As scientists around the world in early 2020 hustled to understand the novel coronavirus SARS-CoV-2, a link emerged between COVID-19 and decreased sense of smell and taste. A year on, a lot more is known about why and how the virus causes these peculiar symptoms.
Dr. Ahmad Sedaghat, an associate professor with UC's College of Medicine and an UC Health physician specializing in diseases of the nose and sinuses, was one doctor who quickly discovered there was indeed a connection.
"Studies have now poured in from all parts of the world reporting how often it happens in patients with COVID," says Sedaghat. "The latest studies looking at thousands upon thousands of patients, again from all over the world, seems to indicate that the prevalence of smell loss in COVID-19 is approximately 50%."
That means half the people around the world with COVID-19 report experiencing loss of smell. What's interesting, Sedaghat points out, is that's a subjective observation number. To simplify, it means the smell loss was so severe that half of all patients noticed it. However, he says objective testing shows a different story.
"There are actually more patients with COVID-19 who if you actually objectively test their sense of smell, while they may tell you that they feel like their sense of smell is normal, it is actually diminished. If you go by objective testing, the prevalence of diminished sense of smell in patients with COVID-19 is actually quite a bit higher. It's closer to maybe 75% or 80% of patients."
Since the prevalence is so high, Sedaghat asserts it makes these particular symptoms especially good for identifying COVID-19. On average, loss of smell occurs within the first three to four days and rarely, if ever, after the first week, according to research. Sedaghat points out somewhere between a quarter to a half of patients have smell loss as their first or one of their first symptoms.
"Things like cough and fever are non-specific symptoms that don't allow us to really differentiate COVID-19 from some other viral illness but this smell loss that seems to be one of the first symptoms is a very specific tip-off that someone might be experiencing COVID-19."
Why Does It Happen?
Here comes the science. When you smell something, odorants in the air make their way up your nose to your smell, or olfactory, nerves, which in turn send signals to your brain and memory centers telling you what you're smelling. A problem anywhere along this process can cause smell loss, and scientists know that the nasal cavity is the primary hotspot for COVID-19 and the virus that causes it.
Sedaghat says there's evidence of three stops along this chain where COVID can cause problems. The first is simple obstruction caused by swelling in the nasal cavity, aka congestion.
The second is destruction of certain important cells targeted by the virus, which in turn causes the olfactory sensory neurons to malfunction. In short, you lose the part of the smell process where the olfactory neurons send signals about odors to the brain.
The third reason isn't entirely understood and appears to be more rare. Sedaghat says the coronavirus may somehow be infecting olfactory sensory neurons and crossing into the central nervous system, resulting in diminished sense of smell in some patients.
Which one of these three reasons may be causing a person's loss of smell may be tied to how severe the illness is and how quickly they regain the lost sense, Sedaghat explains. Someone who recovers their sense of smell quickly - which Sedaghat points out is the majority of patients - could likely be experiencing smell loss caused by congestion, while one of the other two reasons could be at play in someone with a more prolonged loss of smell during and after COVID-19.
Smell Ya Later
Most patients regain their sense of smell within two to four weeks, though some take longer. In extreme illnesses caused by other viruses, people have reported taking one to two years to regain their sense of smell. COVID hasn't been around long enough to know what could happen in the future.
People who don't quickly regain the lost sense can try olfactory training. Sedaghat calls it "physical therapy for your smell nerves." Patients may need to "re-wire" their brains to understand what certain odors smell like. It involves smelling a specific set of odors while thinking about what those odors smell like to essentially reconnect olfactory neurons and memory centers in the brain.
Not So Tasty
Sense of taste is far less studied than sense of smell in relation to COVID-19. Sense of tastespecifically refers to the ability to distinguish sweet, salty, sour, bitter and umami, or savory.
Sedaghat says there is some evidence of dysfunction with the primary sense of taste that may be contributing to diminished ability to taste things during COVID-19. However, in many cases what patients are experiencing is a diminished ability to sense flavor, which is tied to the olfactory senses.
Another component of flavor is chemesthesis, or the ability to sense things like texture, temperature, pain, and acidity. Studies show COVID patients subjectively report diminished ability to sense chemesthesis.
Dr. Sedaghat presented the lecture below on olfactory dysfunction and loss of taste in COVID-19 patients.
