Trigeminal system in Parkinson's disease: A potential avenue to detect Parkinson-specific olfactory dysfunction

Potential convergence of olfactory dysfunction in Parkinson's disease and COVID-19: The role of neuroinflammation

Parkinson's disease (PD) is a prevalent neurodegenerative disorder that affects 7-10 million individuals worldwide. A common early symptom of PD is olfactory dysfunction (OD), and more than 90% of PD patients suffer from OD. Recent studies have highlighted a high incidence of OD in patients with SARS-CoV-2 infection. This review investigates the potential convergence of OD in PD and COVID-19, particularly focusing on the mechanisms by which neuroinflammation contributes to OD and neurological events. Starting from our fundamental understanding of the olfactory bulb, we summarize the clinical features of OD and pathological features of the olfactory bulb from clinical cases and autopsy reports in PD patients. We then examine SARS-CoV-2-induced olfactory bulb neuropathology and OD and emphasize the SARS-CoV-2-induced neuroinflammatory cascades potentially leading to PD manifestations. By activating microglia and astrocytes, as well as facilitating the aggregation of α-synuclein, SARS-CoV-2 could contribute to the onset or exacerbation of PD. We also discuss the possible contributions of NF-κB, the NLRP3 inflammasome, and the JAK/STAT, p38 MAPK, TLR4, IL-6/JAK2/STAT3 and cGAS-STING signaling pathways. Although olfactory dysfunction in patients with COVID-19 may be reversible, it is challenging to restore OD in patients with PD. With the emergence of new SARS-CoV-2 variants and the recurrence of infections, we call for continued attention to the intersection between PD and SARS-CoV-2 infection, especially from the perspective of OD.

Assessing the reproducibility of machine-learning-based biomarker discovery in Parkinson's disease

Feature selection and machine learning algorithms can be used to analyze Single Nucleotide Polymorphisms (SNPs) data and identify potential disease biomarkers. Reproducibility of identified biomarkers is critical for them to be useful for clinical research; however, genotyping platforms and selection criteria for individuals to be genotyped affect the reproducibility of identified biomarkers. To assess biomarkers reproducibility, we collected five SNPs datasets from the database of Genotypes and Phenotypes (dbGaP) and explored several data integration strategies. While combining datasets can lead to a reduction in classification accuracy, it has the potential to improve the reproducibility of potential biomarkers. We evaluated the agreement among different strategies in terms of the SNPs that were identified as potential Parkinson's disease (PD) biomarkers. Our findings indicate that, on average, 93% of the SNPs identified in a single dataset fail to be identified in other datasets. However, through dataset integration, this lack of replication is reduced to 62%. We discovered fifty SNPs that were identified at least twice, which could potentially serve as novel PD biomarkers. These SNPs are indirectly linked to PD in the literature but have not been directly associated with PD before. These findings open up new potential avenues of investigation.

Opportunities and Pitfalls of REM Sleep Behavior Disorder and Olfactory Dysfunction as Early Markers in Parkinson's Disease

During its pre-motor stage, Parkinson's disease (PD) presents itself with a multitude of non-motor symptoms with different degrees of specificity and sensitivity. The most important among them are REM sleep behavior disorder (RBD) and olfactory dysfunction. RBD is a parasomnia characterized by the loss of REM sleep muscle atonia and dream-enacting behaviors. Olfactory dysfunction in individuals with prodromal PD is usually described as hyposmia (reduced sense of smell) or anosmia (complete loss of olfactory function). These symptoms can precede the full expression of motor symptoms by decades. A close comprehension of these symptoms and the underlying mechanisms may enable early screening as well as interventions to improve patients' quality of life. Therefore, these symptoms have unmatched potential for identifying PD patients in prodromal stages, not only allowing early diagnosis but potentially opening a window for early, possibly disease-modifying intervention. However, they come with certain challenges. This review addresses some of the key opportunities and pitfalls of both RBD and olfactory dysfunction as early markers of PD.

