Effects of Olfactory Training in Patients With Postinfectious Olfactory Dysfunction

The efficacy of olfactory training in improving olfactory function: a meta-analysis

PURPOSE

Study the efficacy of olfactory training in smell recovery.

METHODS

An extensive search was performed through different databases in order to find articles analyzing the efficacy of olfactory training as a treatment for olfactory dysfunction. Methodological quality of primary studies within the final sample was assessed following PRISMA guidelines. Standardized mean differences in pre-post olfactory training groups, and also in experimental-control and pre-follow up if possible, were computed by Hedges' g effect size statistic. Each effect size was weighted by its inverse variance.

RESULTS

Final sample was composed of 36 articles (45 pre-post effect sizes). Contrasts were performed separately for odor identification, odor discrimination, odor threshold and general olfactory function. Moderate to large and heterogeneous effect was obtained for olfactory function (g = 0.755, k = 45, SE = 0.093, CI 95% = [0.572, 0.937]), different moderators had a significant effects, such as, training duration, age and anosmia diagnosis.

CONCLUSION

Olfactory training was found to have a positive and significant effect on rehabilitating the olfactory function.

Olfactory training with four and eight odors: comparison with clinical testing and olfactory bulb volumetrics

PURPOSE

Post-infectious olfactory dysfunction (PIOD) is one of the most common causes of olfactory impairment but has limited treatment options. Recently, olfactory training (OT) has been considered an effective treatment method; however, several questions have arisen regarding its optimal scheme. The aim of this study was to assess whether an OT scheme with 8 odors is more effective than the classic OT scheme with 4 odors by comparing psychophysical test results and olfactory bulb (OB) volumetrics.

METHODS

In this prospective cohort study, 72 patients with PIOD were included. The patients followed either the classic 4-odor OT scheme (COT; n = 34 patients) or an extended 8-odor scheme (EOT; n = 38 patients) for 16 weeks. All patients underwent olfactory testing with a Sniffin'Sticks battery test at 0, 8, and 16 weeks. Of the patients, 38 underwent brain magnetic resonance imaging for OB volumetric assessment before and after treatment.

RESULTS

The comparison of the olfactory test results did not show any significant difference between the two study groups, in agreement with the OB volumetrics. The convex OB showed better test results than the non-convex OB, with significantly better improvement after treatment regardless of OT type. The EOT group presented significantly better adherence than the COT group.

CONCLUSION

The number of odors did not appear to play a significant role in the effect of the OT. However, the training scheme with more than four odors showed better adherence among the patients in a long-term treatment plan. The shape of the OB may have prognostic value in clinical assessment and warrants further investigation.

Comparative Analysis of Olfactory and Gustatory Function of Patients With COVID-19 Olfactory Dysfunction and Non-COVID-19 Postinfectious Olfactory Dysfunction

BACKGROUND

Coronavirus disease 2019 (COVID-19) is known to have a high incidence of loss of smell and taste. However, studies in the early stages of the COVID-19 pandemic have evaluated these symptoms using subjective surveys and simple olfactory tests only. Hence, we compared the olfactory and gustatory characteristics of patient groups with COVID-19 olfactory dysfunction (C19OD) and non-COVID-19 postinfectious olfactory dysfunction (PIOD) using an objective olfactory test and evaluated the significance of olfactory training in both patient groups.

METHODS

We retrospectively analyzed the medical records of 14 patients with a decreased sense of smell after having positive COVID-19 polymerase chain reaction results, and 56 patients with PIOD with no history of confirmed COVID-19. Participants were evaluated using the Korean version of the Sniffin' stick (KVSS) II, and chemical gustometry and olfactory training was assessed during their first visit. Olfactory training was then re-evaluated after an average of 8 (± 6) weeks.

RESULTS

The average age of participants in the C19OD group was lower than in those in the non-COVID-19 PIOD group. The proportion of men in the C19OD group was higher than in the non-COVID-19 PIOD group. At baseline assessment, the C19OD group had better olfactory and gustatory functions. After olfactory training, the non-COVID-19 PIOD patient group showed a significant increase in all KVSS II Total, T, D, and I scores, but there was a non-significant increase in all scores in the C19OD group.

