Losing the sense of smell is listed as one of the most common symptoms of COVID-19 infection by the Centers for Disease Control and Prevention (CDC). Many people who contract the virus seem to share this experience of losing smell, yet they differ in the time it takes to regain their precious sense, ranging from days to weeks, numerous months, or, in some cases, never.

Kristin Seiberling, MD, an otolaryngologist at Loma Linda University Health, outlines possible causes behind and treatments for smell loss, called anosmia, after a COVID-19 infection.

Post-viral anosmia isn’t a new phenomenon or unique to COVID-19, she says. Many other viruses can cause the condition, too, including other types of coronaviruses, rhinoviruses, and influenza viruses. Moreover, she says the mechanisms driving COVID-19 to induce smell loss are likely no different from other upper respiratory viruses’ methods of causing it.

Post-viral Smell Loss: What It Is and Why It Happens

First, Seiberling says it is important to note that because taste is heavily reliant on smell, many people who have lost the ability to smell may also feel like they’ve lost their sense of taste. Taste is, after all, 80 percent smell, she says. In other words, our ability to taste the flavor of food, like distinguishing a watermelon from a cherry, depends on our ability to smell it. The only “tastes” left are basic ones that our tongue delivers directly to our brain: sweet, salty, sour, and bitter.

Losing smell after a viral infection can happen when odorants have a hard time reaching the nose and binding to the appropriate receptors within it, Seiberling says, adding that several different factors could be responsible for this kind of conductive post-viral anosmia, including:

• Rhinitis or mucosal edema: Swelling inside the nose or a swollen runny nose can lead to smell loss due to the lack of the odors' ability to get into the nose.

• Neuroepithelial lining damage: The virus may attack tissue made up of sensory cells that receive external stimuli in the ear, nose, and tongue.

• Olfactory nerve damage: Olfactory nerves in the nose are particular sensory nerves for the sense of smell that may come under attack by the virus.

Anosmia induced by the latter two, epithelial or nerve damage, can take months to repair and for the sense of smell to return, though some patients’ smell may never return.

In a small number of cases, the virus may also damage the olfactory system in a retrograde fashion, affecting higher-order neurons and leading patients to experience:

• dyssomnia—a disordered smell perception

• phantosmia—a sort of “olfactory hallucination,” or smelling odors that aren’t present

Olfactory Training to Smell and Taste Again

Doctors will suggest a variety of treatments for patients, depending on the identified cause of the post-viral anosmia. For example, suppose the smell loss hails from rhinitis or mucosal edema. In that case, Seiberling says patients may opt for a nasal steroids spray, a short course of oral steroids, and saltwater rinses in the nasal cavity.

If anosmia persists even after the viral infection is over, patients might begin olfactory sensory retraining, Seiberling says. Patients choose four odors to smell (like rose, eucalyptus, lemon, and clove) several times a day for three months. After three months, they switch to another set of four odors (perhaps menthol, thyme, tangerine, and jasmine).

Seiberling suggests the following guidelines for each three-month cycle of smelling an odor set:

1. Choose one odor and smell it for approximately fifteen seconds while trying to remember what it once smelled like

2. Rest for about ten seconds

3. Smell the next odor for approximately fifteen seconds

4. Rest for about ten seconds

5. Repeat until you have sampled all four odors

Essentially a labor of repetition, persistence, and duration, Seiberling says sensory training helps the neural lining and olfactory nerves identify and adjust to smells again as they regenerate after a viral attack.

Seiberling says patients might repeat this process for up to a year before gradually regaining their sense of smell. She suggests patients purchase essential oils with an array of odors for their sensory training. This allows for ease of use and the option to cap containers to preserve the odors' strength.

For the temporary or permanent duration of anosmia, Seiberling suggests following a set of safety measures:

• Ensure smoke detectors have fresh batteries

• Double-check stoves and ovens are turned off when not in use

• Let others determine if foods have expired

This article was originally published on the Loma Linda University Health news site