Trigeminal nerve stimulation: a current state-of-the-art review

Nearly 5 decades ago, the effect of trigeminal nerve stimulation (TNS) on cerebral blood flow was observed for the first time. This implication directly led to further investigations and TNS' success as a therapeutic intervention. Possessing unique connections with key brain and brainstem regions, TNS has been observed to modulate cerebral vasodilation, brain metabolism, cerebral autoregulation, cerebral and systemic inflammation, and the autonomic nervous system. The unique range of effects make it a prime therapeutic modality and have led to its clinical usage in chronic conditions such as migraine, prolonged disorders of consciousness, and depression. This review aims to present a comprehensive overview of TNS research and its broader therapeutic potentialities. For the purpose of this review, PubMed and Google Scholar were searched from inception to August 28, 2023 to identify a total of 89 relevant studies, both clinical and pre-clinical. TNS harnesses the release of vasoactive neuropeptides, modulation of neurotransmission, and direct action upon the autonomic nervous system to generate a suite of powerful multitarget therapeutic effects. While TNS has been applied clinically to chronic pathological conditions, these powerful effects have recently shown great potential in a number of acute/traumatic pathologies. However, there are still key mechanistic and methodologic knowledge gaps to be solved to make TNS a viable therapeutic option in wider clinical settings. These include bimodal or paradoxical effects and mechanisms, questions regarding its safety in acute/traumatic conditions, the development of more selective stimulation methods to avoid potential maladaptive effects, and its connection to the diving reflex, a trigeminally-mediated protective endogenous reflex. The address of these questions could overcome the current limitations and allow TNS to be applied therapeutically to an innumerable number of pathologies, such that it now stands at the precipice of becoming a ground-breaking therapeutic modality.

Impact of deep brain stimulation (DBS) on olfaction in Parkinson's disease: Clinical features and functional hypotheses

Deep brain stimulation (DBS) is a surgical therapy typically applied in Parkinson's disease (PD). The efficacity of DBS on the control of motor symptoms in PD is well grounded while the efficacity on non-motor symptoms is more controversial, especially on olfactory disorders (ODs). The present review shows that DBS does not improve hyposmia but can affect positively identification/discrimination scores in PD. The functional hypotheses suggest complex mechanisms in terms of cerebral connectivity and neurogenesis process which could act indirectly on the olfactory bulb and olfactory pathways related to specific cognitive olfactory tasks. The functional hypotheses also suggest complex mechanisms of cholinergic neurotransmitter interactions involved in these pathways. Finally, the impact of DBS on general cognitive functions in PD could also be beneficial to identification/discrimination tasks in PD.

Olfactory Bulb Amyloid-β Correlates With Brain Thal Amyloid Phase and Severity of Cognitive Impairment

The Alzheimer disease (AD) neuropathological hallmarks amyloid β (Aβ) and tau neurofibrillary (NF) pathology have been reported in the olfactory bulb (OB) in aging and in different neurodegenerative diseases, which coincides with frequently reported olfactory dysfunction in these conditions. To better understand when the OB is affected in relation to the hierarchical progression of Aβ throughout the brain and whether OB pathology might be an indicator of AD severity, we assessed the presence of OB Aβ and tau NF pathology in an autopsy cohort of 158 non demented control and 173 AD dementia cases. OB Aβ was found in less than 5% of cases in lower Thal phases 0 and 1, in 20% of cases in phase 2, in 60% of cases in phase 3 and in more than 80% of cases in higher Thal phases 4 and 5. OB Aβ and tau pathology significantly predicted a Thal phase greater than 3, a Braak NF stage greater than 4, and an MMSE score lower than 24. While OB tau pathology is almost universal in the elderly and therefore is not a good predictor of AD severity, OB Aβ pathology coincides with clinically-manifest AD and might prove to be a useful biomarker of the extent of brain spread of both amyloid and tau pathology.

Smell tests can discriminate Parkinson's disease patients from healthy individuals: A meta-analysis

BACKGROUND

Olfactory impairment is common in Parkinson's disease (PD). The authors aimed to identify the clinical tests used to assess olfactory function and examine their ability to distinguish PD with different disease duration from healthy individuals with physiological aging.

METHODS

Cross-sectional studies published until May 2020 that assessed the olfaction of individuals with PD using search terms related to PD, olfactory function, and assessment were searched on PubMed, PsycInfo, Cinahl, and Web of Science databases.

RESULTS

Twelve smell tests were identified from the reviewed studies (n = 125) that assessed 8776 individuals with PD. Data of 6593 individuals with PD and 8731 healthy individuals were included in the meta-analyses. Individuals with PD presented worse performance than healthy individuals, regardless of the smell test used. The University of Pennsylvania Smell Identification Test (UPSIT) was used by most studies (n = 2310 individuals with PD) and presented smaller heterogeneity. When the studies were subclassified according to the years of PD duration, there were no significant differences.

