One of the hallmarks of a COVID-19 infection has been a lost sense of smell after the infection ends.
In a new study, researchers blame an ongoing immune assault on the olfactory nerve cells — cells found at the top of the nasal cavity — and a decline in the number of those cells. The study was led by a team at Duke Health in Durham, N.C.
“One of the first symptoms that has typically been associated with COVID-19 infection is loss of smell,” said senior study author Dr. Bradley Goldstein, associate professor of head and neck surgery at Duke.
“Fortunately, many people who have an altered sense of smell during the acute phase of viral infection will recover smell within the next one to two weeks, but some do not,” Goldstein said in a university news release.
“We need to better understand why this subset of people will go on to have persistent smell loss for months to years after being infected with SARS-CoV-2,” he added.
For the study, researchers from Duke, Harvard Medical School in Boston and the University of California, San Diego, analyzed olfactory tissue samples from 24 biopsies, including nine patients with long-term loss of smell after COVID-19.
This approach — paired with sophisticated single-cell analyses in collaboration with Dr. Sandeep Datta at Harvard — revealed a widespread inflammatory response in the tissue where smell nerve cells are located.
Even when there were no detectable COVID levels, this inflammation persisted.
The number of olfactory sensory neurons diminished, possibly due to tissue damage from the ongoing inflammation, the researchers said.
“The findings are striking,” Goldstein said. “It's almost resembling a sort of autoimmune-like process in the nose.”
Information gleaned from the study will be key for designing treatments, he noted.
The researchers were encouraged that neurons appeared to maintain some ability to repair.
“We are hopeful that modulating the abnormal immune response or repair processes within the nose of these patients could help to at least partially restore a sense of smell,” said Goldstein, whose lab is working on this.
The researchers in this study focused on loss of smell, but their work also sheds light on possible underlying causes of other long COVID symptoms, including generalized fatigue, shortness of breath and brain fog.
These findings could inform further research into other symptoms undergoing similar inflammatory processes, Goldstein said.
The findings were published online Dec. 21 in the journal Science Translational Medicine.
The U.S. Centers for Disease Control and Prevention has more on long COVID.
SOURCE: Duke University, news release, Dec. 21, 2022