Not a CNS problem?
Respondents also completed a chemosensory dysfunction home test (CD-HT). They were asked to prepare common household food items, such as peanut butter, sugar, salt, and vinegar, in a particular way – for example, to add sugar or salt to water – and provide feedback on how they smell and taste.
For this CD-HT analysis, 18.4% of respondents reported having persistent loss of smell. This, Dr. Frasnelli said, adds to evidence from self-reported responses and suggests that in some cases, the problem is more than senses not returning to normal.
“From the questionnaires, roughly 50% said their sense of smell is still not back to normal, and when we look at the CD home test, we see that almost 20% of subjects indeed have pretty strong impairment of their sense of smell,” he said.
The results showed no sex differences, although Dr. Frasnelli noted that most of the sample were women. “It’s tricky to look at the data with regard to sex because it’s a bit skewed,” he said.
Male respondents were older than female participants, but there was no difference in impairment between age groups. Dr. Frasnelli said this was “quite interesting,” inasmuch as older people usually lose some sense of smell.
The researchers have not yet examined whether the results differ by type of health care worker.
They also have not examined in detail whether infection severity affects the risk for extended olfactory impairment. Although some research suggests that the problem with smell is more common in less severe cases, Dr. Frasnelli noted this could be because loss of smell is not a huge problem for patients battling grave health problems.
As for other symptoms, many respondents reported lingering fatigue; some reported debilitating fatigue, said Dr. Frasnelli. However, he cautioned that this is difficult to interpret, because the participants were health care workers, many of whom returned to work during the pandemic and perhaps had not fully rested.
He also noted that he and his colleagues have not “made the link” between impaired smell and the degree of fatigue.
The COVID-19 virus appears to attack supporting sustentacular cells in the olfactory epithelium, not nerve cells.
“Right now, it seems that the smell problem is not a central nervous system problem but a peripheral problem,” said Dr. Frasnelli. “But we don’t know for sure; it may be that the virus somehow gets into the brain and some symptoms are caused by the effects of the infection on the brain.”
The researchers will extend their research with another questionnaire to assess senses 10-12 months after COVID-19.
Limitations of the study include the subjective nature of the smell and taste ratings and the single time point at which data were collected.
Confirmatory findings
Commenting on the research in an interview, Thomas Hummel, MD, professor, smell and taste clinic, department of otorhinolaryngology, Technische Universität Dresden (Germany), said the new results regarding loss of smell after COVID-19 are “very congruent” with what he and his colleagues have observed.
Research shows that up to one in five of those infected with SARS-CoV-2 experience olfactory loss. “While the numbers may vary a bit from study to study or lab to lab, I think 5% to 20% of post–COVID-19 patients exhibit long-term olfactory loss,” Dr. Hummel said.
His group has observed that “many more are not back to normal,” which conforms with what Dr. Frasnelli’s study reveals, said Dr. Hummel.
Also commenting on the research, Kenneth L. Tyler, MD, professor of neurology, University of Colorado at Denver, Aurora, and a fellow of the American Academy of Neurology, said the study was relatively large and the results “interesting.”
Although it “provides more evidence there’s a subset of patients with symptoms even well past the acute phase” of COVID-19, the results are “mostly confirmatory” and include “nothing super surprising,” Dr. Tyler said in an interview.
However, the investigators did attempt to make the study “a little more quantitative” and “to confirm the self-reporting with their validated CD home test,” he said.
Dr. Tyler wondered how representative the sample was and whether the study drew more participants with impaired senses. “If I had a loss of smell or taste, maybe I would be more likely to respond to such a survey,” he said.
He also noted the difficulty of separating loss of smell from loss of taste.
“If you lose your sense of smell, things don’t taste right, so it can be confounding as to how to separate out those two,” he noted. The study was supported by the Foundation of the Université du Québec à Trois-Rivières and the Province of Quebec. Dr. Frasnelli has received royalties from Styriabooks in Austria for a book on olfaction published in 2019 and has received honoraria for speaking engagements. Dr. Hummel and Dr. Tyler have disclosed no relevant financial relationships.
A version of this article first appeared on Medscape.com.