Covid-19: The loss of taste, smell, points to neurological underpinnings
- The loss of taste and smell that the the coronavirus causes differs in testable ways from the ways a common cold causes people to lose those senses.
- You can't smell as well with a cold because your nasal passages are blocked; with Covid-19, the symptom seems to be related to the virus's effect on the nervous system.
- The findings of a new study on this difference may help clinicians distinguish between a cold and the coronavirus even if formal diagnostic tools aren't available.
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Losing your senses of taste and smell is a common symptom of Covid-19 — patients have reported being unable to tell that they're wearing perfume or pick up on the nuance of a well-balanced dish.
Anyone who's caught a cold or suffered through the flu might think they can relate, since those ailments also tend to make rich scents and layered flavors less accessible and appealing. (Bring home some bland soup, please.)
But a small study published Tuesday describes key differences between the loss of taste and smell that results from each of those viruses, shedding light on how and why this symptom may arise in Covid cases.
More specifically, it underscores that the coronavirus, unlike other common respiratory infections, affects the brain and nervous system.
"There are altogether different things going on when it comes to smell and taste loss for Covid-19 patients, compared to those with a bad cold," Carl Philpott, the lead researcher and a professor at the University of East Anglia's Norwich Medical School, said in a press release.
"It means that smell and taste tests could be used to discriminate between Covid-19 patients and people with a regular cold or flu," he added.
The study offers more evidence that the coronavirus affects the nervous system
Anywhere from 34% to 98% of hospitalised patients with Covid-19 will experience a loss of smell, clinically known as anosmia. Some will experience it for more than 30 days — and some may never regain it.
For the new study, a group of European smell-disorder experts gave taste and smell tests to 10 Covid-19 patients, as well as 10 people with bad colds and 10 healthy participants. They then compared the results, matched for age and sex.
The researchers found that smell loss was "much more profound" among people infected by the coronavirus — they were less able to identify scents than victims of a cold. (The particular test, "Sniffin' Sticks," asks participants to sniff 12 different everyday smells and choose the correct scent for each out of four multiple-choice options.)
Covid-19 patients' sense of taste was also seriously muted; they couldn't identify sweet or bitter flavors using "taste strips" that react to various tongue areas.
That wasn't so much the case for people with a run-of-the-mill cold.
When you have a cold, smelling and tasting are mostly tough because your nose is stuffed and your airways are clogged. But with Covid-19, the symptoms seem to stem from the way the illness invades the brain and nervous system since the virus doesn't tend to cause a stuffy nose, the researchers suspect.
Other research has also shown that the coronavirus can have potentially severe neurological consequences beyond the olfactory system.
One study found short-term loss of smell may stem from "cleft syndrome," a condition in which swelling prevents aromas from reaching the olfactory neurons.
Another found that some coronavirus patients developed brain swelling and delirium, while others developed nervous-system disorders like Guillain-Barré syndrome that can cause paralysis. Strokes have also been observed as a consequence of Covid-19.
"We're seeing things in the way Covid-19 affects the brain that we haven't seen before with other viruses," Michael Zandi, one of the new study's co-authors, told The Guardian.
The new research may help clinicians differentiate between the coronavirus and colds
The new study found that by giving patients standard and relatively simple tests of taste and smell, it can be pretty straightforward to differentiate between those who have Covid-19 and those with other respiratory conditions.
"Although such tests could not replace formal diagnostic tools such as throat swabs, they could provide an alternative when conventional tests are not available or when rapid screening is needed — particularly at the level of primary care, in emergency departments, or at airports," Philpott said in the release.
Ultimately, more research — including studies done using brain scans — is needed, the authors added, to help understand how exactly the coronavirus affects the senses.
Additional investigations could also tease apart "whether genetic variation in people's bitter and sweet taste receptors might predispose them to Covid-19, or conversely, whether Covid-19 infection changes how these receptors function," Philpott said.
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