Hot topic: Mapping of the human intranasal mucosal thermal sensitivity: A clinical study on thermal threshold and trigeminal receptors

INTRODUCTION

The olfactory and trigeminal system are closely interlinked. Existing literature has primarily focused on characterizing trigeminal stimulation through mechanical and chemical stimulation, neglecting thermal stimulation thus far. The present study aimed to characterize the intranasal sensitivity to heat and the expression of trigeminal receptors (transient receptor potential channels, TRP).

METHODS

A total of 20 healthy participants (aged 21-27 years, 11 women) were screened for olfactory function and trigeminal sensitivity using several tests. Under endoscopic control, a thermal stimulator was placed in 7 intranasal locations: anterior septum, lateral vestibulum, interior nose tip, lower turbinate, middle septum, middle turbinate, and olfactory cleft to determine the thermal threshold. Nasal swabs were obtained in 3 different locations (anterior septum, middle turbinate, olfactory cleft) to analyze the expression of trigeminal receptors TRP: TRPV1, TRPV3, TRPA1, TRPM8.

RESULTS

The thermal threshold differed between locations (p = 0.018), with a trend for a higher threshold at the anterior septum (p = 0.092). There were no differences in quantitative receptor expression (p = 0.46) at the different sites. The highest overall receptor RNA expression was detected for TRPV1 over all sites (p<0.001). The expression of TRPV3 was highest at the anterior septum compared to the middle turbinate or the olfactory cleft. The thermal sensitivity correlated with olfactory sensitivity and results from tests were related to trigeminal function like intensity ratings of ammonium, a questionnaire regarding trigeminal function, nasal patency, and CO2 thresholds. However, no correlation was found between receptor expression and psychophysical measures of trigeminal function.

DISCUSSION

This study provided the first insights about intranasal thermal sensitivity and suggested the presence of topographical differences in thermal thresholds. There was no correlation between thermal sensitivity and trigeminal mRNA receptor expression. However, thermal sensitivity was found to be associated with psychophysical measures of trigeminal and olfactory function.

Olfactory-trigeminal integration in the primary olfactory cortex

Humans naturally integrate signals from the olfactory and intranasal trigeminal systems. A tight interplay has been demonstrated between these two systems, and yet the neural circuitry mediating olfactory-trigeminal (OT) integration remains poorly understood. Using functional magnetic resonance imaging (fMRI), combined with psychophysics, this study investigated the neural mechanisms underlying OT integration. Fifteen participants with normal olfactory function performed a localization task with air-puff stimuli, phenylethyl alcohol (PEA; rose odor), or a combination thereof while being scanned. The ability to localize PEA to either nostril was at chance. Yet, its presence significantly improved the localization accuracy of weak, but not strong, air-puffs, when both stimuli were delivered concurrently to the same nostril, but not when different nostrils received the two stimuli. This enhancement in localization accuracy, exemplifying the principles of spatial coincidence and inverse effectiveness in multisensory integration, was associated with multisensory integrative activity in the primary olfactory (POC), orbitofrontal (OFC), superior temporal (STC), inferior parietal (IPC) and cingulate cortices, and in the cerebellum. Multisensory enhancement in most of these regions correlated with behavioral multisensory enhancement, as did increases in connectivity between some of these regions. We interpret these findings as indicating that the POC is part of a distributed brain network mediating integration between the olfactory and trigeminal systems. PRACTITIONER POINTS: Psychophysical and neuroimaging study of olfactory-trigeminal (OT) integration. Behavior, cortical activity, and network connectivity show OT integration. OT integration obeys principles of inverse effectiveness and spatial coincidence. Behavioral and neural measures of OT integration are correlated.

A scoping review of olfactory interventions for fatigue relief: addressing occupational health hazards

Background

Fatigue poses risks to occupational health and safety, affecting individuals' work efficiency, physical health, and social security, as well as human wellbeing and quality of life. Olfactory interventions, due to their low interference, are considered promising strategies for mitigating fatigue and reducing occupational health hazards.

Objective

The objective of this review is to bridge the current gaps in the literature by conducting a scoping review of olfactory interventions on human alertness. It aims to explore their application in various occupational settings and to provide comprehensive and practical guidance for the practical application of olfactory interventions in mitigating fatigue and reducing occupational risks.

Methods

The literature research was conducted in English using electronic databases such as Web of Science. Keywords related to scent and fatigue and the review followed PRISMA Extension for Scoping Reviews and PICO framework.

Results

28 studies were included in this work. Participant characteristics, fatigue measurement methods, and scent intervention methods, such as types of scents, intervention strategies, and scent presentation systems, are thoroughly investigated and discussed. Additionally, the study places a specific emphasis on the applications and research within the field of scent interventions for fatigue driving. Olfactory interventions have been applied to populations in various occupational fields, demonstrating beneficial effects on both physiological and psychological fatigue.

Conclusions

Olfactory intervention is effective and promising for enhancing alertness and improving the occupational environment. To provide detailed and practical guidance for the actual application of olfactory intervention in fatigue relief and reducing occupational health and safety hazards, further research into the potential mechanisms, applications, and efficacy assessment systems of fatigue-related olfactory interventions is necessary.

Trigeminal function in patients with COVID-associated olfactory loss

PURPOSE

Olfactory dysfunction (OD) can be a long-term consequence of various viral infections, including COVID-19. Dysfunction includes hyposmia/anosmia and parosmia (odor distortions). Interactions of the virus with the olfactory nerve have been extensively researched, but little is known about the interactions of the intranasal trigeminal nerve system in modulating this sensory loss.

METHODS

We investigated intranasal trigeminal function across COVID-19 OD patients with and without parosmia compared to normosmic controls, to determine whether (1) post-viral hyposmia and/or (2) post-viral hyposmia with parosmia is associated with altered trigeminal function. OD patients (n = 27) were tested for olfactory function using the extended Sniffin' Sticks olfactory test and for trigeminal function through three methods-odor lateralization, subjective ratings of nasal patency, and ammonium vapor pain intensity ratings. This group was subsequently compared to controls, normosmic subjects (n = 15).

RESULTS

Our findings revealed that post-COVID OD patients without parosmia experienced decreased sensitivity in ammonium vapor pain intensity ratings and odor lateralization scores-but similar nasal patency ratings-compared to normosmic controls. There were no significant differences in trigeminal function between OD patients with and without parosmia.

CONCLUSIONS

Based on our results, we conclude that the trigeminal nerve dysfunction may partially explain post-viral OD, but does not seem to be a major factor in the generation of parosmia pathophysiology.

