Long Covid: What science knows about the loss of smell and taste

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Imagine waking up one morning after recovering from Covid-19 and discover that your coffee smells like dirty socks, your eggs smell like feces, and your orange juice tastes metallic. Strangely, this is a good thing: it’s a sign that you still have a functioning sense of smell — even if it’s poorly connected in your brain.

Your ability to smell can also disappear completely, a condition called anosmia. Without warning, you can no longer inhale the sweet smell of your baby’s skin, the roses offered by your partner, or the pungent scent of your gym clothes.

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Taste and smell are intertwined, so food can be bland or tasteless. Appetite and enjoyment of life can plummet, which previous studies show can lead to nutritional deficits, cognitive decline and depression.

Danger lurks too. Without a smell, you may not recognize the telltale signs of fires, natural gas leaks, poisonous chemicals, or spoiled food and drink.

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This is reality of about 5% of global Covid-19 survivors who have now developed lasting taste and smell problems according to a 2022 study. More than two years after the pandemic, researchers found that about 15 million people may still have trouble perceiving odors, while 12 million may have problems with taste.

Advocacy and support groups such as AbScent and Fifth Sense have mobilized to help, offering affirmation and hope, tips on smell training, and even recipes to increase appetite.

Olfactory training encourages people to smell essential oils twice a day, said rhinologist Zara Patel, professor of otolaryngology, head and neck surgery at Stanford University School of Medicine in the United States.

“The way I explain this to patients is that if you had a stroke and it made your arm not work, you would do physical therapy, you would do rehab,” Patel said. “This is exactly what olfactory training is for your sense of smell.”

As science learns more about how Covid-19 attacks and disrupts the sense of smell, “I think we’re going to see more targeted interventions,” said rhinologist Justin Turner, associate professor of otolaryngology, head and neck surgery at Vanderbilt University Medical Center in Nashville.

Anyone who is still struggling with the loss of smell and taste “should think positively and assume that their sense of smell will return,” Turner said. “Yes, there are some people who are not going to recover, so for those people, we want them not to ignore it. We want you to take it seriously.”

Cases exploded due to Covid-19

People have been losing their sense of smell and taste for centuries. Common cold and flu viruses, nasal polyps, thyroid disorders, severe allergies, sinus infections and neurological conditions such as Alzheimer’s disease, Parkinson’s disease and multiple sclerosis can impair the ability to smell and taste – sometimes permanently.

As well as head trauma, exposure to harmful chemicals, cancer treatments, smoking, gum disease, antibiotics and various medications for blood pressure, cholesterol, reflux and allergy, according to the Cleveland Clinic.

Aging is one of the main causes of loss of smell, as the ability of olfactory neurons to regenerate decreases. One study carried out in 1984 found that more than 50% of people between 65 and 80 years old suffered from “great olfactory deficiency”. The number has risen to over 75% for people over 80.

When the virus that causes Covid-19 invaded our lives, a relatively rare condition among people under the age of 50 expanded exponentially, affecting all ages.

“Covid-19 has affected young people far more than other forms of post-viral loss of smell,” said surgeon Eric Holbrook, associate professor of otolaryngology and head and neck surgery at Harvard Medical School. “You wouldn’t see much loss of smell in the pediatric population, for example, and now it’s very common.”

In fact, the loss of smell was so prevalent early in the pandemic that it was considered an early sign of Covid-19 infection, even in the absence of other symptoms.

This is not true today. One study published in May indicated that 17% of people lost their sense of smell when infected with the Ômicron variant, which became the predominant strain of the disease-causing virus in late 2021. (This could change again if the virus mutates).

In comparison, people infected with the two original variants, Alpha and Beta, were 50% more likely to lose their sense of smell or taste. Delta was almost as bad — 44% of people were affected, according to the study.

Statistics show that most people regain their sense of taste and smell. An August analysis of 267 people who had lost their sense of smell and taste for at least two years revealed that the majority had fully (38.2%) or partially (54.3%) regained their ability to smell and taste. This was especially true for people under 40, according to the study.

But 7.5% did not regain their sense of smell and taste two years after the Covid-19 infection was cleared. Those who were least likely to recover included people with existing nasal congestion, more women than men, and those who had a greater initial severity of smell loss, the study found.

How the damage occurs

How does Covid-19 damage the olfactory system? At first, scientists believed that the virus infected neurons in the nose responsible for transmitting smells from the environment to the brain. These neurons sit in the olfactory bulbs at the top of each nostril and send axons, or cables, to unique sensory points in the brain.

Soon studies showed that the virus does not enter these neurons. Instead, it attacks support cells, also known as support cells, which provide nutrition and protection to nerve cells from birth. Unlike many other cells, neurons in the nose are reborn every two to three months.

“Infection (Covid-19) of these support cells likely has some sort of long-term effect on the ability of these neurons to regenerate over time,” Turner said.

“That’s one of the reasons we sometimes see a delayed effect: people may have some loss of smell that recovers, and then later have a second wave of loss of smell, parosmia, or other symptoms because that regenerative ability is at an end. defect,” he said.

Parosmia is the medical term for distorted smells, which can often be quite disgusting, Zara said.

“Unfortunately, there are these classic categories of really terrible smells and flavors,” she said. “Sometimes it’s feces, garbage or dirty old socks. There may be a kind of sick and sweet chemical smell and taste. Oh, and rotten meat is another common category.”

For many people, parosmia tends to recur or recur at the three-month mark, around the time when olfactory neurons would be naturally regenerating, experts told CNN .

“If the reconnection misses the mark and hits a different spot in the brain reserved for a different odor, your sense of smell will be totally impaired,” Holbrook said.

“You have to trust the ability of these axons to retract and then find their way to the right place,” he added. “Or, if they’re not correct, wait for those neurons to die and for new ones to come back and find the right spot.”

Science continues to discover ways in which the virus attacks. One study February indicated that it can also damage olfactory receptors that sit on the surface of nerve cells in the nose. These receptors bind smells and trigger nerve impulses that transmit information to the brain.

There may also be a genetic component. One study January showed a mutation in two overlapping genes, UGT2A1 and UGT2A2, which play a role in odor metabolism. People with this mutation may be more susceptible to losing their sense of smell, but more studies are needed to determine the virus’ association with genes – if any.

Older people and people with chronic diseases that affect the nervous system, such as diabetes, are often more susceptible to olfactory damage, Zara said.

“It’s the very small vessels in the body, including the nose, that are affected by diabetes, disrupting the flow of blood, nutrients and oxygen to these olfactory nerves,” she said. “People with chronic sinusitis or allergic inflammation in the nose — anything that makes it harder for our system to recover will likely also be at greater risk.”

Source: CNN Brasil

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