Why do viruses like the coronavirus sometimes steal our sense of smell?

When you catch a severe cold, your nose stuffs up, you can’t smell anything and food tastes funny. Fortunately, most people regain their sense of smell once the cold runs its course. But for others, the complete (anosmia) or partial (hyposmia) loss of the sense of smell is permanent.

I spoke with Zara Patel, MD, a Stanford associate professor of otolaryngology, head and neck surgery, and director of endoscopic skull base surgery, to learn more about her research on olfactory disorders. In particular, we discussed her recent study on the possible association between post-viral olfactory loss and other cranial neuropathies, which are disorders that impair your nerves and ultimately your ability to feel or move. She also described how her work pertains to the COVID-19 pandemic.  

How does a virus impair someone’s sense of smell?

A variety of viruses can attack the cranial nerves related to smell or the mucosal tissue that surrounds those nerves. Cranial nerves control things in our head and neck — such as the nerves that allow us to speak by using our vocal cords, control our facial motion, hear and smell.

For example, COVID-19 is just one type of disease caused by a coronavirus. There are many other types of coronaviruses that cause colds and upper respiratory illnesses, as well as rhinoviruses and influenza viruses. Any of these viruses are known to cause inflammation, either directly around the nerve in the nasal lining or within the nerve itself. When the nerve is either surrounded by inflammatory molecules or has a lot of inflammation within the nerve cell body, it cannot function correctly — and that is what causes the loss or dysfunction of smell. And it can happen to anyone: young and old, healthy and sick.

How did your study investigate olfactory loss?

In my practice, I see patients who have smell dysfunction. But I’m also a sinus and skull base surgeon, so I have a whole host of other patients with sinus problems and skull-based tumors who don’t have an olfactory loss. So we did a case-control study to compare the incidence of cranial neuropathies — conditions in which nerves in the brain or brain stem are damaged — in two patient groups. Ninety-one patients had post-viral olfactory loss and 100 were controls; and they were matched as closely as possible for age and gender.

We also looked at family history of neurologic diseases — such as Alzheimer’s disease, Parkinson’s disease and stroke.

What did you find?

Patients with post-viral olfactory loss had six-times higher odds of having other cranial neuropathies than the control group — with an incidence rate of other cranial nerve deficits of 11% and 2%, respectively. Family history of neurologic diseases was associated with more than two-fold greater odds of having a cranial nerve deficit. Although we had a small sample size, the striking difference between the groups implies that it is worthwhile to research this with a larger population.

Our findings suggest that patients experiencing these pathologies may have inherent vulnerabilities to neural damage or decreased ability of nerve recovery — something beyond known risk factors like age, body mass index, co-morbidities and the duration of the loss before intervention. For example, there may be a genetic predisposition, but that is just an untested theory at this point.

How does this work pertain to COVID-19?

Smell loss can be one of the earliest signs of a COVID-19 infection. It can sometimes be the only sign. Or it can present after other symptoms. Although it may not affect every patient with COVID-19, loss of smell and taste is definitely associated with the disease. In some countries, including France, they’ve used this as a triage mechanism. People need to know that these symptoms can be related to the COVID-19 disease process so they aren’t going about their lives like normal and spreading the virus.

The pandemic also might impact how we treat patients with olfactory dysfunction in general. When someone has a viral-induced inflammation of the nerve, we sometimes treat it with steroids to decrease the inflammation. But treating COVID-19 patients with steroids might be a bad idea because of its effect on the inflammatory processes going on in their heart and lungs.

What advice do you have for people who have an impaired sense of smell?  

First, if you lose your sense of smell and it isn’t coming back after all the other symptoms have gone away, seek care as soon as possible. If you wait too long, there is much less that we can do to help you. Interventions, including olfactory training and medications, are more effective when you are treated early.

Second, if you lose your sense of smell or taste during this pandemic and you don’t have any other symptoms, contact your doctor. The doctor can decide whether you need to be tested for COVID-19 or whether you need to self-isolate to avoid being a vector of the virus in your family or community.

Image by carles

This is a reposting of my Scope story, courtesy of Stanford School of Medicine.

Author: Jennifer Huber

As a Ph.D. physicist and research scientist at the Lawrence Berkeley National Laboratory, I gained extensive experience in medical imaging and technical writing. Now, I am a full-time freelance science writer and science-writing instructor. I've lived in the San Francisco Bay Area most of my life and I frequently enjoy the eclectic cultural, culinary and outdoor activities available in the area.

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