CONCLUSION

All smell tests were able to discriminate the olfactory function of PD from that of healthy individuals, although the UPSIT was widely used. The abnormal olfaction was not related to the disease duration. Systematic review protocol registration (PROSPERO/2020-CRD42020160878).

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.

Systematic review and meta-analysis of Sniffin Sticks Test performance in Parkinson's disease patients in different countries

INTRODUCTION

Olfaction impairment occurs in about 90% of patients with Parkinson's disease. The Sniffin Sticks Test is a widely used instrument to measure olfactory performance and is divided into three subtests that assess olfactory threshold, discrimination and identification. However, cultural and socioeconomic differences can influence test performance.

OBJECTIVES

We performed a systematic review and meta-analysis of the existent data about Sniffin Sticks Test performance of Parkinson's disease patients and healthy controls in different countries and investigated if there are other cofactors which could influence the olfactory test results. A subgroup analysis by country was performed as well as a meta-regression using age, gender and air pollution as covariates.

RESULTS

Four hundred and thirty studies were found and 66 articles were included in the meta-analysis. Parkinson's disease patients showed significantly lower scores on the Sniffin Sticks Test and all its subtests than healthy controls. Overall, the heterogeneity among studies was moderate to high as well as the intra-country heterogeneity. The subgroup analysis, stratifying by country, maintained a high residual heterogeneity.

CONCLUSION

The meta-regression showed a significant correlation with age and air pollution in a few subtests. A high heterogeneity was found among studies which was not significantly decreased after subgroup analysis by country. This fact signalizes that maybe cultural influence has a small impact on the Sniffin Sticks Test results. Age and air pollution have influence in a few olfactory subtests.

Orthonasal, but not Retronasal Olfaction Is Specifically Impaired in Parkinson's Disease

Olfactory dysfunction (OD) in Parkinson's disease (PD) appears several years before the presence of motor disturbance. Olfactory testing has the potential to serve as a tool for early detection of PD, but OD is not specific to PD as it affects up to 20% of the general population. Olfaction includes an orthonasal and a retronasal components; in some forms of OD, retronasal olfactory function is preserved. We aimed to evaluate whether combined testing components allows for discriminating between PD-related OD and non-Parkinsonian OD (NPOD). The objective of this study is to orthonasal and retronasal olfactory function in PD patients and compare them to a NPOD group and to healthy controls. We hypothesized that this combined testing allows to distinguish PD patients from both other groups. We included 32 PD patients, 25 NPOD patients, and 15 healthy controls. Both olfactory components were impaired in PD and NPOD patients, compared with controls; however, NPOD patients had significantly better orthonasal scores than PD patients. Furthermore, the ratio of retronasal/orthonasal score was higher in PD than in both other groups. In the NPOD group, orthonasal and retronasal scores were significantly correlated; no such correlation could be observed in PD patients. In summary, PD patients seem to rely on compensatory mechanisms for flavor perception. Combined orthonasal and retronasal olfactory testing may contribute to differentiate PD patients from patients with NPOD.

Machine Learning for the Diagnosis of Parkinson's Disease: A Review of Literature

Diagnosis of Parkinson's disease (PD) is commonly based on medical observations and assessment of clinical signs, including the characterization of a variety of motor symptoms. However, traditional diagnostic approaches may suffer from subjectivity as they rely on the evaluation of movements that are sometimes subtle to human eyes and therefore difficult to classify, leading to possible misclassification. In the meantime, early non-motor symptoms of PD may be mild and can be caused by many other conditions. Therefore, these symptoms are often overlooked, making diagnosis of PD at an early stage challenging. To address these difficulties and to refine the diagnosis and assessment procedures of PD, machine learning methods have been implemented for the classification of PD and healthy controls or patients with similar clinical presentations (e.g., movement disorders or other Parkinsonian syndromes). To provide a comprehensive overview of data modalities and machine learning methods that have been used in the diagnosis and differential diagnosis of PD, in this study, we conducted a literature review of studies published until February 14, 2020, using the PubMed and IEEE Xplore databases. A total of 209 studies were included, extracted for relevant information and presented in this review, with an investigation of their aims, sources of data, types of data, machine learning methods and associated outcomes. These studies demonstrate a high potential for adaptation of machine learning methods and novel biomarkers in clinical decision making, leading to increasingly systematic, informed diagnosis of PD.

