Presence of olfactory event-related potentials predicts recovery in patients with olfactory loss following upper respiratory tract infection

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.

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.

Omega-3 supplementation in postviral olfactory dysfunction: a pilot study

BACKGROUND

This study aimed to examine whether omega-3 supplementation would support olfactory recovery among postviral olfactory dysfunction patients.

METHODOLOGY

Patients with postviral olfactory dysfunction were included in this non-blinded, prospective pilot study. Structured medical history was taken from the patients, including the following: age, sex, history of COVID-19 infection, and duration of symptoms. Patients were randomly assigned to receive olfactory training only (control group) versus olfactory training with omega-3 supplementation (treatment group). All patients exposed themselves twice a day to four odours (phenyl ethyl alcohol [rose], eucalyptol [eucalyptus], citronellal [lemon], and eugenol [cloves]). Olfactory function was measured before and after training using "Sniffin' Sticks", comprised of tests for odour threshold, discrimination, and identification. The average interval between olfactory tests was 3 months.

RESULTS

Fifty-eight patients were included in the study, 25 men and 33 women. Generally, an improvement in olfactory scores was observed. Compared to the control group, the improvement in odour thresholds was more pronounced in the omega-3 group. Age, sex, and duration of symptoms had no effect on olfactory scores among both control and treatment groups.

CONCLUSION

Overall, the present results indicate that omega-3 supplementation may be an option for adjunct therapy with olfactory training in patients with postviral olfactory dysfunction.

Immunological status of the olfactory bulb in a murine model of Toll-like receptor 3-mediated upper respiratory tract inflammation

BACKGROUND

Postviral olfactory dysfunction (PVOD) following a viral upper respiratory tract infection (URI) is one of the most common causes of olfactory disorders, often lasting for over a year. To date, the molecular pathology of PVOD has not been elucidated.

METHODS

A murine model of Toll-like receptor 3 (TLR3)-mediated upper respiratory tract inflammation was used to investigate the impact of URIs on the olfactory system. Inflammation was induced via the intranasal administration of polyinosinic-polycytidylic acid (poly(I:C), a TLR3 ligand) to the right nostril for 3 days. Peripheral olfactory sensory neurons (OSNs), immune cells in the olfactory mucosa, and glial cells in the olfactory bulb (OB) were analyzed histologically. Proinflammatory cytokines in the nasal tissue and OB were evaluated using the quantitative real-time polymerase chain reaction (qPCR) and enzyme-linked immunosorbent assay (ELISA).

RESULTS

In the treated mice, OSNs were markedly reduced in the olfactory mucosa, and T cell and neutrophil infiltration therein was observed 1 day after the end of poly(I:C) administration. Moreover, there was a considerable increase in microglial cells and slight increase in activated astrocytes in the OB. In addition, qPCR and ELISA revealed the elevated expression of interleukin-1 beta, interleukin-6, tumor necrosis factor-alpha, and interferon-gamma both in the OB and nasal tissue.

CONCLUSIONS

Taken together, the decreased peripheral OSNs, OB microgliosis, and elevated proinflammatory cytokines suggest that immunological changes in the OB may be involved in the pathogenesis of PVOD.

[Predictors of posttreatment olfactory improvement in patients with postviral olfactory dysfunction]

Objective:To analyzed the results of olfactory function test in patients with post-viral olfactory dysfunction（PVOD）, and evaluated the prognostic factors, so as to provide a basis for clinical diagnosis and treatment. Methods:This study included patients who were diagnosed with PVOD at least one year ago in Beijing Anzhen Hospital and whose telephone interviews of subjective olfactory function were available. The general condition of the patients, the results Sniffin' Sticks olfactory test and the event-related potentials（ERPs） were analyzed in different improvement groups. This study retrospectively analyzed PVOD patients treated in the outpatient department of Beijing Anzhen Hospital. They were given olfactory training for 4 months. The Sniffin' Sticks test was performed on the patients before and after the treatment. The Sniffin' Sticks test and event-related potentials（ERPs） results were used to evaluate the prognostic factors. Results:In this study, the olfactory improvement rate of 63 PVOD patients was 52.38%（33/63）. Compared to the non-improvement group, the course of disease in the group with improved subjective olfactory function was significantly shorter（P<0.001）, the initial olfactory function was significantly better（P<0.001）, and the olfactory threshold was much lower（P<0.001）. The presence of olfactory event-related potentials and trigeminal ERPs（tERPs） were 52.38%（33/63） and 87.30%（55/63）, respectively. The presence of oERPs in the olfactory function improvement group was significantly higher than that in the non-improvement group（P<0.05）, but there was no difference in the presence of tERPs（P>0.05）. Latency of N1 and P2 waves in oERPs with improvement group（ON1L, OP2L） were longer than those in the non-improvement group（P<0.05）, N1 and P2 wave amplitudes（ON1A, OP2A） had no difference（P>0.05）. The N1 and P2 amplitudes and latency of tERPs showed no difference between the two groups. Multivariate Logistic regression analysis showed that threshold value before treatment（OR=21.376, 95%CI: 2.172-210.377, P=0.009）; ON1L（OR=0.994, 95%CI: 0.988-0.999, P=0.029） and course of disease（OR=0.607, 95%CI: 0.405-0.920, P=0.016） was significantly associated with olfactory prognosis. Conclusion:The course of olfactory dysfunction, the severity of olfactory dysfunction, the threshold of olfactory function, and the latency of N1 wave of oERPs can be used to evaluate the prognosis of PVOD patients. However, age, olfactory discrimination, recognition ability, oERPs amplitude and tERPs wave value had less prognostic value.