Olfactory dysfunction and its related molecular mechanisms in Parkinson's disease

Changes in olfactory function are considered to be early biomarkers of Parkinson's disease. Olfactory dysfunction is one of the earliest non-motor features of Parkinson's disease, appearing in about 90% of patients with early-stage Parkinson's disease, and can often predate the diagnosis by years. Therefore, olfactory dysfunction should be considered a reliable marker of the disease. However, the mechanisms responsible for olfactory dysfunction are currently unknown. In this article, we clearly explain the pathology and medical definition of olfactory function as a biomarker for early-stage Parkinson's disease. On the basis of the findings of clinical olfactory function tests and animal model experiments as well as neurotransmitter expression levels, we further characterize the relationship between olfactory dysfunction and neurodegenerative diseases as well as the molecular mechanisms underlying olfactory dysfunction in the pathology of early-stage Parkinson's disease. The findings highlighted in this review suggest that olfactory dysfunction is an important biomarker for preclinical-stage Parkinson's disease. Therefore, therapeutic drugs targeting non-motor symptoms such as olfactory dysfunction in the early stage of Parkinson's disease may prevent or delay dopaminergic neurodegeneration and reduce motor symptoms, highlighting the potential of identifying effective targets for treating Parkinson's disease by inhibiting the deterioration of olfactory dysfunction.

Assessment of Intranasal Function of the Trigeminal Nerve in Daily Clinical Practice

BACKGROUND

The trigeminal nerve is a mixed cranial nerve responsible for the motor innervation of the masticatory muscles and the sensory innervation of the face, including the nasal cavities. Through its nasal innervation, we perceive sensations, such as cooling, tingling, and burning, while the trigeminal system mediates the perception of airflow. However, the intranasal trigeminal system has received little attention in the clinical evaluation of patients with nasal pathology.

SUMMARY

Testing methods that enable the clinical assessment of intranasal trigeminal function have recently been developed. This study aims to present the current clinical methods that can be utilised in everyday practice, as described in the literature. These methods include four assessment techniques: (1) the quick screening test of trigeminal sensitivity involves patients rating the intensity of ammonium vapour presented in a lipstick-like container. (2) The lateralisation test requires subjects to identify which nasal cavity is being stimulated by a trigeminal stimulus, such as eucalyptol or menthol, while the other side receives an odourless stimulus. (3) The trigeminal sticks test evaluates the trigeminal function similarly to the olfactory function using sticks filled with trigeminal stimulant liquids. (4) The automated CO2 stimulation device is used for measuring trigeminal pain thresholds, utilising intranasal CO2 stimuli to define the pain threshold.

KEY MESSAGES

Assessing intranasal trigeminal function clinically may prove useful in evaluating rhinology patients, particularly those who encounter nasal obstruction without anatomical blockage and those experiencing olfactory disorders with suspected trigeminal dysfunction. Despite their limitations, the presented methods may provide useful information about nasal patency, chemosensitivity, and pain sensation in the daily clinical practice of such patients, leading to better therapeutic decisions.

In-vehicle fragrance administration as a countermeasure for driver fatigue

Driver fatigue is a contributing factor in about 10-30% of all fatal crashes. Prevention of fatigue-related crashes relies on robust detection of driver fatigue and application of effective countermeasures. A potential countermeasure is fragrance administration since odors can have alerting effects on humans. The aim here was to investigate if a fragrance incorporating trigeminal components could be used as an in-vehicle countermeasure for driver fatigue. The fragrance was tested in a driving simulator with 21 healthy but sleep-deprived participants. Each participant performed a monotonous driving task twice, once with active fragrance containing a trigeminal component and once with olfactory fragrance, in a cross-over single-blind design. The order of trigeminal/olfactory fragrance was randomized and blinded to the participants. Both fragrances (trigeminal/olfactory) were administered either when the participant fell asleep (defined as eye closure > 3 s) or after approximately 45 min if the participant did not fall asleep. Self-reported sleepiness was assessed using the Karolinska Sleepiness Scale (KSS) every 5 min during driving. Variability in speed and lateral position and line crossing frequency were logged for each drive to measure driving performance. Heart rate measurements (ECG) and eye blinks (EOG) were collected to investigate potential arousing effects of the fragrance and to track objective signs of sleepiness. Mean blink duration, which was used as an objective measure of sleepiness, decreased significantly, after fragrance exposure, as did the frequency of line crossings, but there were no statistically significant differences between the fragrance with trigeminal stimulus and the pure olfactory fragrance. The results are in line with the effects found for other commonly used fatigue countermeasures, like playing loud music. These countermeasures can restore alertness and driving performance for a short while. Whether this is sufficient to support driving performance until the driver can make a safe stop in real traffic remains a topic for future studies.

Quantifying Peripheral Modulation of Olfaction by Trigeminal Agonists

In the mammalian nose, two chemosensory systems, the trigeminal and the olfactory mediate the detection of volatile chemicals. Most odorants are able to activate the trigeminal system, and vice versa, most trigeminal agonists activate the olfactory system as well. Although these two systems constitute two separate sensory modalities, trigeminal activation modulates the neural representation of an odor. The mechanisms behind the modulation of olfactory response by trigeminal activation are still poorly understood. We addressed this question by looking at the olfactory epithelium (OE), where olfactory sensory neurons (OSNs) and trigeminal sensory fibers co-localize and where the olfactory signal is generated. Our study was conducted in a mouse model. Both sexes, males and females, were included. We characterize the trigeminal activation in response to five different odorants by measuring intracellular Ca2+ changes from primary cultures of trigeminal neurons (TGNs). We also measured responses from mice lacking TRPA1 and TRPV1 channels known to mediate some trigeminal responses. Next, we tested how trigeminal activation affects the olfactory response in the olfactory epithelium using electro-olfactogram (EOG) recordings from wild-type (WT) and TRPA1/V1-knock out (KO) mice. The trigeminal modulation of the olfactory response was determined by measuring responses to the odorant, 2-phenylethanol (PEA), an odorant with little trigeminal potency after stimulation with a trigeminal agonist. Trigeminal agonists induced a decrease in the EOG response to PEA, which depended on the level of TRPA1 and TRPV1 activation induced by the trigeminal agonist. This suggests that trigeminal activation can alter odorant responses even at the earliest stage of the olfactory sensory transduction.SIGNIFICANCE STATEMENT Most odorants reaching the olfactory epithelium (OE) can simultaneously activate olfactory and trigeminal systems. Although these two systems constitute two separate sensory modalities, trigeminal activation can alter odor perception. Here, we analyzed the trigeminal activity induced by different odorants proposing an objective quantification of their trigeminal potency independent from human perception. We show that trigeminal activation by odorants reduces the olfactory response in the olfactory epithelium and that such modulation correlates with the trigeminal potency of the trigeminal agonist. These results show that the trigeminal system impacts the olfactory response from its earliest stage.

