The Effect of Olfactory Training Using Korean Version Odorants: A Preliminary Study

Olfactory training - Thirteen years of research reviewed

The sense of smell is interrelated with psychosocial functioning. Olfactory disorders often decrease quality of life but treatment options for people with olfactory loss are limited. Additionally, olfactory loss accompanies and precedes psychiatric and neurodegenerative diseases. Regular, systematic exposure to a set of odors, i.e., olfactory training (OT) has been offered for rehabilitation of the sense of smell in clinical practice. As signals from the olfactory bulb are directly projected to the limbic system it has been also debated whether OT might benefit psychological functioning, i.e., mitigate cognitive deterioration or improve emotional processing. In this review we synthesize key findings on OT utility in the clinical practice and highlight the molecular, cellular, and neuroanatomical changes accompanying olfactory recovery in people with smell loss as well as in experimental animal models. We discuss how OT and its modifications have been used in interventions aiming to support cognitive functions and improve well-being. We delineate main methodological challenges in research on OT and suggest areas requiring further scientific attention.

Clinical Features of Olfactory Dysfunction in COVID-19 Patients

BACKGROUND

The aim of this study was to evaluate the subjective and objective olfactory function in coronavirus disease 2019 (COVID-19) patients and the effect of olfactory training.

METHODS

A prospective cohort study was performed in 53 patients who recovered from COVID-19 and visited our tertiary hospital. Subjective olfactory function was evaluated using the 11-point Likert scale (0-10) and the Korean version of the Questionnaire of Olfactory Disorders (QOD). Objective olfactory function was evaluated using Cross-Cultural Smell Identification Test (CC-SIT). Confirmed patients were followed up after 2 months of olfactory training.

RESULTS

The median, interquartile range (Q1-Q3) score of subjective olfactory function significantly deteriorated in patients with olfactory dysfunction (OD) than in those without OD, even after 3 months of onset (11-point Likert scale, 8, 6-9 vs. 10, 10-10; short version of QOD-negative statements, 19, 16-21 vs. 21, 21-21; QOD-visual analogue scale, 7, 1-13 vs. 0, 0-0; all P < 0.001). However, the objective olfactory function was not significantly different between the two groups (median, interquartile range; 11, 9-11 vs. 11, 9-11, P = 0.887). The percentage of patients with objective hyposmia (CC-SIT ≤ 10) was also not significantly different (47.4% vs. 40%, P = 0.762). OD in COVID-19 was normalized after 2 months of olfactory training in 70% of patients even after 3 months of olfactory impairment.

CONCLUSION

Although subjective olfactory function is significantly decreased in the OD group, the objective olfactory function was not significantly different. Moreover, olfactory training is effective in COVID-19 patients with OD.

Clinical Olfactory Working Group consensus statement on the treatment of postinfectious olfactory dysfunction

BACKGROUND

Respiratory tract viruses are the second most common cause of olfactory dysfunction. As we learn more about the effects of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), with the recognition that olfactory dysfunction is a key symptom of this disease process, there is a greater need than ever for evidence-based management of postinfectious olfactory dysfunction (PIOD).

OBJECTIVE

Our aim was to provide an evidence-based practical guide to the management of PIOD (including post-coronavirus 2019 cases) for both primary care practitioners and hospital specialists.

METHODS

A systematic review of the treatment options available for the management of PIOD was performed. The written systematic review was then circulated among the members of the Clinical Olfactory Working Group for their perusal before roundtable expert discussion of the treatment options. The group also undertook a survey to determine their current clinical practice with regard to treatment of PIOD.

RESULTS

The search resulted in 467 citations, of which 107 articles were fully reviewed and analyzed for eligibility; 40 citations fulfilled the inclusion criteria, 11 of which were randomized controlled trials. In total, 15 of the articles specifically looked at PIOD whereas the other 25 included other etiologies for olfactory dysfunction.

CONCLUSIONS

The Clinical Olfactory Working Group members made an overwhelming recommendation for olfactory training; none recommended monocycline antibiotics. The diagnostic role of oral steroids was discussed; some group members were in favor of vitamin A drops. Further research is needed to confirm the place of other therapeutic options.

Differences in Mechanisms of Steroid Therapy and Olfactory Training for Olfactory Loss in Mice

Objective

Steroid therapy and olfactory training are common treatments for olfactory loss. Systemic steroid treatment is the most effective approach for treating sinonasal olfactory loss. Olfactory training is typically effective for treating sensorineural olfactory loss. However, the differences in mechanisms of steroid therapy and olfactory training for olfactory dysfunction are unclear. The aim of this study was thus to evaluate the differences in mechanisms of olfactory training and steroid therapy.

