- One of the strangest coronavirus symptoms is also a sign that’s widely recognized as a likely COVID-19 indicator.
- Several studies attempted to explain what causes the sudden loss of smell and taste in COVID-19 patients. A new study employed brain MRI and PET scans to identify potential issues that can coincide with anosmia in COVID-19.
- The researchers think that glucose metabolism inside the brain regions associated with processing smell might partially explain the mysterious COVID-19 symptom.
Doctors have observed all sorts of unexpected symptoms during the first months of the COVID-19 pandemic, with the sudden loss of smell (anosmia) and taste (ageusia) being the strangest ones. Anosmia has been observed in other respiratory illnesses, but scientists soon realized that the novel coronavirus is the likeliest cause for losing the sense of smell this year. The symptom also happens to be the only COVID-19 symptom that might help patients and doctors tell COVID-19 apart from the flu without a diagnostic test. Doctors have been trained to recognize the sign, and it’s very likely they’d recommend immediate COVID-19 testing if you’re experiencing it.
Other scientists have tried to explain why the sense of smell suddenly vanishes, and some of the published papers out there indicate that the virus infects cells in the olfactory region of the nose. Those studies said the resulting inflammation and swelling prevents odors from reaching neurons that would otherwise pick them up. But a new study now says that people lose their sense of smell in COVID-19 because of some changes in the brain.
Researchers from Belgium used imaging techniques to map the brain of 12 COVID-19 patients with anosmia and 26 healthy individuals. For seven of the patients, the loss of smell was their main coronavirus symptom. Five patients recovered within 10 weeks after beating COVID-19, Newsweek reports. Seven still had problems with smell up to 16 weeks after recovering from coronavirus infections.
The researchers concluded that there might be a direct effect on parts of the brain involved with processing odors. They found that certain regions — including the olfactory cleft, the olfactory bulbs, and adjacent neural tissue — were indeed affected in some COVID-19 patients.
The team also indicated that the infection could impact sugar metabolism in brain cells that help process the sense of smell. And they said the nerve cell connections that collect the information about smell might be interrupted in COVID-19 patients.
Intriguingly, severe loss of smell (as documented by a low score on the “identification test”) was associated with a higher glucose metabolism in some olfactory core (e.g., orbitofrontal cortex) and high-order brain areas ([bilateral frontal eye fields] and [the left posterior parietal cortex]), while patients with more preserved olfactory function had lower glucose metabolism in these brain areas.
Although these results appear rather counterintuitive, they are in line with those of a previous FDG-PET study, which also showed a negative correlation of regional glucose consumption with the level of loss of smell. By contrast, longer duration of loss of smell was associated with a higher glucose metabolism in some core (e.g., orbitofrontal cortex) and high-order brain areas ([bilateral dorsolateral prefrontal cortex], [right medial prefrontal cortex], and [the left posterior parietal cortex]). Importantly, no correlation was found between the intensity and the duration of smell loss.
Experts who did not work on the study agree there might be different mechanisms that contribute to anosmia in COVID-19. But some of them told Newsweek that it’s too soon to draw conclusions from just 12 patients, and that more research is required. It would be “wrong” to attribute the cause of smell loss to glucose metabolism with “so little data,” said professor Aytug Altundag, an ENT expert from Turkey’s Biruni University.
The study was published in pre-print form on medRxiv, which means it hasn’t yet been peer reviewed.