According to researchers, the most common reason for chronic loss of smell (anosmia) is a virus, making up 19%-43% of cases. It’s anticipated that the COVID-19 pandemic will leave in its wake over 700,000 new anosmia cases in the U.S. To address post-viral smell loss, clinicians have been recommending olfactory training (OT), but results of a new clinical trial conducted by Washington University School of Medicine in St. Louis indicates that olfactory training for smell loss may not be effective on its own.*

The nose smells what the eyes see

OT can involve more than just the nose. In the journal JAMA Otolaryngology–Head & Neck Surgery, the study authors explain that the olfactory pathway can recover with training after injury through neuroplasticity, or the brain’s ability to rewire itself. 

A strong connection between our senses of smell and sight has also been supported by numerous studies, and data suggests that OT with an added visual component can promote rewiring through a process called sensory transfer.

The objective of the recent study was to compare and contrast the effectiveness of dual visual-OT versus conventional single-mode OT. Studies have also shown that modifying OT to enhance patient adherence can increase OT effectiveness, so the researchers also evaluated the effecacy of patient-selected versus clinician-assigned scents.

What the researchers did

The randomized, single-blinded trial (only the researchers knew who was receiving the treatment) involved 275 adults with OL.  that included a control group with no smell loss for comparison.

Average time of smell loss was six months, with most reporting poor or absent smell.

• Average participant age was 41 years, with 86% women.
• 68% also had parosmia (distorted smell) or phantosmia (smelling a non-existent odor).

Smell loss was defined by the University of Pennsylvania Smell Identification Test (UPSIT) as a score of less than 34 for men and less than 35 for women (on a 40-point scale) for three  months or longer. Average UPSIT score was 24.9 at enrollment. 

Participants were randomly divided into four treatment groups for comparison against the control group.

• Visual-OT with patient-selected scents
• Conventional OT with clinician-assigned scents
• Visual-OT with clinician-assigned scents
• Conventional OT with patient-selected scents

OT consisted of participants sniffing four essential oils for 15 seconds, followed by 30 seconds rest, for three months. Assigned scents were eucalyptus, lemon, rose, and clove. In the visual-OT groups, participants were shown images of what they were smelling.

What they found

Lead investigator Jay Piccirillo, M.D. explained that:

• No treatment group saw significant improvement in smell.
• There was no significant difference between treatment groups.
• There was no real difference between treatment groups and controls.

Among the four groups, those receiving dual visual-OT with self-selected scents saw the greatest improvement (an increase of four or more points) in their UPSIT scores, with 53% of that group showing improvement versus 24% of the control group. The actual

Meanwhile, participants with the greatest self-reported improvement were those in the visual-OT/clinician-assigned scent group. Self-reported improvement included:

• 46% of visual-OT/clinician-assigned 
• 37% of conventional OT/clinician-assigned 
• 35% of visual-OT/patient-selected 
• 32% of conventional OT/patient-selected 
• 19% of control participants

What it means

The researchers stated that, despite the findings, the amount of variation in both UPSIT and self-reporting results makes it difficult to draw any definitive conclusions, as a person can have a slightly improved test score but still have trouble smelling. This highlights an area of difficulty in smell loss research. An additional limitation is that fewer than half the participants adhered to the study protocols, even when allowed to self-select scents. 

Still, the researchers said, dual visual-OT with self-selected scents is a non-invasive, low-cost therapeutic that may be beneficial for post-COVID smell loss as part of an overall treatment plan.

*Kneisel, K. (2022, December 29). COVID Smell Loss: ‘Retraining’ Mostly Flops in Trial. MedPage Today. https://www.medpagetoday.com/infectiousdisease/longcovid/102438