Correlations between gustatory, trigeminal, and olfactory functions and nasal airflow

PURPOSE

To determine the relationship of chemosensory screening and nasal airflow tests among the same set of participants, and to determine other factors that are related to the outcomes of these tests.

METHODS

Participants had no chemosensory complaints. Structured medical history was taken. Participants underwent 5 screening tests: q-sticks (orthonasal olfaction), q-powders (retronasal olfaction), trigeminal lateralization test, taste sprays, and peak nasal inspiratory flow (PNIF). Ratings of smell/taste ability and nasal airflow were obtained using visual analogue scales (VAS). Composite sinusitis symptoms and significance of olfaction questionnaire scores were also determined.

RESULTS

Four hundred participants were included in the study, 156 men, 244 women; aged 18-82 years (mean: 46). The q-powders and taste spray scores were weakly positively correlated with all the other chemosensory tests and PNIF. However, chemosensory test scores were not correlated with VAS, composite sinusitis symptoms, and significance of olfaction questionnaire scores. Various tests showed significant decrease starting at specific ages (in years, PNIF and trigeminal lateralization: 40, q-powders: 60, and q-sticks: 70).

CONCLUSION

Chemosensory screening tests and self-rated chemosensory function showed no correlation in participants without chemosensory complaints. In addition, gustatory function appeared to be correlated with olfactory and trigeminal function but also with nasal airflow, and nasal airflow was related not only to olfactory but also to trigeminal and taste function. Over all, the results suggest that chemosensory functions (orthonasal olfactory, trigeminal, retronasal olfactory, gustatory) and nasal airflow are correlated with each other, which we propose may be possibly mediated, at least in part, through central nervous system interactions.

Hyposmia in COVID-19: Temporal Recovery of Smell: A Preliminary Study

Background and Objectives: Hypo/anosmia is a characteristic symptom of COVID-19 infection. The aim of this study is to investigate the time of smell recovery and to identify a possible order of perception recovery of different odors in COVID-19 patients. Materials and Methods: A prospective observational study was conducted on not hospitalized COVID-19 patients, selected according to eligible criteria. The study was approved by the Ethical Committee. A questionnaire formulated by our team was submitted to patients in order to know the duration of the hypo/anosmia and hypo/ageusia and the order of odor recovery: vanillin (mixed olfactory/gustatory substances), phenyl ethyl alcohol (rosewater) (pure olfactory substances), eucalyptol (mixed olfactory/trigeminal substances), and eugenol (mixed olfactory/trigeminal/gustatory substances). Results: 181 patients were included. Hypo/ageusia and hypo/anosmia lasted on average 10.25 (±8.26) and 12.8 (±8.80) days, respectively. The most frequent odor recovery sequence was: (1) phenyl ethyl alcohol; (2) eucalyptol; (3) vanillin; and (4) eugenol. In COVID-19 patients, hypo/anosmia occurs more often in women and at a young age. Conclusions: This preliminary investigation highlighted novel data: there is a chronological order in perception recovery of different olfactory substances and, therefore, in the restoration of the various sensitive nerve pathways involved in the sense of smell.

Reduction of Odor-causing Compounds in Wastewater using Biochar: A Review

Wastewater contains chemical compounds that cause malodors, such as ammonium cation, dimethyl sulfide, and volatile organic compounds. Biochar-based reduction in the odorants has been proposed as an effective approach along with maintaining environmental neutrality as biochar is a sustainable material made from biomass and biowaste. Biochar can have high specific surface area and microporous structure with proper activation, appropriate for sorption purposes. Recently, various research directions have been proposed to determine the removal efficiency of biochar for different odorants contained in wastewater. This article is aimed at providing the most updated review of biochar-based removal of odor-causing compounds in wastewater while highlighting the current advances. It was distinguished that the odorant removal performance of biochar is highly associated with the raw material and modification method of biochar, and the kind of odorants. Further research should be required for more practical use of biochar for the reduction of odorants in wastewater.

Intranasal trigeminal function in chronic rhinosinusitis: a review

INTRODUCTION

Chronic rhinosinusitis (CRS) affects 5-12% of the general population with significant effects on quality of life. Chronic inflammation also seems to affect intranasal trigeminal sensitivity.

AREAS COVERED

A systematic literature search was done in Scopus, Web of Science, and PubMed in February 2023. The review addressed intranasal trigeminal function in patients with CRS and summarized current knowledge on trigeminal function as it relates to the symptoms, assessment, and treatment of CRS.

EXPERT OPINION

Olfaction and trigeminal function are synergistic and this interaction may contribute to trigeminal dysfunction in CRS. Aside from anatomic blockage through polypoid mucosal changes, trigeminal dysfunction may affect the perception of nasal obstruction in CRS. Upregulated immune defense mechanisms leading to damage of nerve endings, changes in nerve growth factor release or other mechanisms may be responsible for trigeminal dysfunction in CRS. Since the pathophysiology of trigeminal dysfunction in CRS is poorly understood, current treatment recommendations are directed toward the therapy of CRS as an underlying cause, although the effect of surgery and corticosteroids on trigeminal function remains unclear. A standardized and validated trigeminal test that is accessible and easy to use in clinical settings would be beneficial for future studies.

Brain electrophysiological recording during olfactory stimulation in mild cognitive impairment and Alzheimer disease patients: An EEG dataset

The dataset presented in this article contains preprocessed cleaned electroencephalography (EEG) recording from 35 participants including 13 Alzheimer's disease (AD) patients, 7 amnestic mild cognitive impairment (aMCI) patients, and 15 healthy elderly. All participants performed the same olfactory task which consisted of 120 trials of 2 s olfactory stimulation and 8 s rest (no odorant). The olfactory stimulation consisted of rose and lemon odorants. Odor trials were presented randomly with a probability of 0.75 presenting lemon and 0.25 presenting rose. The impedance of the electrodes was kept under 15 KΩ during the experiment. The data was filtered from 0.5 to 40 Hz using a bandpass filter and epoched from 1 s pre-stimulus to 2 s post-stimulus. Artifacts related to eye blinks were removed by running independent component analysis (ICA) and the remaining noisy trials were identified by eye and removed from further analysis. Mini Mental State Examination (MMSE) test scores for all participants are also provided in the dataset. Olfactory dysfunction has been shown to be associated with neurodegenerative diseases such as dementia and Alzheimer's disease. Therefore, studying the response of the olfactory system may lead to identifying early biomarkers for related brain disorders.