CONCLUSION

The C19OD group had better olfactory and gustatory function than the non-COVID-19 PIOD group at the initial assessment. After olfactory training, there was an increase in olfactory function test scores in both groups. Olfactory training may be helpful in C19OD, as in non-COVID-19 PIOD.

The Olfactory System: Basic Anatomy and Physiology for General Otorhinolaryngologists

Olfaction is one of the five basic human senses, and it is known to be one of the most primitive senses. The sense of olfaction may have been critical for human survival in prehistoric society, and although many believe its importance has diminished over time, it continues to have an impact on human interaction, bonding, and propagation of the species. Even if we are unaware of it, the sense of smell greatly affects our lives and is closely related to overall quality of life and health. Nonetheless, olfaction has been neglected from a scientific perspective compared to other senses. However, olfaction has recently received substantial attention since the loss of smell and taste has been noted as a key symptom of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. Studies investigating olfaction loss in association with coronavirus disease 2019 (COVID-19) have revealed that olfactory dysfunction can be both conductive and sensorineural, possibly causing structural changes in the brain. Olfactory training is an effective treatment for olfactory dysfunction, suggesting the reorganization of neural associations. A reduced ability to smell may also alert suspicion for neurodegenerative or psychiatric disorders. Here, we summarize the basic knowledge that we, as otorhinolaryngologists, should have about the sense of smell and the peripheral and central olfactory pathways for managing and helping patients with olfactory dysfunction.

Post-viral olfactory loss and parosmia

The emergence of SARS-CoV-2 has brought olfactory dysfunction to the forefront of public awareness, because up to half of infected individuals could develop olfactory dysfunction. Loss of smell-which can be partial or total-in itself is debilitating, but the distortion of sense of smell (parosmia) that can occur as a consequence of a viral upper respiratory tract infection (either alongside a reduction in sense of smell or as a solo symptom) can be very distressing for patients. Incidence of olfactory loss after SARS-CoV-2 infection has been estimated by meta-analysis to be around 50%, with more than one in three who will subsequently report parosmia. While early loss of sense of smell is thought to be due to infection of the supporting cells of the olfactory epithelium, the underlying mechanisms of persistant loss and parosmia remain less clear. Depletion of olfactory sensory neurones, chronic inflammatory infiltrates, and downregulation of receptor expression are thought to contribute. There are few effective therapeutic options, so support and olfactory training are essential. Further research is required before strong recommendations can be made to support treatment with steroids, supplements, or interventions applied topically or injected into the olfactory epithelium in terms of improving recovery of quantitative olfactory function. It is not yet known whether these treatments will also achieve comparable improvements in parosmia. This article aims to contextualise parosmia in the setting of post-viral olfactory dysfunction, explore some of the putative molecular mechanisms, and review some of the treatment options available.

Liu D, A. Müller C, A. Stuck B, Welge-Lüssen A, Hähner A. Olfactory Dysfunction: Etiology, Diagnosis, and Treatment

BACKGROUND

Disorders of the sense of smell have received greater attention because of the frequency with which they occur as a symptom of SARS-CoV-2 infection. Olfactory dysfunction can lead to profound reduction in quality of life and may arise from many different causes.

METHODS

A selective literature review was conducted with consideration of the current version of the guideline issued by the Association of the Scientific Medical Societies in Germany.

RESULTS

The cornerstones of diagnosis are the relevant medical history and psychophysical testing of olfactory function using standardized validated tests. Modern treatment strategies are oriented on the cause of the dysfunction. While treatment of the underlying inflammation takes precedence in patients with sinunasal dysosmia, olfactory training is the primary treatment option for other forms of the disorder. The prognosis is determined not only by the cause of the olfactory dysfunction and the patient's age, but also by the olfactory performance as measured at the time of diagnosis.

