he and his colleagues did not “establish a connection” between the weakened smell and the degree of fatigue.

The COVID virus appears to attack supporting sensitive cells in the olfactory epithelium, not nerve cells.

“Right now, the smell problem doesn’t seem to be a central nervous system problem, but a peripheral problem,” Frasnelli said. “But we don’t know for sure; it could be that the virus somehow enters the brain, and some symptoms are caused by the effects of the infection on the brain.”

The researchers will expand their research with another sensory assessment questionnaire 10 to 12 months after COVID.

Limitations of the study include the subjective nature of odor and taste ratings and the unique time point at which data were collected.

Confirmatory findings

Commenting on the research for Medscape Medical News, Thomas Hummel, Ph.D. Med., Professor, Odor and Taste Clinic, Department of Otorhinolaryngology, Technische Universität Dresden, Dresden, Germany, said the new results regarding odor loss after COVID were “very consistent” with what he and his coleagues.

Studies show that up to 1 in 5 infected SARS-CoV-2 has odor loss. “Although the numbers may vary slightly from study to study or from laboratory to laboratory, I think 5% to 20% of post-COVID patients show long-term loss of odor,” Hummel said.

His group noted that “a lot of them haven’t returned to normal yet,” which is in line with what Frasnelli’s study reveals, Hummel said.

I also comment for Medscape Medical News, Kenneth L. Tyler, Ph.D. Med., A professor of neurology at the University of Colorado School of Medicine in Denver, Colorado, and a member of the American Academy of Neurology, said the study was relatively large and the results “interesting.”

Although it “provides more evidence that there is a subset of patients with symptoms even after the acute phase” of COVID, the results are “mostly affirmative” and include “nothing super surprising,” Tyler said.

However, investigators tried to make the study “a little more quantitative” and “to confirm self-registration with their verified home CD test,” he said.

Tyler wondered how representative the sample was and whether the study attracted multiple participants with impaired senses. “If I had a loss of smell or taste, I might be more likely to respond to such research,” he said.

He also noted difficulties in separating odor loss from taste loss.

“If you lose your sense of smell, things don’t taste good, so it can be confusing how to separate the two,” he noted.

The study was supported by the University of Quebec Foundation in Trois-Rivieres and the Province of Quebec. Frasnelli received a fee from the Austrian Styriabooks for a fragrance book published in 2019 and fees for speaking engagements. Hummel and Tyler did not disclose relevant financial relationships.

Annual Meeting of the American Academy of Neurology (AAN) 2021: Summary 2192. Introduced April 20, 2021.

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