Prognostic value of olfactory evoked potentials in patients with post-infectious olfactory dysfunction

PURPOSE

Prognostic assessment of patients with post-infectious olfactory dysfunction (PIOD) poses a challenge for clinicians. While there have been some studies on the prognostic factors of PIOD focusing on demographic factors, the aim of this study was to investigate whether event-related potentials (ERPs) could be used as a new predictor of olfactory recovery in PIOD.

METHODS

This was a retrospective study involving patients who underwent olfactory examinations using Sniffin' Sticks test before treatment and after 1 year of follow-up. The responder group was defined by an increase of threshold-discrimination-identification (TDI) score of ≥ 6 points. All patients underwent ERP examination and the amplitude and latency of each wave of ERPs were recorded before treatment.

RESULTS

A total of 61 patients (age 47.50 ± 11.04 years, 27 males) were analyzed. The presence of olfactory ERPs (oERPs) was greater in the responder group than in the non-responder group (P = 0.007), while that of trigeminal ERPs (tERPs) did not differ between the two groups (P = 0.346). Logistic-regression analyses showed that factors associated with improvement of subjective olfactory function were duration (OR, 1.604; 95% CI, 1.062-2.423; P = 0.025), initial threshold (odds ratio [OR], 0.043; 95% confidence interval [CI], 0.004-0.439; P = 0.008), and latency of N1 in oERPs (OR, 1.007; 95% CI, 1.001-1.013; P = 0.021).

CONCLUSION

Our study shows that duration of OD, initial threshold, and latency of N1 in oERPs were associated with olfactory improvement in PIOD patients, which may provide guidance for clinicians.

Electrophysiologic assessment of olfactory and gustatory function

This chapter reviews approaches for assessing human and gustatory function using electrophysiologic methods. Its focus is on changes in electrical signals, including summated generator potentials that occur after nasal or oral stimulation. In the first part of the review, we describe tools available to the clinician for assessing olfactory and nasotrigeminal function, including modern electroencephalography (EEG) analysis of brain responses both in the time domain and in the time-frequency (TF) domain. Particular attention is paid to chemosensory event-related potentials (CSERPs) and their potential use in medical-legal cases. Additionally, we focus on the changes of summated generator potentials from the olfactory and respiratory nasal epithelium that could provide new diagnostic insights. In the second part, we describe gustatory event-related potentials (gCSERPs) obtained using a relatively new computer controlled gustometer. A device for presenting different pulses of electrical current to the tongue is also described, with weaker pulses likely reflecting gCSERPs and stronger ones trigeminal CSERPs. Finally, summated generator potentials from the surface of the tongue during gustatory stimulation are described that may prove useful for examining peripheral taste function.