The contribution of subliminal perceptions, dietary habits, and psychological traits to the perception of oral tingling and burning sensations

The tingling evoked by Sichuan pepper and the burning elicited by chili pepper constitutes the typical flavor of Sichuan cuisine and is a component of leisure food. Although factors affecting the burning sensation have extensively been studied, few studies have examined the factors of individual sensitivity, personality traits, and dietary habits that contribute to the perception of oral tingling sensation, which hinders the formulation of tingling products and the development of new products. In contrast, many studies have examined the factors influencing the burning sensation. In this web-based survey, 68 participants disclosed their dietary habits, liking for tingling and hot foods, and psychological traits. Individual sensitivity to the tingling and burning sensation produced by a range of Sichuan pepper oleoresin and capsaicin solutions was determined using rated differences from control, generalized labeled magnitude scale method and ranking test. The consistency score indicated the accuracy of individual ranking results while also providing an indirect response to the sensitivity of the participant to supra-threshold for burning or tingling. Individual ratings for medium Sichuan pepper oleoresin concentrations significantly correlated with the just noticeable difference (p < 0.01), and ratings for medium and high capsaicin concentrations correlated significantly with 6-n-propylthiouracil ratings (p < 0.01). Notably, the power exponent of burning was significantly correlated with the burning recognition threshold (p < 0.01), and the power exponent of tingling and burning were significantly correlated (r = 0.340, p < 0.05). There was a negative correlation between supra-threshold tingling and burning sensation perceptions and life satisfaction ratings. Further, intensity ratings for oral tingling and burning sensation did not always correspond with individual sensitivity indicators (e.g., recognition threshold, 6-n-propylthiouracil, just noticeable difference, and consistency score). Thus, this study provides new insight into establishing a sensory selection method for chemesthetic sensation panelists and theoretical guidelines for formulation design and in-depth analysis of popular tingling dishes and foods.

Trigeminal Sensory Neuropathy Predicts Chemosensory Dysfunction After Skull Base Surgery

BACKGROUND

Trigeminal sensory neuropathy, characterized by diminished or exaggerated/painful sensation along the distribution of trigeminal branches, is one of the complications after skull base surgery. These procedures can also affect chemosensory functions and compromise the patients' quality of life.

OBJECTIVE

To examine those postoperative complications in chemosensory and facial cutaneous sensory functions as well as their interaction.

METHODS

A total of 61 patients being operated for various pathologies in the vicinity of trigeminal pathway (17 with trigeminal neuralgia, 20 with meningiomas, and 24 with vestibular schwannomas) and 50 healthy controls were enrolled in this case-control study. Postoperative trigeminal neuropathic symptoms were evaluated. The olfactory, gustatory, and intranasal trigeminal functions were assessed using the Sniffin' Stick olfactory test, lateralized taste strip test, and the lateralized intranasal trigeminal CO 2 detection thresholds, respectively.

RESULTS

Patients with trigeminal neuralgia and meningioma had relatively decreased olfactory function, whereas those with vestibular schwannoma had a relative decrease in taste function. As for the relationship between the distribution of trigeminal neuropathy and chemosensory dysfunction, we found that V3 involvement predicted lower side-specific taste function. In addition, V2/V3 involvement predicted relatively lower side-specific intranasal trigeminal function.

CONCLUSION

The present results suggest a differential involvement of the chemical senses in relation to the site and pathology of the lesions after surgery. The presence of trigeminal neuropathy is associated with taste and intranasal trigeminal dysfunction. The results serve as a basis for consultation regarding outcome prediction and patient education.

Exploratory Study on Chemosensory Event-Related Potentials in Long COVID-19 and Mild Cognitive Impairment: A Common Pathway?

People affected by the Long COVID-19 (LC) syndrome often show clinical manifestations that are similar to those observed in patients with mild cognitive impairments (MCI), such as olfactory dysfunction (OD), brain fog, and cognitive and attentional diseases. This study aimed to investigate the chemosensory-evoked related potentials (CSERP) in LC and MCI to understand if there is a common pathway for the similarity of symptoms associated with these disorders. Eighteen LC patients (mean age 53; s.d. = 7), 12 patients diagnosed with MCI (mean age 67; s.d. = 6), and 10 healthy control subjects (mean age 66; s.d. = 5, 7) were recruited for this exploratory study. All of them performed a chemosensory event-related potentials (CSERP) task with the administration of trigeminal stimulations (e.g., the odorants cinnamaldehyde and eucalyptus). Study results highlighted that MCI and LC showed reduced N1 amplitude, particularly in the left frontoparietal network, involved in working memory and attentional deficits, and a reduction of P3 latency in LC. This study lays the foundations for evaluating aspects of LC as a process that could trigger long-term functional alterations, and CSERPs could be considered valid biomarkers for assessing the progress of OD and an indicator of other impairments (e.g., attentional and cognitive impairments), as they occur in MCI.

Quantifying peripheral modulation of olfaction by trigeminal agonists

In the mammalian nose, two chemosensory systems, the trigeminal and the olfactory mediate the detection of volatile chemicals. Most odorants in fact are able to activate the trigeminal system, and vice versa, most trigeminal agonists activate the olfactory system as well. Although these two systems constitute two separate sensory modalities, trigeminal activation modulates the neural representation of an odor. The mechanisms behind the modulation of olfactory response by trigeminal activation are still poorly understood. In this study, we addressed this question by looking at the olfactory epithelium, where olfactory sensory neurons and trigeminal sensory fibers co-localize and where the olfactory signal is generated. We characterize the trigeminal activation in response to five different odorants by measuring intracellular Ca2+ changes from primary cultures of trigeminal neurons (TGNs). We also measured responses from mice lacking TRPA1 and TRPV1 channels known to mediate some trigeminal responses. Next, we tested how trigeminal activation affects the olfactory response in the olfactory epithelium using electro-olfactogram (EOG) recordings from WT and TRPA1/V1-KO mice. The trigeminal modulation of the olfactory response was determined by measuring responses to the odorant, 2-phenylethanol (PEA), an odorant with little trigeminal potency after stimulation with a trigeminal agonist. Trigeminal agonists induced a decrease in the EOG response to PEA, which depended on the level of TRPA1 and TRPV1 activation induced by the trigeminal agonist. This suggests that trigeminal activation can alter odorant responses even at the earliest stage of the olfactory sensory transduction.

