Why does COVID-19 impair your sense of smell?

BIRMINGHAM, Ala. — There are still many unknowns regarding COVID-19, but one common occurrence among those infected is smell loss. There are many different events that can trigger smell loss, and University of Alabama at Birmingham Assistant Professor Jessica Grayson, M.D., provides insight as to why this might be happening.

Hyposmia or anosmia, a decreased or complete loss of smell, has been widely reported in patients with COVID-19, often as a symptom noticed early on prior to other symptoms or even as the only symptom present in an otherwise asymptomatic patient.

“There have been a significant number of people who have been treated or tested at UAB who have had loss of smell in the setting of their COVID diagnosis,” Grayson said. “However, given the isolation protocols, most of these have only been diagnosed subjectively.”

Grayson, an assistant professor with the UAB Department of Otolaryngology, says post-viral smell loss is the most common reason for loss of smell. Approximately 40 percent of anosmia is due to post-viral causes. More than 200 different viruses can cause alterations in smell and taste due to temporary or long-term injury to the neural structures. 

“If the nose is congested or full of polyps, then the odorant particulates cannot reach the olfactory fibers, which sense the smell and communicate with the brain,” she said. “In other instances, the nose looks relatively normal, but there is local inflammation in the olfactory mucosa that leads to adverse effects on smell.”

Grayson adds that, apart from COVID-19, some people are born with hyposmia or anosmia. These disorders are typically diagnosed once the person is old enough to communicate that they cannot smell, typically during adolescence. In people suffering with obesity, there is also an increased risk of potential smell loss due to benign intracranial hypertension, which causes thinning of the skull base particularly in the region of the cribriform plate because it is already the thinnest region where the olfactory nerves penetrate into the nasal cavity.

“Patients with BIH have also been found to suffer from alterations in smell,” she said. “Trauma, particularly high-velocity head trauma, can cause loss of smell in patients either due to shearing of the olfactory nerves from coup and contrecoup injuries to the head or from direct injury to the olfactory region of the skull base.”

Grayson adds that surgery in the region of the olfactory mucosa that does not preserve this mucosa can also cause smell loss. Tumors arising from the olfactory mucosa, olfactory fibers and anterior cranial fossa can also cause smell loss. Neurodegenerative diseases and inflammatory conditions, like Parkinson’s disease and Sjogrens disease, can also negatively affect smell.

Retraining the nose

Although losing the sense of smell can be quite jarring, Grayson says there are ways to regain that lost sense.

“Patients with post-viral smell loss have roughly a 60-80 percent chance of regaining some of their smell function at one year,” Grayson said. “However, people with traumatic injuries often do not regain smell.”

Smell retraining consists of exposure to certain scents in a repeated nature over many weeks. Patients smell four odor categories — flowery, fruity, aromatic and resinous — every day for 12 weeks and potentially up to six months.

“Studies have shown improvement in smell when patients utilize smell retraining, and some studies have suggested changing the four scents at 12-week intervals,” she said. “When patients performing smell retraining were compared to patients who were not, there were more patients who had improvement in their sense of smell.”

Keen on smelling

Grayson says sense of smell is important for flavoring of food as well as safety reasons.

“The ability to smell is important, because if something is burning, the house is on fire, food has gone bad, a gas leak, brakes burning in your car, you need to be able to have that sense,” she said. “Patients who cannot smell have to have protections in place to prevent bad things from happening like checking smoke detectors monthly, following expiration dates strictly or having someone else in the home who can smell, or having natural gas detectors.”

She says, if you begin to experience a lost sense of smell or taste, COVID-19 testing may be an option.

“Right now, if you lose your sense of smell and taste, you can look for COVID testing, if available,” she said. “However, you can also isolate for 10 days, or 72 hours symptom-free, and follow CDC guidelines.”

Grayson adds that those concerned can reach out to the department, which can send a smell test to complete.

“If we need to do smell retraining based on loss, our team will also get you set up for a follow-up appointment.” 

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