CONCLUSION

Options for the treatment of olfactory dysfunction are available but limited, depending on the cause. It is therefore important to carry out a detailed diagnostic work-up and keep the patient informed of the expected course and prognosis.

Olfactory training assists in olfactory recovery after sinonasal surgery

Objectives

Olfactory dysfunction is a common complaint in patients with chronic rhinosinusitis (CRS). The study aimed to evaluate the effectiveness of olfactory training (OT) in patients with CRS after sinonasal surgery.

Methods

We enrolled 111 patients with CRS who underwent sinonasal surgery. Prior to surgery and 3 months after starting OT, the participants were assessed by both an olfactory function test and endoscopy. The Korean version of the Sniffin' Stick II (KVSS-II) was used to perform the olfactory function test. Over the course of 3 months, five odorants were used in OT (rose, lemon, cinnamon, orange, and peach).

Results

Over a 12-week duration, 37% of the participants in the OT group showed clinically relevant increase in olfactory function. The OT group had significantly higher olfactory outcomes for the total KVSS-II and identification scores than the non-OT group between the initial and follow-up assessments. The initial score influenced the degree of olfactory improvement after OT.

Conclusions

Patients with OT exhibited significantly higher total KVSS-II scores compared with non-OT patients following sinonasal surgery; in particular, the odor identification score was different between the two groups. The results of this study demonstrated that a 12-week period of repeated short-term exposure to various odors could be useful in enhancing olfactory activity in patients who underwent sinonasal surgery for the improvement of sensory-neural olfactory impairment.

Level of evidence

2c.

Case Report: Acupuncture is an effective treatment for olfactory dysfunction in the post COVID-19 condition

Olfactory dysfunction in the post COVID-19 condition reported worldwide are refractory for some patients. For this reason, appropriate treatment is desired. In this article, we describe two cases of olfactory dysfunction in the post COVID-19 condition that was improved by traditional acupuncture treatment. By using the Yingxiang point (LI20), which is said to improve the sense of smell since ancient times, acupuncture treatment was performed 1-2 times a week in two patients about 6 and 7 months after the diagnosis of COVID-19. Acupuncture needles with a body length of 30 mm and a body diameter of 0.16 mm were inserted about 10 mm deep into the skin. We stimulated LI20 of the right and left sides until the patients felt the de qi sensation (acupuncture resonance), and left needles in the points for about 15 min. Immediately after the acupuncture treatment, the symptoms of olfactory dysfunction were alleviated, and the improvement in olfactory dysfunction lasted for 2-4 days. As the number of acupuncture treatments increased, the time until the flareup of olfactory dysfunction was prolonged, and the symptoms tended to decrease. In our experience, the acupuncture treatment was effective in a short period for treating residual olfactory dysfunction of the post COVID-19 condition, suggesting that acupuncture may serve as an adjunct to modern medical treatment, and it may also be a new option for patients who are resistant to Western medical treatment or unable to continue treatment because of side effects. In conclusion, acupuncture may be a new option for patients who are resistant to modern medical treatment or who are unable to continue treatment because of side effects.

Intensive Olfactory Training in Post-COVID-19 Patients: A Multicenter Randomized Clinical Trial

BACKGROUND

Olfactory dysfunction (OD)-including anosmia and hyposmia-is a common symptom of COVID-19. Previous studies have identified olfactory training (OT) as an important treatment for postinfectious OD; however, little is known about its benefits and optimizations after SARS-CoV-2 infection.

OBJECTIVE

This study aimed to assess whether olfactory training performance can be optimized using more fragrances over a shorter period of time in patients with persistent OD after COVID-19. In addition, we determined the presence of other variables related to OD and treatment response in this population.

METHODS

This multicenter randomized clinical trial recruited 80 patients with persistent OD and prior COVID-19 infection for less than 3 months. The patients were divided into 2 groups receiving either 4 or 8 essences over 4 weeks. Subjective assessments and the University of Pennsylvania Smell Identification Test (UPSIT) were performed before and after the treatment.