Examining the Influence of Chemosensation on Laryngeal Health and Disorders

Inhaled airborne stimuli are associated with laryngeal disorders affecting respiration. Clinically, several themes emerged from the literature that point to specific gaps in the understanding and management of these disorders. There is wide variation in the types of airborne stimuli that trigger symptoms, lack of standardization in provocation challenge testing using airborne stimuli, and vague reporting of laryngeal symptoms. Scientifically, evidence exists outside the field of voice science that could prove useful to implement among patients with impaired laryngeal-respiration. To expand this area of expertise, here we provide a thematic overview of relevant evidence and methodological tools from the discipline of chemosensory sciences. This review provides distinctions across the three chemosensory systems of olfaction, trigeminal chemesthesis, and gustation, guidance on selecting and delivering common chemosensory stimuli for clinical testing, and methods of quantifying sensory experiences using principles of human psychophysics. Investigating the science of chemosensation reveals that laryngeal responses to inhaled airborne stimuli have explanations involving physiological mechanisms as well as higher cognitive processing. Fortunately, these findings are consistent with current pharmacological and nonpharmacological interventions for impaired laryngeal-respiration. Based on the close relationships among inhaled airborne stimuli, respiration, and laryngeal function, we propose that new perspectives from chemosensory sciences offer opportunities to improve patient care and target areas of future research.

Olfactory stimulation may modulate the sensation of nasal patency

BACKGROUND

The sensation of nasal patency can be induced by inhaling menthol, which predominantly produces trigeminal stimulation. It remains unclear whether olfactory stimulation can also induce or modulate the sensation of nasal patency.

METHODOLOGY

A total of 118 participants (normosmia: n=67, olfactory dysfunction: n=51) were exposed to four odors in a randomized order: 1) phenylethanol (PEA), 2) menthol, 3) a mixture of PEA and menthol, 4) nearly odorless propylene glycol. The odors were presented by nasal clips. After the nasal clip had been removed, the participants rated relative nasal patency (RNP) from - 50 to +50, and their peak nasal inspiratory flow (PNIF) was measured. Repeated measures analysis of variance was used to examine the difference of RNP and PNIF among the four conditions and the influence of olfactory function.

RESULTS

The RNPs, other than PNIFs, differed between the four conditions. Menthol induced the highest RNP, followed by the mixed solution, PEA and the odorless condition. Normosmic participants, but not those with olfactory dysfunction, responded to PEA significantly higher than odorless condition with regard to RNP. The correlation analysis showed that the better the subjective or measured olfactory performance, the greater the PEA-induced sensation of nasal patency.

CONCLUSIONS

A specific olfactory stimulant that selectively induces olfactory perception can also evoke and modulate the sensation of nasal patency. Hence, patients might benefit from exposing themselves to odors in order to relieve the annoying nasal obstruction.

[Involvement of the trigeminal nerve system in the sense of smell]

A systematic review of literature on the issue of involvement in the sense of smell, as well as the interaction between the trigeminal and olfactory nerves, was carried out. The article discusses the features of the chemical perception systems, as well as the treatment of olfactory disorders using transcranial electrical stimulation of the trigeminal nerve.

Not Only COVID-19: Involvement of Multiple Chemosensory Systems in Human Diseases

Chemosensory systems are deemed marginal in human pathology. In appraising their role, we aim at suggesting a paradigm shift based on the available clinical and experimental data that will be discussed. Taste and olfaction are polymodal sensory systems, providing inputs to many brain structures that regulate crucial visceral functions, including metabolism but also endocrine, cardiovascular, respiratory, and immune systems. Moreover, other visceral chemosensory systems monitor different essential chemical parameters of “milieu intérieur,” transmitting their data to the brain areas receiving taste and olfactory inputs; hence, they participate in regulating the same vital functions. These chemosensory cells share many molecular features with olfactory or taste receptor cells, thus they may be affected by the same pathological events. In most COVID-19 patients, taste and olfaction are disturbed. This may represent only a small portion of a broadly diffuse chemosensory incapacitation. Indeed, many COVID-19 peculiar symptoms may be explained by the impairment of visceral chemosensory systems, for example, silent hypoxia, diarrhea, and the “cytokine storm”. Dysregulation of chemosensory systems may underlie the much higher mortality rate of COVID-19 Acute Respiratory Distress Syndrome (ARDS) compared to ARDSs of different origins. In chronic non-infectious diseases like hypertension, diabetes, or cancer, the impairment of taste and/or olfaction has been consistently reported. This may signal diffuse chemosensory failure, possibly worsening the prognosis of these patients. Incapacitation of one or few chemosensory systems has negligible effects on survival under ordinary life conditions but, under stress, like metabolic imbalance or COVID-19 pneumonia, the impairment of multiple chemosensory systems may lead to dire consequences during the course of the disease.

Electrical stimulation of the trigeminal nerve improves olfaction in healthy individuals: A randomized, double-blind, sham-controlled trial

BACKGROUND

Both activated by environmental odorants, there is a clear role for the intranasal trigeminal and olfactory nerves in smell function. Unfortunately, our ability to perceive odorants decreases with age or with injury, and limited interventions are available to treat smell loss.

OBJECTIVE

We investigated whether electrical stimulation of the trigeminal nerve via trigeminal nerve stimulation (TNS) or transcranial direct current stimulation (tDCS) modulates odor sensitivity in healthy individuals.

METHODS

We recruited 20 healthy adults (12 Female, mean age = 27) to participate in this three-visit, randomized, double-blind, sham-controlled trial. Participants were randomized to receive one of three stimulation modalities (TNS, tDCS, or sham) during each of their visits. Odor detection thresholds were obtained at baseline, immediately post-intervention, and 30-min post-intervention. Furthermore, participants were asked to complete a sustained attention task and mood assessments before odor detection testing.

RESULTS

Findings reveal a timeXcondition interaction for guaiacol (GUA) odorant detection thresholds (F (3.188, 60.57) = 3.833, P = 0.0125), but not phenyl ethyl alcohol (PEA) odorant thresholds. At 30-min post-stimulation, both active TNS and active tDCS showed significantly increased sensitivity to GUA compared to sham TNS (Sham TNS = -8.30% vs. Active TNS = 9.11%, mean difference 17.43%, 95% CI 5.674 to 29.18, p = 0.0044; Sham TNS = -8.30% vs. Active tDCS = 13.58%, mean difference 21.89%, 95% CI 10.47 to 33.32, p = 0.0004).

