The doctor explains how COVID-19 suppresses the sense of smell
The doctor explains how COVID-19 suppresses the sense of smell

The doctor explains how COVID-19 suppresses the sense of smell

In December 2020, I received an unexpected Christmas present: a dry, persistent cough.

Just as vaccines were rolling out, I was given COVID-19. Super.

My symptoms were quite predictable – cough and fatigue. I thought that would be the extent of it, but one morning, a few days in isolation, I made a cup of tea, a sweet and spicy mixture with a strong scent. I lifted the mug to my nose and inhaled. Nothing.

Like many people who lost their sense of smell – a condition referred to as anosmia – due to COVID-19, I spent the next 10 minutes smelling various things from the back of my refrigerator. Could I detect chopped garlic? No. Dijon mustard? Nix. Blue cheese? Nothing.

A day or two later, my taste buds also slipped away. Soup was hot water with chunks in; biscuits nothing but crunchy cardboard. I could still see a rudimentary taste: Cookies were vaguely sweet, and chips were a little salty. But that was where it ended.

Through the weeks with my dull senses, I could not help but wonder: “What is happens? Are my poor olfactory neurons – the cells responsible for treating odor – dying? Are they temporarily damaged? Or is this a permanent loss? “

Months later, after I – spoiler alert – had a full recovery, including my sense of taste and smell, Zara PatelMD, en Stanford Medicine medical scientist and associate professor of otolaryngology and others published one paper who dug into the latest research on odor, including the effect of COVID-19 on the sense.

Even though my COVID-19 stay had become a distant memory, I was still wondering about the science behind how I improved my sense of smell, so I contacted Patel. She helped shed light on what was going on molecularly, how I regained my senses, and how people who have not yet recovered might be able to retrain their olfactory system to smell again.

“Most people with previous variants of COVID-19 experienced odor loss, but there are so many different reasons why this happens; it’s not just one mechanism,” Patel said. “Unfortunately, few physicians specialize in treating, evaluating, or diagnosing anosmia.”

To help physicians and patients with more knowledge, Patel has made her publication open to all, in the hopes that she can demystify a rather disturbing phenomenon.

The roads to meaninglessness

Some studies suggest that congestion and swelling of tissues in the nose are major factors leading to anosmia. That may be the case for some, Patel said, but congestion is not always a symptom of COVID-19. (It was not for me.)

What is more likely, she said, is that SARS-CoV-2 damages sustained support cells, which act as helper cells for the major neurons that pick up chemicals in the air and send signals to the brain. They help maintain a healthy ecosystem where neurons can thrive, and they help guide neurons to grow and make the right connections.

SARS-CoV-2 locks to specific receptors to infect a cell, but olfactory neurons do not have that receptor. So do the support cells. “Our body’s natural immune response is to bring inflammatory molecules to the site of trying to kill the attacker,” Patel said.

But that response can overwhelm the structural integrity of the support cells, and they end up as side damage. Without the right support, olfactory neurons cannot successfully transmit chemical signals to the brain, effectively attenuating the odor.

What about loss of taste? Is it the same mechanism? Yes, and … maybe not.

“Sweet, salty, bitter, umami and sour are the flavors that actually come from your tongue,” Patel said. “Anything else that adds to the flavor of your food, like distinguishing vanilla from chocolate or raspberry, is completely dependent on your ability to smell it.”

Some preliminary data suggest COVID-19 may attack the nerves in the tongue directly, Patel said, but for most people who have loss of taste, it is more likely due to odor problems.

Some even assume that odor loss is a benevolent sacrifice made to protect something even more critical: the central nervous system. Early studies of animals suggest that the damage seen in the olfactory pathway creates a roadblock for the virus, eliminating a channel for infection in the brain and the wider nervous system.

The theory has not yet been demonstrated in humans, but it is an interesting concept, Patel said.

Recovering odor

I distinctly remember the day, about five days after the first anosmia, when I began to regain my sense of smell. I searched for an indication of improvement, held a partially peeled mandarin to my nose, and unexpectedly got a hint of something sweet and spicy. Excited, I pulled the peel further back, pressed it closer to my nose and sniffed. Even though it was faint, it was really there – sour citrus.

(Later that year, I remembered it as one of the best moments of 2021 – and I got engaged that summer.)

My return to the smell stretched over 15-20 days. Seven days after the mandarin, I mostly felt recovered, but another week passed before the recovery was complete.

“Within the first month, about 70% to 75% of people who have COVID-19-related odor loss recover,” Patel said. That leaves 25% to 30% who do not and struggle to find answers.

A recurring, sometimes complicated problem

Patel also told me something quite disturbing: COVID-19-induced odor loss is not always a one-time thing. You can lose your sense of smell, recover it and then lose it again – and you do not need to have more COVID-19 infections.

And it’s not always just a loss of smell a second time; sometimes it is a distortion of smell and taste.

In these bizarre cases, something normal, like a flower or a smoldering stew, can assume the smell of something disgusting – think of rotten meat, chemicals or other repulsive things. The distortion is a result of damaged support cells.

Olfactory neurons regenerate naturally, but when they do so among glitchy support cells, the neurons do not receive the nutrients and structural help needed to grow into healthy cells. Unable to properly sense odor particles, neurons send signals that are unrecognizable to the brain.

“It’s never a good smell,” Patel said. “It’s probably because our brains do not recognize the signal, which naturally causes the person to avoid the substance that causes the unrecognizable odor. It is a kind of evolutionary defense mechanism.” Cue, old meat.

Olfactory neurons can only perform their work after the virus has finally been removed and the supporting cells recover.

Smell rehab

Patel has hope for patients whose symptoms linger – and for their doctors who may not know what they are doing. In her paper, she has released an algorithm that collects information about symptoms, duration of illness and other COVID-19-related details, and then spits out guidance to physicians and patients while navigating care and treatment options, such as olfactory training – in other words, smell rehabilitation.

“It’s a structured odor protocol that sounds simple, but if you think about it, it really looks like what to do if they get a stroke – they would go to rehab,” Patel said. Patients can sniff different blends of essential oils or herbal fragrances in a specific order several times a day as part of their workout.

“You just train your sense of smell over and over for about six months.” During that time, the olfactory nerves regenerate and the support cells can slowly recover, allowing the olfactory sensory neurons to process chemicals and transmit odor signals correctly.

Odor detoxification is not just for people who have lost their odor – it is for anyone whose ability to smell has been distorted by the virus.

In addition, Patel hopes that patients who have lost their sense of smell and feel despair can benefit from the paper: “I really want both patients and doctors to have access to this and use it as a source of information and treatment guidance.”

Photo of DimaBerlin

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