RESULTS

Significant olfactory improvement was measured subjectively and using the UPSIT in both groups; however, no significant differences between the groups were observed. Additionally, the presence of olfactory fluctuations was associated with higher UPSIT scores.

CONCLUSION

These data suggest that training intensification by increasing the number of essences for 4 weeks does not show superiority over the classical method. Moreover, fluctuant olfaction seems to be related to a higher score on the UPSIT.

International consensus statement on allergy and rhinology: Olfaction

BACKGROUND

The literature regarding clinical olfaction, olfactory loss, and olfactory dysfunction has expanded rapidly over the past two decades, with an exponential rise in the past year. There is substantial variability in the quality of this literature and a need to consolidate and critically review the evidence. It is with that aim that we have gathered experts from around the world to produce this International Consensus on Allergy and Rhinology: Olfaction (ICAR:O).

METHODS

Using previously described methodology, specific topics were developed relating to olfaction. Each topic was assigned a literature review, evidence-based review, or evidence-based review with recommendations format as dictated by available evidence and scope within the ICAR:O document. Following iterative reviews of each topic, the ICAR:O document was integrated and reviewed by all authors for final consensus.

RESULTS

The ICAR:O document reviews nearly 100 separate topics within the realm of olfaction, including diagnosis, epidemiology, disease burden, diagnosis, testing, etiology, treatment, and associated pathologies.

CONCLUSION

This critical review of the existing clinical olfaction literature provides much needed insight and clarity into the evaluation, diagnosis, and treatment of patients with olfactory dysfunction, while also clearly delineating gaps in our knowledge and evidence base that we should investigate further.

Research Progress of Olfactory Nerve Regeneration Mechanism and Olfactory Training

The olfactory nerve (ON) is the only cranial nerve exposed to the external environment. Hence, it is susceptible to damage from head trauma, viral infection, inflammatory stimulation, and chemical toxins, which can lead to olfactory dysfunction. However, compared with all other cranial nerves, the ON is unique due to its inherent ability to regenerate. This characteristic provides a theoretical basis for treatment of olfactory dysfunction. Olfactory training (OT) is one of the main treatments for olfactory dysfunction. It is easy to apply and has few side-effects, and has been shown to be efficacious for patients with olfactory dysfunction of various causes. To further understand the application value of ON regeneration and OT on olfactory dysfunction, we review the research progress on the mechanism of ON regeneration and OT.

Long-Term Persistence of Olfactory and Gustatory Disorders in COVID-19 Patients

Smell and taste disorders are frequent symptoms during acute COVID-19 and may persist long after the resolution of the initial phase. This study aims to estimate the proportion and risk factors for smell and/or taste disorders at the onset of symptoms and their persistence after more than 6 months of follow-up in COVID-19 patients. We analyzed a prospective cohort of COVID-19 patients admitted to our institute in Marseille, France in early 2020. After being discharged from the hospital, patients with smell and/or taste disorders were contacted for a telephone interview. Logistic regression analysis was performed to determine the risk factors for smell and/or taste disorders. A total of 3,737 patients were included, of whom 1,676 reported smell and/or taste disorders at the onset of symptoms. Taste and/or smell disorders were independently associated with being younger and female, a lower likelihood of suffering from diabetes, cardiovascular diseases and cancer, a longer delay between the onset of symptoms and consultation, and non-severe forms of COVID-19 at admission. Of the 605 patients with smell and/or taste disorders who were followed-up, 154 (25.5%) reported the persistence of symptoms for more than 6 months. At the time of follow-up, being female, having a chronic respiratory disease and using angiotensin-converting enzyme inhibitors (ACEis) were factors independently associated with the persistence of smell and/or taste disorders. In conclusion, the long-term persistence of olfactory and gustative disorders is frequent among COVID-19 patients, notably affecting female patients and patients who suffered from chronic respiratory diseases before infection. The role of ACEis needs to be further evaluated in larger numbers of patients.