CONCLUSION

TNS is a safe, simple, noninvasive method for boosting olfaction. Future studies should investigate the use of TNS on smell function across different stimulation parameters, odorants, and patient populations.

A non-randomized comparative study of olfactory and gustatory functions in children who recovered from COVID-19 (1-year follow-up)

The experimental group included 68 children over 6 years of age who had recovered from COVID-19. The control group included 22 children over 6 years of age who have never had COVID-19. Research methods included neurological examination, verification of cognitive status, examination by an otolaryngologist, and smell and taste assessment. The examination was performed 6-8 weeks after COVID-19 recovery and after 1 year in some patients. Children who recovered from COVID-19 had a reduction in their ability to smell compared to children who had never had COVID-19. The olfactory thresholds and taste identification scores after recovery from COVID-19 were identical, whether the parents had reported anosmia in their children during COVID-19 or not, and irrespective of hyperthermia level and the presence or absence of headache and hyperhidrosis during COVID-19. Analysis of correlation with neuropsychiatric symptoms showed no differences in the olfactory thresholds in children irrespective of the presence of neuropsychiatric symptoms (tics, tremors, enuresis, compulsive movements, seizures, speech disorders, attention deficit, and easy fatigability) both in general, and in particular among subjects performing or not any compulsive movements, and experiencing or not a combination of easy fatigability and daytime sleepiness. Evidence suggests that in children and adolescents, partial hyposmia is associated with depressive symptoms, varying in severity from low to high, but symptoms of depression were not caused by COVID-19 infection itself. Analysis in subgroups with different degrees of state and trait anxiety did not reveal any significant differences in the olfactory threshold. A re-examination of 21 children was performed after 1 year. An objective olfactometric examination showed that the sensitivity to odorants increased significantly. In 1 year, we compared the thresholds of smell in children who had COVID-19 and those who did not have this disease: olfactory sensitivity after COVID-19 in children is restored to normal values. Schulte correction test showed that none of 14 children with asthenic manifestations in the form of fluctuations or exhaustion when performing the test immediately after COVID-19 had these manifestations after 1 year. Thus, asthenization of cognitive activity was recorded within the next 1.5 months after suffering from COVID-19 but was absent after 1 year.

Parkinson's Disease Affects Functional Connectivity within the Olfactory-Trigeminal Network

BACKGROUND

Olfactory dysfunction (OD) is a frequent symptom of Parkinson's disease (PD) that appears years prior to diagnosis. Previous studies suggest that PD-related OD is different from non-parkinsonian forms of olfactory dysfunction (NPOD) as PD patients maintain trigeminal sensitivity as opposed to patients with NPOD who typically exhibit reduced trigeminal sensitivity. We hypothesize the presence of a specific alteration of functional connectivity between trigeminal and olfactory processing areas in PD.

OBJECTIVE

We aimed to assess potential differences in functional connectivity within the chemosensory network in 15 PD patients and compared them to 15 NPOD patients, and to 15 controls.

METHODS

Functional MRI scanning session included resting-state and task-related scans where participants carried out an olfactory and a trigeminal task. We compared functional connectivity, using a seed-based correlation approach, and brain network modularity of the chemosensory network.

RESULTS

PD patients had impaired functional connectivity within the chemosensory network while no such changes were observed for NPOD patients. No group differences we found in modularity of the identified networks. Both patient groups exhibited impaired connectivity when executing an olfactory task, while network modularity was significantly weaker for PD patients than both other groups. When performing a trigeminal task, no changes were found for PD patients, but NPOD patients exhibited impaired connectivity. Conversely, PD patients exhibited a significantly higher network modularity than both other groups.

CONCLUSION

In summary, the specific pattern of functional connectivity and chemosensory network recruitment in PD-related OD may explain distinct behavioral chemosensory features in PD when compared to NPOD patients and healthy controls.

Possible Use of Phytochemicals for Recovery from COVID-19-Induced Anosmia and Ageusia

The year 2020 became the year of the outbreak of coronavirus, SARS-CoV-2, which escalated into a worldwide pandemic and continued into 2021. One of the unique symptoms of the SARS-CoV-2 disease, COVID-19, is the loss of chemical senses, i.e., smell and taste. Smell training is one of the methods used in facilitating recovery of the olfactory sense, and it uses essential oils of lemon, rose, clove, and eucalyptus. These essential oils were not selected based on their chemical constituents. Although scientific studies have shown that they improve recovery, there may be better combinations for facilitating recovery. Many phytochemicals have bioactive properties with anti-inflammatory and anti-viral effects. In this review, we describe the chemical compounds with anti- inflammatory and anti-viral effects, and we list the plants that contain these chemical compounds. We expand the review from terpenes to the less volatile flavonoids in order to propose a combination of essential oils and diets that can be used to develop a new taste training method, as there has been no taste training so far. Finally, we discuss the possible use of these in clinical settings.

Olfaction in patients with Parkinson's disease: a new threshold test analysis through turning points trajectories

Olfactory deficit is a widely documented non-motor symptom in Parkinson’s disease (PD). Abnormal turning points trajectories through olfactory threshold testing have been recently reported in patients with olfactory dysfunction, who seem to adapt faster to olfactory stimuli, but data on PD patients are lacking. The aim of this study is to perform olfactory threshold test and explore the turning points trajectories in PD patients in comparison to normal controls. We recruited 59 PD patients without dementia, and no conditions that could influence evaluation of olfaction and cognition. Sixty healthy subjects served as controls. Patients and controls underwent a comprehensive olfactory evaluation with the Sniffin’ Sticks extended test assessing threshold, discrimination and identification and a full neuropsychological evaluation. Besides, threshold test data were analyzed examining all the turning points trajectories. PD patients showed a different olfactory threshold test pattern, i.e., faster olfactory adaptation, than controls with no effect of age. Normosmic PD patients showed different olfactory threshold test pattern, i.e., better threshold score, than normosmic controls. Visuospatial dysfunction was the only factor that significantly influenced this pattern. Olfactory threshold trajectories suggested a possible adaptation phenomenon in PD patients. Our data offered some new insights on normosmic PD patients, which appear to be a subset with a specific psychophysical profile. The analysis of the turning points trajectories, through an olfactory threshold test, could offer additional information on olfactory function in PD patients. Future larger studies should confirm these preliminary findings.