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.

Patterns of olfactory dysfunctions in patients with Parkinson disease

Background

Olfactory dysfunction (OD) is a well-established nonmotor manifestations (NMM) of Parkinson disease (PD) which needs objective assessment for better understanding of the disease pathogenesis. The aim of this work was quantitative and qualitative assessment of olfactory performance in newly diagnosed PD patients.

Methods

This study was performed on 32 recently diagnosed PD patients and 24 healthy controls subjects (HCS) submitted to unified Parkinson’s disease rating scale–III (UPDRS–III), extended n-butanol Sniffin’ Sticks test (SST) and olfactory bulbs volumetry (OBV).

Results

There were significant decreases in SST threshold, discrimination, identification, and TDI variables as well as OBV in PD patients compared to HCS. The olfactory performance was negatively correlated with disease duration but had no relation with PD severity as well as motor subtype.

Conclusion

OD is highly prevalent during the early stages of PD which is both measurable and specific with identification and discrimination impairments to certain odors which makes smell performance testing an important step in PD patients’ evaluation.

Olfactory bulb surroundings can help to distinguish Parkinson's disease from non-parkinsonian olfactory dysfunction

BACKGROUND

The olfactory bulb is one of the first regions of insult in Parkinson's disease (PD), consistent with the early onset of olfactory dysfunction. Investigations of the olfactory bulb may, therefore, help early pre-motor diagnosis. We aimed to investigate olfactory bulb and its surrounding regions in PD-related olfactory dysfunction when specifically compared to other forms of non-parkinsonian olfactory dysfunction (NPOD) and healthy controls.

METHODS

We carried out MRI-based olfactory bulb volume measurements from T2-weighted imaging in scans from 15 patients diagnosed with PD, 15 patients with either post-viral or sinonasal NPOD and 15 control participants. Further, we applied a deep learning model (convolutional neural network; CNN) to scans of the olfactory bulb and its surrounding area to classify PD-related scans from NPOD-related scans.

RESULTS

Compared to controls, both PD and NPOD patients had smaller olfactory bulbs, when measured manually (both p < .001) whereas no difference was found between PD and NPOD patients. In contrast, when a CNN was used to differentiate between PD patients and NPOD patients, an accuracy of 88.3% was achieved. The cortical area above the olfactory bulb which stretches around and into the olfactory sulcus appears to be a region of interest in the differentiation between PD and NPOD patients.

CONCLUSION

Measures from and around the olfactory bulb in combination with the use of a deep learning model may help differentiate PD patients from patients with NPOD, which may be used to develop early diagnostic tools based on olfactory dysfunction.

Anatomical assessment of trigeminal nerve tractography using diffusion MRI: A comparison of acquisition b-values and single- and multi-fiber tracking strategies

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Investigation of the performance of multiple dMRI acquisitions and fiber models for trigeminal nerve (TGN) identification.

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Expert rating study of over 1000 TGN visualizations using seven proposed expert rating anatomical criteria.

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The two-tensor tractography method had better performance on identifying true positive structures, while generating more false positive streamlines in comparison to the single-tensor tractography method.

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TGN tracking performance was significantly different across the three b-values for almost all structures studied.

Background

The trigeminal nerve (TGN) is the largest cranial nerve and can be involved in multiple inflammatory, compressive, ischemic or other pathologies. Currently, imaging-based approaches to identify the TGN mostly rely on T2-weighted magnetic resonance imaging (MRI), which provides localization of the cisternal portion of the TGN where the contrast between nerve and cerebrospinal fluid (CSF) is high enough to allow differentiation. The course of the TGN within the brainstem as well as anterior to the cisternal portion, however, is more difficult to display on traditional imaging sequences. An advanced imaging technique, diffusion MRI (dMRI), enables tracking of the trajectory of TGN fibers and has the potential to visualize anatomical regions of the TGN not seen on T2-weighted imaging. This may allow a more comprehensive assessment of the nerve in the context of pathology. To date, most work in TGN tracking has used clinical dMRI acquisitions with a b-value of 1000 s/mm² and conventional diffusion tensor MRI (DTI) tractography methods. Though higher b-value acquisitions and multi-tensor tractography methods are known to be beneficial for tracking brain white matter fiber tracts, there have been no studies conducted to evaluate the performance of these advanced approaches on nerve tracking of the TGN, in particular on tracking different anatomical regions of the TGN.

