Overview

When COVID-19 spread throughout the world, one of the earliest symptoms that nobody expected was anosmia. This refers to a sudden loss of smell. This symptom was unusual because when you have a cold or the flu, you tend to lose your sense of smell due to a stuffy or blocked nose. With COVID-19, many people were unable to smell anything, even when their noses were not blocked.

This made the world ask an important question: How does COVID-19 cause anosmia differently from other viruses? Over the past few years, since COVID, researchers have been studying the virtues effects on the nose to understand this question. This article will go through what we already know about the mechanisms behind COVID-19-induced anosmia.

What is anosmia

Anosmia means that you have completely lost your sense of smell. This can happen due to many different reasons, such as infections, head injuries or even neurological diseases. With COVID-19, anosmia became so common that it was eventually used as a sign of infection - sometimes even before a person had a cough or fever.¹

Whilst other viruses cause a blocked nose, COVID-19-related anosmia often happens without congestion, and in most cases it comes on suddenly. From this, it may suggest that the virus affects the sense of smell in a completely different way.¹

How does the sense of smell work?

To understand how COVID-19 can affect our sense of smell, we need to understand how the sense of smell works:

• Inside the nose, there is a small area known as the olfactory epithelium. It sits in the nasal cavity²
• In this area, there are neurons known as olfactory sensory neurons. These are special nerve cells that detect smells²
• These neurons then send signals to the olfactory bulb in the brain. Here, the signs are worked on and smells are found out² 

The neurons are supported by sustentacular cells and basal stem cells. These cells help repair any damage and keep the neuron's environment healthy.²

How does COVID-19 affect the nose?

Infection of supporting cells, not neurons

COVID-19 is caused by a virus called SARS-CoV-2. This virus uses proteins called ACE2 and TMPRSS2 to enter human cells. 

Studies have shown that:

• You won’t find entry proteins on the olfactory neurons
• Instead, you can mainly find it on sustentacular cells (helper cells in the olfactory epithelium)

This means that the virus doesn't directly hurt the smell neurons; instead, it harms the supporting cells around them. When these cells are infected, they can no longer protect and maintain the neurons. This causes the neurons to stop working temporarily.³

Local inflammation in the nose

People with COVID-19 might not have a runny nose or congestion. However, there will be inflammation in the nasal tissue. This will cause the body's immune system to react to the virus and release cytokines. These are chemical messengers that cause swelling and irritation. 

This inflammation can:

• Affects how the smell neurons function
• Damage cells in the olfactory epithelium
• Delay the neuron’s ability to send smell signals to the brain⁴

Could COVID-19 affect the brain?

There was concern early on that the virus might travel from the nose to the olfactory bulb in the brain and cause damage there. However, studies now suggest that:

• The virus mostly stays in the nasal cavity
• Direct infection of the brain through the olfactory pathway is not common
• Inflammation and immune reactions might indirectly affect the olfactory bulb, leading to longer-lasting anosmia in some cases

So, although brain involvement is possible, it is not the main reason for smell loss in most people.⁵

Could blood vessels be involved?

Another theory is that COVID-19 affects the small blood vessels in the nose. The virus is known to damage the cells that line blood vessels (endothelial cells), which also have ACE2 receptors.

• This may mean there will be less oxygen and blood that reaches the olfactory epithelium
• When the cells lack oxygen, they won't be able to function or heal normally
• This can explain why certain people take more time to get their sense of smell back⁶

Why do some people recover quickly, and others don’t?

Most people who lose their smell because of COVID-19 usually recover within a few weeks. This is because the olfactory system can repair itself. The basal stem cells in the nose can create new supporting cells and neurons. 

However, in some people: 

• The damage may be more severe
• There may be longer-lasting inflammation
• The regeneration of neurons might be slower or incomplete

These people could take months to recover, and in some cases, they can develop parosmia. This is where smells become distorted or unpleasant.⁷

Treatments and therapies

As more information is being researched on COVID-19-induced anosmia, treatment options are being looked at:

Olfactory training

This is the most recommended therapy. It involves:

• Smelling certain scents (like rose or lemon) twice a day
• Training the brain to “re-learn” smells by stimulating the olfactory neurons
• Many patients will see some improvement after a few weeks of regular olfactory training⁸ 

Steroids or anti-inflammatory medication 

Doctors sometimes recommend nasal steroid sprays. The sprays help with inflammation in the nose. But it’s important to remember it might not work for everyone.⁹

Vitamin and zinc supplements

Some studies suggest that vitamins and minerals like vitamin A, vitamin D, and zinc might help the olfactory system heal.¹⁰

Summary

COVID-19-induced loss of smell has always been a unique and early symptom of the virus, often occurring without nasal congestion. Common colds can block the nose, but COVID-19 affects smell by targeting the supporting cells in the nose. They do not target the smell nerves themselves. This affects the sense of smell, even if the nerves are fine. Inflammation and, in some cases, damage to blood vessels in the nose might also have a part in it.

Most people get their sense of smell back in a few weeks. Both the nose and brain naturally repair themselves. However, some experience longer-lasting symptoms or distorted smells. Fortunately, there's a widely recommended approach called olfactory training—regularly smelling certain scents—that has been shown to help many patients get their sense of smell back over time. Other treatments are being researched, but no single solution works for everyone yet.

As more studies come out, we learn how COVID-19 affects the sense of smell. Knowing the early signs and supporting recovery is an important part of managing the disease. All the research and findings can be useful. It can help improve treatment for smell loss caused by different illnesses in the future.