Chemosensory Loss during a Traumatic Brain Injury Suggests a Central Pathway for the Rehabilitation of Anosmia

Currently, no method has been developed for rehabilitating olfaction in anosmic patients following a traumatic brain injury (TBI). Here a method for rehabilitation is described which is based on a recent finding that the human posterior pyriform cortex (PPC) generates predictive odor "search images" in advance of an encounter with an olfactory stimulus. The search image enhances perceptual sensitivity and allows the odor it represents to be identified without input occurring from the olfactory receptors or bulbs. Furthermore, based on the finding here that anosmics with a TBI often have normal trigeminal and gustatory function, it is proposed that normality in these chemosensory systems may indicate that key cortical regions including the PPC are intact in anosmics and capable of processing olfactory information. In addition, the results of chemosensory tests of the olfactory, gustatory, and intranasal trigeminal systems of 18 patients with a TBI are given that identify which patients would most likely benefit from the rehabilitation procedure.

Olfactory-Trigeminal Interactions in Patients with Parkinson's Disease

Olfactory dysfunction (OD) is a highly frequent early non-motor symptom of Parkinson's disease (PD). An important step to potentially use OD for the development of early diagnostic tools of PD is to differentiate PD-related OD from other forms of non-parkinsonian OD (NPOD: postviral, sinunasal, post-traumatic, and idiopathic OD). Measuring non-olfactory chemosensory modalities, especially the trigeminal system, may allow to characterize a PD-specific olfactory profile. We here review the literature on PD-specific chemosensory alteration patterns compared with NPOD. Specifically, we focused on the impact of PD on the trigeminal system and particularly on the interaction between olfactory and trigeminal systems. As this interaction is seemingly affected in a disease-specific manner, we propose a model of interaction between both chemosensory systems that is distinct for PD-related OD and NPOD. These patterns of chemosensory impairment still need to be confirmed in prodromal PD; nevertheless, appropriate chemosensory tests may eventually help to develop diagnostic tools to identify individuals at risks for PD.

Discrimination of Complex Odor Mixtures: A Study Using Wine Aroma Models

There are key unanswered questions when it comes to multicomponent odor discrimination. This study was designed to assess discrimination of odorant mixtures that elicit a singular percept. We collected data to address the following two questions: (1) What odor features do humans notice when attempting to discriminate between subtly different odor mixtures? (2) Are odor mixtures easier to discriminate when an odorant is added, compared with when a component is removed? Using modern aroma chemistry techniques, an odor mixture resembling a generic white wine was constructed. This wine odor mixture was modified using a series of three esters which are commonly found in white wines that vary in chain length and branching. Participants performed a sequence of discrimination tasks for the addition/subtraction of modifiers to the base wine at different concentrations. Only one of the esters (ethyl propanoate) led to a discriminable odor mixture. As concentration of the modifying odorant was increased, discrimination of odor mixtures was first reported because of changes in odor mixture familiarity and then intensity. We found similar sensitivity to changes in odor mixtures regardless whether the modifying compound was added or subtracted, suggesting that perceptual stability of odor mixtures is equally dependent on both imputing missing information (pattern completion) and disregarding extraneous information.

Olfactory response as a marker for Alzheimer's disease: Evidence from perceptual and frontal lobe oscillation coherence deficit

High-frequency oscillations of the frontal cortex are involved in functions of the brain that fuse processed data from different sensory modules or bind them with elements stored in the memory. These oscillations also provide inhibitory connections to neural circuits that perform lower-level processes. Deficit in the performance of these oscillations has been examined as a marker for Alzheimer's disease (AD). Additionally, the neurodegenerative processes associated with AD, such as the deposition of amyloid-beta plaques, do not occur in a spatially homogeneous fashion and progress more prominently in the medial temporal lobe in the early stages of the disease. This region of the brain contains neural circuitry involved in olfactory perception. Several studies have suggested that olfactory deficit can be used as a marker for early diagnosis of AD. A quantitative assessment of the performance of the olfactory system can hence serve as a potential biomarker for Alzheimer's disease, offering a relatively convenient and inexpensive diagnosis method. This study examines the decline in the perception of olfactory stimuli and the deficit in the performance of high-frequency frontal oscillations in response to olfactory stimulation as markers for AD. Two measurement modalities are employed for assessing the olfactory performance: 1) An interactive smell identification test is used to sample the response to a sizable variety of odorants, and 2) Electroencephalography data are collected in an olfactory perception task with a pair of selected odorants in order to assess the connectivity of frontal cortex regions. Statistical analysis methods are used to assess the significance of selected features extracted from the recorded modalities as Alzheimer's biomarkers. Olfactory decline regressed to age in both healthy and mild AD groups are evaluated, and single- and multi-modal classifiers are also developed. The novel aspects of this study include: 1) Combining EEG response to olfactory stimulation with behavioral assessment of olfactory perception as a marker of AD, 2) Identification of odorants most significantly affected in mild AD patients, 3) Identification of odorants which are still adequately perceived by mild AD patients, 4) Analysis of the decline in the spatial coherence of different oscillatory bands in response to olfactory stimulation, and 5) Being the first study to quantitatively assess the performance of olfactory decline due to aging and AD in the Iranian population.

Systematic evaluation of olfaction in patients with hereditary cystic kidney diseases/renal ciliopathies

BACKGROUND

Hereditary cystic kidney diseases such as nephronophthisis, polycystic kidney disease and Bardet-Biedl syndrome (BBS) are caused by a dysfunction of primary cilia. Cilia are involved in a variety of cellular functions and perceptions, with one of them being the sense of smell. Hyposmia is a typical feature found in patients with BBS. However, reports of olfactory dysfunction in other cystic kidney diseases are sparse. Here we provide a systematic survey on olfaction in a large cohort of patients displaying genetically determined renal ciliopathies.

METHODS

We performed a match-controlled systematic olfactory evaluation in a group of 75 patients with a defined genetic background using age adapted and validated odour identification tests.

RESULTS

Test results revealed a significant olfactory deficit in patients carrying TMEM67 variants (n=4), while all other genetic disorders causing nephronophthisis (n=25) or polycystic kidney disease (n=18) were not associated with an impaired sense of smell. Also in patients with BBS, olfactory performance was depending on the underlying molecular defect. While defects in the BBS1 gene (n=9) had no impact on the sense of smell, all other BBS gene disorders (n=19) were associated with significant hyposmia. Noteworthy, there was no correlation of the olfactory deficit with the level of renal impairment.