Objective

We compare TGN tracking performance using dMRI data with different b-values, in combination with both single- and multi-tensor tractography methods. Our goal is to assess the advantages and limitations of these different strategies for identifying the anatomical regions of the TGN.

Methods

We proposed seven anatomical rating criteria including true and false positive structures, and we performed an expert rating study of over 1000 TGN visualizations, as follows. We tracked the TGN using high-quality dMRI data from 100 healthy adult subjects from the Human Connectome Project (HCP). TGN tracking performance was compared across dMRI acquisitions with b = 1000 s/mm², b = 2000 s/mm² and b = 3000 s/mm², using single-tensor (1T) and two-tensor (2T) unscented Kalman filter (UKF) tractography. This resulted in a total of six tracking strategies. The TGN was identified using an anatomical region-of-interest (ROI) selection approach. First, in a subset of the dataset we identified ROIs that provided good TGN tracking performance across all tracking strategies. Using these ROIs, the TGN was then tracked in all subjects using the six tracking strategies. An expert rater (GX) visually assessed and scored each TGN based on seven anatomical judgment criteria. These criteria included the presence of multiple expected anatomical segments of the TGN (true positive structures), specifically branch-like structures, cisternal portion, mesencephalic trigeminal tract, and spinal cord tract of the TGN. False positive criteria included the presence of any fibers entering the temporal lobe, the inferior cerebellar peduncle, or the middle cerebellar peduncle. Expert rating scores were analyzed to compare TGN tracking performance across the six tracking strategies. Intra- and inter-rater validation was performed to assess the reliability of the expert TGN rating result.

Results

The TGN was selected using two anatomical ROIs (Meckel's Cave and cisternal portion of the TGN). The two-tensor tractography method had significantly better performance on identifying true positive structures, while generating more false positive streamlines in comparison to the single-tensor tractography method. TGN tracking performance was significantly different across the three b-values for almost all structures studied. Tracking performance was reported in terms of the percentage of subjects achieving each anatomical rating criterion. Tracking of the cisternal portion and branching structure of the TGN was generally successful, with the highest performance of over 98% using two-tensor tractography and b = 1000 or b = 2000. However, tracking the smaller mesencephalic and spinal cord tracts of the TGN was quite challenging (highest performance of 37.5% and 57.07%, using two-tensor tractography with b = 1000 and b = 2000, respectively). False positive connections to the temporal lobe (over 38% of subjects for all strategies) and cerebellar peduncles (100% of subjects for all strategies) were prevalent. High joint probability of agreement was obtained in the inter-rater (on average 83%) and intra-rater validation (on average 90%), showing a highly reliable expert rating result.

Conclusions

Overall, the results of the study suggest that researchers and clinicians may benefit from tailoring their acquisition and tracking methodology to the specific anatomical portion of the TGN that is of the greatest interest. For example, tracking of branching structures and TGN-T2 overlap can be best achieved with a two-tensor model and an acquisition using b = 1000 or b = 2000. In general, b = 1000 and b = 2000 acquisitions provided the best-rated tracking results. Further research is needed to improve both sensitivity and specificity of the depiction of the TGN anatomy using dMRI.

Machine-learning-derived rules set excludes risk of Parkinson's disease in patients with olfactory or gustatory symptoms with high accuracy

BACKGROUND

Chemosensory loss is a symptom of Parkinson's disease starting already at preclinical stages. Their appearance without an identifiable etiology therefore indicates a possible early symptom of Parkinson's disease. Supervised machine-learning was used to identify parameters that predict Parkinson's disease among patients having sought medical advice for chemosensory symptoms.

METHODS

Olfactory, gustatory and demographic parameters were analyzed in 247 patients who had reported for chemosensory symptoms. Unsupervised machine-learning, implanted as so-called fast and frugal decision trees, was applied to map these parameters to a diagnosis of Parkinson's disease queried for in median 9 years after the first interview.

