Efficacy of Bimodal Visual-Olfactory Training in Participants With COVID-19 Resultant Hyposmia or Anosmia Using Participant-Preferred Scents

Over 200,000 people visit physicians yearly for taste and smell disorders and given the well-documented prevalence of olfactory dysfunction in COVID-19 infection, there is likely to be an increased need to address these concerns. The loss of the sense of smell has been shown to be linked to decreased quality of life, depression, decreased enjoyment of the flavor of foods, and may even be a contributing factor in the physiologic anorexia of aging.

Some of the most common causes of olfactory dysfunction include post-infectious, post-traumatic, and neurodegenerative. Of these, post-viral olfactory dysfunction is the leading cause, accounting for an estimated 18.6 to 42.5% of individuals with olfactory dysfunction. Respiratory viruses found to be responsible for olfactory loss include common respiratory viruses including rhinovirus, coronavirus, parainfluenza virus, adenovirus, and influenza virus. It is then no surprise, that olfactory dysfunction is a defining symptom of COVID-19 infection. Estimates for the prevalence of smell dysfunction in COVID-19 infection vary. In a cross-sectional survey of 59 patients with COVID-19, 34% (20/59) self-reported a smell and/or taste disorder. In a multi-center European study, 85.6% (357/417) of cases with confirmed COVID-19 experienced olfactory dysfunction. Only an estimated 44% of these patients experienced recovery of olfaction after 2 weeks of convalescence from COVID-19 infection. Although it is impossible to know the long-term recovery rates of this newly emerging pathogen, as the total number of confirmed COVID-19 cases approaches 19 million in the United States, unpublished data generated by Amish Mustafa Khan in Dr. Jay F. Piccirillo's lab at Washington University estimates nearly 250,000 to 500,000 new cases of chronic olfactory dysfunction.

There is no gold standard set of guidelines for the diagnosis and treatment of post-viral hyposmia or anosmia. Most evidence for pharmacological interventions is weak, with very few controlled studies that account for spontaneous improvement overtime. Moreover, treatments that are effective for sino-nasal disease such as topical corticosteroids are not effective for sensorineural post-viral olfactory loss. A systemic review of post-viral olfactory dysfunction studied eight commonly utilized pharmacological treatments: Oral corticosteroids, local corticosteroids, zinc sulfate, alpha-lipoic acid, caroverine, Vitamin A, Gingko Bilboa, Minocycle. Improvement was noted for study participants receiving oral corticosteroids, local corticosteroids, alpha lipoid acid, and caroverine. However, these studies were of poor quality, and the authors conclude that there is no strong evidence supporting the use of any pharmacological intervention for the treatment of post-viral olfactory dysfunction.

One proposed treatment shown to be beneficial for a wide variety of etiologies of olfactory dysfunction, including post-viral upper respiratory infection, is olfactory training. The theoretical basis for olfactory training emerges from multiple experimental and clinical studies suggesting that the olfactory pathway has neuroplasticity to recover, both peripherally, due to the regenerative capacity of olfactory receptor cells, and centrally. In a study using fMRI after olfactory training, there were increased functional connections in olfactory areas such as the anterior entorhinal cortex, inferior prefrontal gyrus, and the primary somatosensory cortex, suggesting that the olfactory pathways are capable of reorganization with training. In another study, increased exposure by anosmic participants to androstenone resulted in an increase in amplitude of the olfactory evoked potential and the olfactory event-related potential, suggesting that that the peripheral olfactory receptor cells are also neuroplastic, likely due to an increase in expression of olfactory neuron receptors in response to training.

The investigators believe that patients experiencing olfactory dysfunction secondary to COVID-19 are especially good candidates for olfactory training for two reasons. Firstly, the pathophysiology of COVID-19 olfactory dysfunction is mediated through damage to the peripheral olfactory receptor cells located in the nasal epithelium lining the nasal cavity and central pathways via neuro-invasion through the olfactory pathway. This suggests that interventions most likely to be efficacious in this patient population target both central and peripheral pathways, as olfactory training does. Secondly, relative to other causes of olfactory dysfunction, post-viral olfactory dysfunction more commonly presents with hyposmia, rather than anosmia. Residual olfactory function is an important prognosticator that improves the likelihood of improvement. Furthermore, patients with post-viral olfactory dysfunction more commonly present with concurrent dysosmia than other common causes of olfactory dysfunction. It is likely that dysomia may be a result of disordered axonal regeneration. This further suggests that patients with post-viral olfactory loss are most likely to benefit from olfactory training.