CONCLUSION

Hyposmia is a part of the clinical spectrum of BBS and of other renal ciliopathies. Depending on the genetic background, clinicians should be aware of this subtle and so far underappreciated symptom when clinically assessing patients with BBS or TMEM67 gene variants.

Olfactory Costimulation Influences Intranasal Somatosensory Perception

Olfactory sensitivity is influenced by intranasal trigeminal sensation. For instance, sniffing is central to how humans and animals perceive odorants. Here, we investigated the influence of olfactory costimulation on the perception of intranasal somatosensory stimulation. In this study, 22 healthy human subjects, with normal olfactory function, performed a localization task for stimulation using weak air puffs, a pure odorant, phenyl ethyl alcohol (PEA; rose odor), or their combination. Visual cues were used to inform participants to briefly hold their breath while weak, poorly localizable, air puffs and/or PEA were delivered to either nostril. Although PEA alone could not be localized to the correct nostril, when it accompanied a weak air puff in the ipsilateral nostril, localization accuracy significantly improved, relative to presentation of the air puff without the odorant. The enhancement of localization was absent when the air puff and PEA were presented to opposite nostrils. Since ipsilateral but not contralateral costimulation with PEA increased the accuracy of weak air puff localization, the results argue against a non-specific alerting effect of PEA. These findings suggest an interaction between olfactory and intranasal somatosensory stimuli leading to their integration.

Chemical intolerance: involvement of brain function and networks after exposure to extrinsic stimuli perceived as hazardous

Background

Chemical intolerance (CI) is a chronic condition characterized by recurring and severe symptoms triggered by exposure to low levels of odorous or pungent substances. The etiology of CI has been a controversial subject for a long time. The aim of this review is to summarize findings on the neurological processing of sensory information during and after exposure to low levels of odorous or pungent substances in individuals with CI, focusing on the brain function and networks.

Methods

Scientific studies on CI published between 2000 and 2019 in academic peer-reviewed journals were systematically searched using medical and scientific literature databases. Only peer-reviewed articles reporting original research from experimental human studies directly associated with CI, and involving related neurological responses or brain imaging after exposure to odorous or pungent substances (i.e., in chemical provocation tests), were considered.

Results

Forty-seven studies were found to be eligible for a full-text review. Twenty-three studies met the selection criteria and were included in this review. Evidence indicated that differences between subjects with CI and healthy controls were observed by brain imaging during and after exposure to odorous or pungent substances. Differences in brain imaging were also observed between initial exposure and after exposure to these substances. Neurological processing of sensory information after exposure to extrinsic stimuli in the limbic system and related cortices were altered in subjects with CI. A previous documentable exposure event was likely to be involved in this alteration.

Conclusions

This review documents consistent evidence for the altered neurological processing of sensory information in individuals with CI. Further neurophysiological research exploring the processing of extrinsic stimuli and cognition of sensation through the limbic system and related cortices in CI, and the appearance of symptoms in individuals with CI, are required.

Photoactivation of olfactory sensory neurons does not affect action potential conduction in individual trigeminal sensory axons innervating the rodent nasal cavity

Olfactory and trigeminal chemosensory systems reside in parallel within the mammalian nose. Psychophysical studies in people indicate that these two systems interact at a perceptual level. Trigeminal sensations of pungency mask odour perception, while olfactory stimuli can influence trigeminal signal processing tasks such as odour localization. While imaging studies indicate overlap in limbic and cortical somatosensory areas activated by nasal trigeminal and olfactory stimuli, there is also potential cross-talk at the level of the olfactory epithelium, the olfactory bulb and trigeminal brainstem. Here we explored the influence of olfactory and trigeminal signaling in the nasal cavity. A forced choice water consumption paradigm was used to ascertain whether trigeminal and olfactory stimuli could influence behaviour in mice. Mice avoided water sources surrounded by both volatile TRPV1 (cyclohexanone) and TRPA1 (allyl isothiocyanate) irritants and the aversion to cyclohexanone was mitigated when combined with a pure odorant (rose fragrance, phenylethyl alcohol, PEA). To determine whether olfactory-trigeminal interactions within the nose could potentially account for this behavioural effect we recorded from single trigeminal sensory axons innervating the nasal respiratory and olfactory epithelium using an isolated in vitro preparation. To circumvent non-specific effects of chemical stimuli, optical stimulation was used to excite olfactory sensory neurons in mice expressing channel-rhodopsin (ChR2) under the olfactory marker protein (OMP) promoter. Photoactivation of olfactory sensory neurons produced no modulation of axonal action potential conduction in individual trigeminal axons. Similarly, no evidence was found for collateral branching of trigeminal axon that might serve as a conduit for cross-talk between the olfactory and respiratory epithelium and olfactory dura mater. Using direct assessment of action potential activity in trigeminal axons we observed neither paracrine nor axon reflex mediated cross-talk between olfactory and trigeminal sensory systems in the rodent nasal cavity. Our current results suggest that olfactory sensory neurons exert minimal influence on trigeminal signals within the nasal cavity.

Correlation between olfactory function, trigeminal sensitivity, and nasal anatomy in healthy subjects

PURPOSE

Few studies have investigated the correlation between chemosensory function (trigeminal and olfactory) and nasal volume in humans, even though nasal anatomy is crucial for the sense of smell. Aim of this study was to evaluate these correlations in normosmic subjects.

METHODS

Two hundred and fifty-six healthy volunteers (age range 19-69 years) participated. Olfactory function was investigated for (the rose-like) phenylethyl alcohol odor threshold and odor identification (OI) using the Sniffin' Sticks test, while nasal structure was evaluated by acoustic rhinometry (AR); trigeminal sensitivity was assessed in terms of detection "thresholds" for the odorless carbon dioxide (CO₂).

RESULTS

There were negative correlations between olfactory sensitivity at threshold level and minimum cross-sectional area (MCSA) in both nostrils. No significant correlations were found between OI and nasal anatomy. Similar to olfactory sensitivity, with regard to the trigeminal stimulus CO₂ for the right nostril subjects were the more sensitive the smaller the MCSA.

CONCLUSIONS

The current results emphasize the significance of nasal anatomy for trigeminal/olfactory threshold perception. Interestingly, correlations were not found between suprathreshold odor identification and nasal anatomy. Other than odor identification, odor thresholds appear to depend on subtle differences in nasal anatomy.