Effects of Tokishakuyakusan on Regeneration of Murine Olfactory Neurons In Vivo and In Vitro

Post-upper respiratory tract infection related olfactory dysfunction typically occurs due to neural damage after an upper respiratory tract infection associated with a common cold or influenza. At present, Tokishakuyakusan, a Japanese traditional Kampo medicine, has been found to be effective for post-viral olfactory dysfunction. However, the pharmacodynamics of Tokishakuyakusan in the treatment of post-viral olfactory dysfunction remains unresolved. We investigated the effects of Tokishakuyakusan on the regeneration of olfactory neurons and expression of nerve growth factor (NGF) in neural systems, using in vivo murine studies and in vitro cell culture studies. Eight-week-old BALB/C female mice were fed a pellet diet with or without Tokishakuyakusan. Degeneration of cells in olfactory epithelium was induced by intraperitoneal methimazole injection. Regeneration of olfactory neurons was observed by histological and immunohistochemical procedures. NGF expression in the olfactory bulb was measured by enzyme-linked immunosorbent assay. NGF gene and protein expression were measured using rat primary cultured astrocytes by real-time polymerase chain reaction and enzyme-linked immunosorbent assay. We found that olfactory marker protein, Ki-67, and NGF were more highly expressed in the olfactory epithelium during the regeneration period in mice receiving Tokishakuyakusan. In cultured astrocytes, Tokishakuyakusan as well as its individual components, Atractylodes lancea rhizome and Japanese angelica root, increased NGF expression. Screening assays revealed that NGF production was increased by atractylodin and levistolide A, which are ingredients in Atractylodes lancea rhizome and Japanese angelica root, respectively. These results suggest that Tokishakuyakusan promotes regeneration of olfactory neurons by increasing NGF expression in the olfactory bulb.

Disrupted Odor Perception

Olfactory loss is frequent. However, in public not many people complain of that, or they are even not (fully) aware of it. This indicates that it is possible to live a life without a sense of smell, albeit it is more dangerous, less pleasant, and food tastes much less interesting. Most common causes for smell loss are sinunasal disease (chronic rhinosinusitis with and without nasal polyps), acute infections of the upper airways, head trauma, and neurodegenerative disorders. In many people smell loss seems to be due to the aging process. Before treatment olfactory disorders are diagnosed according to cause with the medical history being a big portion of the diagnostic process. Olfactory disorders are in principle reversible, with a relatively high degree of spontaneous improvement in olfactory loss following infections of the upper respiratory tract. Medical treatment is according to cause. It also involves surgical approaches as well as conservative treatments including the use of corticosteroids, antibiotics, or smell training. Because today olfactory dysfunction seems to receive more attention than in previous years it can be expected that tomorrow we will have more specific and effective treatment options available.

Non-sinonasal-related olfactory dysfunction: A cohort of 496 patients

INTRODUCTION AND AIM

There is a high prevalence of olfactory dysfunction in the general population. Several causes of olfactory dysfunction have been reported and this disorder is classically divided into sinonasal and non-sinonasal-related olfactory dysfunction. The aims of this study were firstly, to evaluate the frequency of the various aetiologies of olfactory dysfunction in a population of patients with non-sinonasal-related olfactory dysfunction and secondly, to evaluate the degree of olfactory impairment associated with these various aetiologies.

MATERIAL AND METHODS

We retrospectively reviewed a cohort of 496 patients with non-sinonasal-related olfactory dysfunction. The aetiology of the olfactory dysfunction was recorded for each patient. The aetiology was determined by a complete clinical assessment, including medical history, complete otorhinolaryngological examination, psychophysical testing of olfactory function, recording of olfactory event-related potentials and brain magnetic resonance imaging. Six groups of patients were defined on the basis of the aetiology of the disease and orthonasal and retronasal psychophysical olfactory performances were evaluated in each group.

RESULTS

Post-infectious and post-traumatic aetiologies were the most common causes, representing 37.9% and 33.1% of patients, respectively, followed by idiopathic (16.3%), congenital (5.9%), toxic (3.4%) and neurological (3.4%) olfactory dysfunction. Anosmia was significantly more frequent in congenital (93.1%) and post-traumatic (62.8%) olfactory dysfunction, whereas hyposmia was more frequent in the post-infectious group (59.6%). Orthonasal and retronasal olfactory function tests were significantly correlated in all groups except for the congenital group.

CONCLUSIONS

The data of this study confirm that the most common causes of non-sinonasal-related olfactory dysfunction are post-infectious and post-traumatic. Post-infectious olfactory dysfunction is mainly observed in middle-aged women and is mainly associated with hyposmia, whereas post-traumatic olfactory dysfunction is mainly observed in young men and is associated with a high rate of anosmia.

Prognostic Value of Olfactory Bulb Volume Measurement for Recovery in Postinfectious and Posttraumatic Olfactory Loss

Objectives

Several prognostic factors influencing the recovery from olfactory dysfunction have been described. The aim of this study was to investigate whether olfactory bulb volume could be used as a new predictor of olfactory recovery in postinfectious and posttraumatic olfactory loss.