RESULTS

A symbolic hierarchical decision rule-based classifier was created that comprised d = 5 parameters, including scores in tests of odor discrimination, odor identification and olfactory thresholds, the age at which the chemosensory loss has been noticed, and a familial history of Parkinson's disease. The rule set provided a cross-validated negative predictive performance of Parkinson's disease of 94.1%; however, its balanced accuracy to predict the disease was only 58.9% while robustly above guessing.

CONCLUSIONS

Applying machine-learning techniques, a classifier was developed that took the shape of a set of six hierarchical rules with binary decisions about olfaction-related features or a familial burden of Parkinson's disease. Its main clinical strength lies in the exclusion of the possibility of developing Parkinson's disease in a patient with olfactory or gustatory loss.

Specific intranasal and central trigeminal electrophysiological responses in Parkinson's disease

Olfactory dysfunction is a frequent early non-motor symptom of Parkinson's disease (PD). There is evidence that with regard to trigeminal perception, PD-related olfactory dysfunction is different from other olfactory disorders. More specifically, trigeminal sensitivity, when measured behaviorally, was unimpaired in PD patients as opposed to patients with non-Parkinsonian olfactory dysfunction (NPOD). We sought to investigate the trigeminal pathway by measuring electrophysiological recordings from the nasal epithelium and EEG-derived event-related potentials in response to a specific trigeminal stimulus in 21 PD patients and compare them to 23 patients with NPOD and 25 controls (C). The peripheral trigeminal response, as measured by the negative-mucosa potential, showed no difference between patients with PD and controls whereas PD patients showed faster responses than patients with NPOD, the latter having shown slower and larger responses than controls (18 PD, 14 NPOD, 20 C). The central trigeminal response, as measured by event-related potentials, revealed larger early component response in PD patients compared to patients with NPOD (15 PD, 21 NPOD, 23 C). As expected, psychophysical olfactory testing showed impaired olfactory function in both groups of patients as opposed to controls. Discriminant analysis revealed a model that could predict group membership for 80% of participants based on the negative-mucosa potential latency, olfactory threshold and discrimination tests. These results provide novel insights into the pattern of trigeminal activation in PD which will help to differentiate PD-related olfactory loss from NPOD, a crucial step towards establishing early screening batteries for PD including smell tests.

The intranasal trigeminal system

Many odors activate the intranasal chemosensory trigeminal system where they produce cooling and other somatic sensations such as tingling, burning, or stinging. Specific trigeminal receptors are involved in the mediation of these sensations. Importantly, the trigeminal system also mediates sensitivity to airflow. The intranasal trigeminal and the olfactory system are closely connected. With regard to central nervous processing, it is most interesting that trigeminal stimuli can activate the piriform cortex, which is typically viewed as the primary olfactory cortex. This suggests that interactions between the two systems may form at a relatively early stage of processing. For example, there is evidence showing that acquired olfactory loss leads to reduced trigeminal sensitivity, probably on account of the lack of interaction in the central nervous system. Decreased trigeminal sensitivity may also be responsible for changes in airflow perception, leading to the impression of congested nasal airways.

Chemosensory perception is specifically impaired in Parkinson's disease

Patients with Parkinson's Disease (PD) exhibit a considerably diminished sense of smell. The olfactory system is intimately connected to the trigeminal system, responsible for the perception of sensations such as freshness, warmth or piquancy in odorants. Usually, olfactory impairment is associated with a similar reduction of trigeminal sensitivity. A recent study suggests that the trigeminal system is not affected in patients with PD. To test this, we evaluated perception of mixed olfactory/trigeminal stimuli in 23 patients with idiopathic PD and compared them to 22 healthy matched controls. More specifically, we evaluated the trigeminal dimensions of coolness, warmth and piquancy and the olfactory dimensions of pleasantness, familiarity and edibility of 10 mixed olfactory/trigeminal odorants using Likert scale. We show that PD patients perceive trigeminal sensations of coolness, warmth, and piquancy of odorants equally well as controls, as opposed to olfactory dimensions that are perceived significantly less compared to controls (p < 0.001). Moreover, Chi-square Tests show that equal number of participants in both groups perceive the trigeminal dimensions of odorants. Thus, we provide further evidence that the trigeminal system, as opposed to the olfactory system, is not impaired in PD patients reflecting a specific pattern of chemosensory impairment in PD.