The nose knows: Anosmia and parosmia. What’s the difference?

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“Hundreds of receptors in the nose send signals to the brain that identifies scents. When the sense of smell is lost, the brain can’t distinguish between a lime and lemon. They just both taste sour,” says Dr. Nancy Rawson. Photo courtesy of Shutterstock.

For many people who have lost their senses of taste or smell due to COVID, the most terrifying part can be not knowing when, if ever, those faculties will return. To demystify some of the science behind the smell disorders anosmia and parosmia, host Evan Kleiman speaks with Dr. Nancy Rawson, a scientist at the Monell Institute of Smell in Philadelphia. She is also an advisor to the newly formed Smell and Taste Association of North America, a patient advocacy group that aims to improve the quality-of-life for people with smell and taste disorders. 

The following interview has been edited for length and clarity.

KCRW: What goes on in the nose when someone loses their sense of smell due to COVID? 

Nancy Rawson: “The cells that detect odors live in the upper part of your nose, roughly between your eyes and under the bridge of your nose. And when the virus goes into this tissue, it actually doesn't directly affect those cells. But what it does is bind to receptors that are present on the cells that support those detector cells. So these supporting cells have the receptors for the COVID virus. … The virus binds very actively, very tightly to those cells and gets into the cell, and uses those cells to replicate. 

Those cells are so important to the detection of odors. What they do is help to maintain the correct environment around the receptor cells. And if that environment is disrupted by the death of the supporting cells, the detector cells just can't function. And so the whole structure sort of collapses. When we look at tissue that's been infected by this virus, it looks kind of like Swiss cheese, very disorganized, and a lot of dying cells. And once that structure falls apart, the whole tissue just deteriorates and has to regenerate from scratch.”

Is the nose the only organ involved with smell? What about the tongue and the brain?

“The tongue and the brain are very important in flavor perception. Odors involve things that can go up your nose, things that are very volatile. So they're small molecules. And this really is a very important part of what we call flavor, and what most people say when they refer to taste. So ... if you're drinking a cup of coffee, the coffee has bitterness, and that's taste. But the aroma of coffee, the thing that makes you know this is coffee, is smell. So these are two different senses. They're carried by different nerves to the brain. 

But the brain integrates and collects all this information into a flavor perception. So if you're talking about food, smell and taste are intimately linked. And if you're talking about something like ... smelling a flower or a perfume, that's our sense of smell. And the first step is detecting these small chemical compounds that float up your nose and interact with hundreds of receptors that are in that upper part of your nose. And when they get activated, they send signals to the brain that tells the brain it's a rose scent, or it's a lime scent, or it's a lemon scent. If you don't have a sense of smell, you can't discriminate lemons from limes because they both just taste sour.”

Can you define the terms anosmia and parosmia?

“Anosmia is when you lose your sense of smell completely. … Parosmia happens when there's an odor present, but your brain recognizes it as something different than what it is. And for whatever reason, the different thing the brain picks is usually something that's very unpleasant. For instance, coffee will now smell like gasoline fumes. 

Before COVID, the number of cases of parosmia that were seen clinically were really extremely small. … Now we have COVID, and all of a sudden, so many of the people, as they're recovering from COVID, are reporting parosmia. And it's something that just hasn't been studied that much because if it was, it used to be very rare.”

Do you know what proportion of people with COVID experienced long-term loss of smell?

“We're still collecting data on that. And it does look like … probably 60% of people recover [their sense of smell] within … one to three months. But there is a significant proportion of people that still have smell loss six months later. And based on historical data, experts are estimating that at least 10% of people who lose their sense of smell with COVID may not ever regain it. 

But again, the data are still coming in. This is still a new condition. We do know that for other types of viral-induced smell loss, where there is damage to the tissue. Some of the factors that reduce the likelihood of recovery include things like aging and other types of repeated exposure to viruses. So if you've had multiple viral sensory loss, the likelihood of recovery diminishes over time.”

Does it tend to impact adults more than children who contract COVID? How worrying is a disease like COVID-19 to someone like you who's been studying smell for so long?

“Yes, it does. It's very worrying. It also, in some ways, has presented us with some opportunities because of people being more aware of how important their sense of smell is. It's very easy to tell someone that your sense of smell is important. But if you haven't experienced it, and you haven't talked to somebody who's experienced it personally, it's really hard to appreciate just how important it really is in your quality of life and your safety. 

I had someone talking with me, really at her wit's end, because she had a new baby, and she couldn't smell her baby. It's awful. 

And it's hard for us as researchers because we've been pushing for years to increase the amount of funding available for this type of research. And we've been competing with other health conditions. There are so many important health conditions, and people say, ‘Well, it's not that important. It's not going to kill you.’ But the adverse impact on your quality of life is really very sad.

I will say that there is hope, even after many months, for people that have lost their sense of smell. It's a remarkable system. It's remarkably regenerative, more so than many other parts of the body that don't have that ability. And we'll keep researching and working to try to make a difference.”