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The Olfactory System

The sense of smell and taste (gustatory system) are often referred together as the chemosensory system; they provide information to the brain about the chemical composition of substances through a process called transduction. Humans possess a main olfactory system, which detects airborne substances, and an accessory system which senses fluid-phase stimuli.

Olfaction occurs when odor molecules (odorants) reach the nasal cavity during inhalation or through retro-nasal olfaction which occurs during chewing when the tongue pushes air into the nasopharynx. The middle turbinate and superior turbinate are erectile organs within the nasal cavity that provide variable resistance to dissect nasal airflow and allow a small portion of our nasal airflow to lose velocity and allow for diffusion of odorants. Inside the nasal cavity, odorants dissolve through the mucociliary blanket and in some cases are metabolized by enzymes secreted by the olfactory neuroepithelium.

COVID-19 and Olfaction

In addition to the 12% of the U.S. population that reports some level of impairment in sense of taste or smell, patients with COVID-19 are 27 times more likely to develop smell impairment compared to non COVID-19 infected individuals. Up to 85% percent of patients with COVID-19 develop some smell or taste dysfunction, and 25% of patients may report smell dysfunction as their first symptom.

Smell Training Protocol

In a recent trial by Hummel et al, higher concentrations of odorants led to improved smell compared to patients receiving lower concentrations of odorants. Current recommendations include deliberate smelling of essential oils placed on a cotton tipped applicator and smelled vigorously for 20 seconds at a time at least twice a day for 3 months. Essential oils are available for purchase online in smell regeneration kits; rose, lemon, clove, and eucalyptus are commonly used.

Summary

Olfactory loss is a common affliction, affecting approximately 12% of the population and up to 85% of patients suffering from COVID-19. Fortunately, the neuroepithelium is supported by a robust layer of stem cells and is capable of regenerating from many insults, including post-infectious damage, particularly if rehabilitative therapy is initiated within the first year. Patients who recover from COVID-19 should be queried for any residual symptoms. For those who report olfactory or taste dysfunction, these patients should be referred to otolaryngology for further evaluation via nasal endoscopy and/or imaging to rule out sinonasal, posttraumatic, malignant, or neurodegenerative causes of olfaction. For those in which other causes have been ruled out and presumed to have postinfectious olfactory loss, therapy with safety counseling, olfactory training, and adjuvant medical therapies should be implemented.

 

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