Olfactory training for Olfactory dysfunction in COVID-19: A promising mitigation amidst looming neurocognitive sequelae of the pandemic

Olfactory dysfunction (OD) is a recognized symptom of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and is independently associated with neurodegenerative disorders. Moreover, the central nervous system manifestations in patients infected with the coronavirus -2019 (COVID-19) have demonstrated cognitive decline and neuropsychiatric manifestations. Hence, OD in COVID -19 necessitates perusal of its' mechanism and available treatment options to avert possible development of neurocognitive sequelae of the pandemic. The article presents a literature review organized from the published information about olfactory training (OT) for OD during COVID-19. The methodology comprised retrieval of available literature from database searches and subsequent scrutinization of relevant information. Inferentially, Injury to the sustentacular cells, possessing angiotensin-converting enzyme 2 (ACE-2) receptors, is an important mechanism causing OD in COVID-19. OD may be prolonged in severe cases of anosmia predisposing to neurodegenerative and cognitive impairment in COVID-19 infection. OT demonstrates an effective treatment for OD based on human and animal-derived evidence through recent studies. It curtails the progression of OD, besides inducing neural rearrangement and changes in functional connectivity in patients receiving OT. Additionally, contemporary reports support that the administration of OT for COVID-induced anosmia is effective and encompasses no significant adverse effects. The present review highlights the prominence of olfactory training as a recommended intervention for OD in COVID-19. This review can guide the clinicians in curbing neurological repercussions of COVID besides enhancing cognitive rehabilitation through olfactory training.

Impact of olfactory dysfunction on quality of life in coronavirus disease 2019 patients: a systematic review

OBJECTIVE

To outline the impact on quality of life in coronavirus disease 2019 patients with olfactory dysfunction.

METHODS

Five databases were searched for articles referring to the impact on quality of life in coronavirus disease 2019 patients with olfactory dysfunction. The search was conducted for the period from November 2019 to April 2021. The search was conducted over one month (May 2021).

RESULTS

Four studies that met the objective were included. Altogether, there were 1045 patients. Various questionnaires were used to assess quality of life. Overall, the quality of life deficit affected 67.7 per cent of patients. Quality of life domains investigated include overall quality of life (four studies), food and taste dysfunction (two studies), mental health (two studies), cognitive function (one study), functional outcome (one study) and safety domains (one study).

CONCLUSION

Quality of life deficit was reported to be 67.7 per cent among coronavirus disease 2019 patients with olfactory dysfunction. The high prevalence of persistent olfactory dysfunction prompts more serious research, as the long-standing consequences of olfactory dysfunction are detrimental.

Olfactory training with Aromastics: olfactory and cognitive effects

Purpose

The olfactory system can be successfully rehabilitated with regular, intermittent stimulation during multiple daily exposures to selected sets of odors, i.e., olfactory training (OT). OT has been repeatedly shown to be an effective tool of olfactory performance enhancement. Recent advancements in studies on OT suggest that its beneficial effects exceed olfaction and extend to specific cognitive tasks. So far, studies on OT provided compelling evidence for its effectiveness, but there is still a need to search for an optimal OT protocol. The present study examined whether increased frequency of OT leads to better outcomes in both olfactory and cognitive domains.

Method

Fifty-five subjects (28 females; M_age = 58.2 ± 11.3 years; 26 patients with impaired olfaction) were randomly assigned to a standard (twice a day) or intense (four times a day) OT. Olfactory and cognitive measurements were taken before and after OT.

Results

OT performed twice a day was more effective in supporting olfactory rehabilitation and interventions targeted to verbal semantic fluency than OT performed four times a day, even more so in subjects with lower baseline scores.

Conclusions

OT is effective in supporting olfactory rehabilitation and interventions targeted to verbal semantic fluency. However, it may be prone to a ceiling effect, being efficient in subjects presenting with lower baseline olfactory performance and lower verbal semantic fluency.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00405-021-06810-9.

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.