Subjects and Methods

Mice in each group were administered 3-methylindole at a dose of 300 mg/kg. Olfactory function was evaluated with a food-finding test once a week. The olfactory neuroepithelium was harvested for histologic examination and protein analysis. Subsequently, data analysis, gene ontology and pathway analysis, quantitative real-time polymerase chain reaction of mRNA, and western blot analysis were conducted.

Results

Mice were divided into four groups according to treatment. Control, anosmia, training, and steroid groups resumed food-finding. MMP27, CCL22 and IL18rap mRNA expression were significantly increased in the training group compared to that in the steroid group. IL1R2 mRNA expression was significantly higher in the olfactory neuroepithelium of steroid-treated mice than in that of the training group mice.

Conclusions

Steroid therapy improved olfactory function via anti-inflammatory effects, unlike olfactory training which involved cell regeneration and tissue remodeling. Protein and gene analyses revealed that steroid therapy and olfactory training are underpinned by distinct mechanisms. Selection of the most appropriate treatment will be dependent on the cause of olfactory loss.

Olfactory Ensheathing Cells Mediate Neuroplastic Mechanisms After Olfactory Training in Mouse Model

Background

Several studies have reported beneficial effects of olfactory training (OT) on the olfactory nervous system. However, the mechanisms underlying the regeneration of the olfactory system induced by OT are still under investigation.

Objectives

To determine the key mechanisms involved in the olfactory system recovery and to assess the neuroplastic effects of OT.

Methods

Thirty healthy female C57BL/6 mice were randomly allocated to 4 groups: control, n = 6; anosmia (no treatment), n = 8; OT, n = 8; and steroid treatment; n = 8. Except for the control group, mice were administered 3-methylindole. Anosmia was assessed using a food-finding test (FFT). The olfactory neuroepithelium was for histological examinations, gene ontology with pathway analyses, RNA, and protein studies.

Results

FFT was significantly reduced at 3 weeks in the OT mice versus steroids (78.27 s vs 156.83 s, P < .008) and controls (78.27 s vs 13.14 s, P < .003), although final outcome in the FFT was similar in these groups. Expression of olfactory and neurogenesis marker was higher in the olfactory neuroepithelium of the OT group than in the anosmia group without treatment. The mechanisms underlying olfactory regeneration might be related to early olfactory receptor stimulation, followed by neurotrophic factor stimulation of neuronal plasticity.

Conclusion

OT can improve olfactory function and accelerate olfactory recovery. The mechanisms underlying olfactory regeneration might be related to an initial stimulation of olfactory receptors followed by neurogenesis. Olfactory ensheathing cells might play an important role in olfactory regeneration following OT, based on the observed changes in messenger ribonucleic acid (mRNA) and protein expression, as well as the findings of the gene analysis.

The neuroplastic effect of olfactory training to the recovery of olfactory system in mouse model

Background

Several studies have reported the benefits of olfactory training (OT) in the olfactory nervous system of mouse models. Therefore, in this study we performed next‐generation sequencing to evaluate the effects of OT on mRNA sequencing in the olfactory area.

Methods

Mice in each group were administered 300 mg of 3‐methylindole per kilogram of mouse weight. The olfactory function was evaluated by a food‐finding test once a week. The olfactory neuroepithelium was harvested for histologic examination and protein analysis. Subsequently, data analysis, gene ontology and pathway analysis, quantitative real‐time polymerase chain reaction of mRNA, and Western blot analysis were conducted.

Results

Mice were divided into 4 groups according to treatment. Control, anosmia, training, and steroid group mice resumed food finding. Olfactory Maker Protein, olfr1507, ADCY3, and GNAL mRNA expression was higher in the olfactory neuroepithelium of OT than anosmia group mice. In total, 26,364 mRNAs were analyzed. Comparison of the results of OT vs anosmia revealed that ADCY8,10, GFAP, NGF, NGFR, GFAP, and BDNF mRNAs were upregulated in the gene ontology.

Conclusion

OT improved olfactory function, as indicated by the food‐finding test. OT improved the olfactory recovery time to stimulate olfactory nerve regeneration. OT may initially stimulate the olfactory receptor, followed by neurogenesis. Steroid therapy and OT operated under completely different mechanisms in the upregulated gene study. These results indicate that OT may be one of the future modalities for treating olfactory impairment.