Study Design

Cohort study; Level of evidence, 4.

Setting

Tertiary university clinic, department of otolaryngology.

Subjects and Methods

A cohort of 60 patients with postinfectious (n = 28) and posttraumatic olfactory loss (n = 32) was investigated. Assessment of olfactory function was performed using orthonasal (Sniffin’ Sticks test) and retronasal psychophysical olfactory tests, at the time of the diagnosis (t1) and 15 months later (t2). All patients were examined on 3 tesla magnetic resonance imaging, and the olfactory bulbs volume was assessed using planimetric contouring at the time of the diagnosis (t1).

Results

Recovery rate was 25% in patients with posttraumatic olfactory loss and 36% in patients with postinfectious olfactory loss. There was a correlation between both orthonasal and retronasal olfactory testing and the initial measurement of the total olfactory bulb volume. In addition, we observed a significant correlation between changes in olfactory functions and initial measurement of the total olfactory bulb volume, with larger volumes relating to higher improvement of olfactory function. Finally, we found that none of the patients with a total olfactory bulb volume of 40 mm3 or less exhibited recovery of olfactory function.

Conclusion

Olfactory bulb volume seems to be a predictor of olfactory recovery in patients with postinfectious and posttraumatic olfactory loss.

Intranasal trigeminal chemosensitivity in patients with postviral and post-traumatic olfactory dysfunction

CONCLUSIONS

The olfactory and intranasal trigeminal systems are closely connected. With regard to intranasal trigeminal event-related potential (ERP), patients with olfactory dysfunction (OD) showed longer latency and lower amplitude, which indicated decreased trigeminal sensitivity. Different age, etiology, and olfactory status also affect trigeminal sensitivity differently.

OBJECTIVE

OD is a common symptom in the rhinology clinic. ERP is considered an important method to evaluate chemosensitivity. The aim of this study was to investigate changes of intranasal trigeminal sense in patients with postviral OD (PVOD) and post-traumatic OD (PTOD).

METHODS

A total of 96 participants (30 healthy adults and 66 patients with OD) aged 20-65 years were investigated. The T&T olfactometer, the Sniffin' Sticks olfactory test, and trigeminal ERPs (tERPs) were used. We evaluated trigeminal sensitivity influenced by different factors (age, etiology, and olfactory status) in healthy subjects with normal olfactory function and in patients with PVOD and PTOD.

RESULTS

Patients with OD showed higher trigeminal thresholds than normal controls. Compared with controls, N1/P2 latencies of tERPs increased and amplitudes decreased in patients with OD. Older subjects showed longer latencies and lower amplitudes than young subjects in both controls and the OD group. Patients with PTOD exhibited worse psychophysical olfactory function and decreased trigeminal sensitivity.

Time-frequency analysis of chemosensory event-related potentials to characterize the cortical representation of odors in humans

Background

The recording of olfactory and trigeminal chemosensory event-related potentials (ERPs) has been proposed as an objective and non-invasive technique to study the cortical processing of odors in humans. Until now, the responses have been characterized mainly using across-trial averaging in the time domain. Unfortunately, chemosensory ERPs, in particular, olfactory ERPs, exhibit a relatively low signal-to-noise ratio. Hence, although the technique is increasingly used in basic research as well as in clinical practice to evaluate people suffering from olfactory disorders, its current clinical relevance remains very limited. Here, we used a time-frequency analysis based on the wavelet transform to reveal EEG responses that are not strictly phase-locked to onset of the chemosensory stimulus. We hypothesized that this approach would significantly enhance the signal-to-noise ratio of the EEG responses to chemosensory stimulation because, as compared to conventional time-domain averaging, (1) it is less sensitive to temporal jitter and (2) it can reveal non phase-locked EEG responses such as event-related synchronization and desynchronization.

Methodology/Principal Findings

EEG responses to selective trigeminal and olfactory stimulation were recorded in 11 normosmic subjects. A Morlet wavelet was used to characterize the elicited responses in the time-frequency domain. We found that this approach markedly improved the signal-to-noise ratio of the obtained EEG responses, in particular, following olfactory stimulation. Furthermore, the approach allowed characterizing non phase-locked components that could not be identified using conventional time-domain averaging.

Conclusion/Significance

By providing a more robust and complete view of how odors are represented in the human brain, our approach could constitute the basis for a robust tool to study olfaction, both for basic research and clinicians.