Olfactory dysfunction in frontotemporal dementia and psychiatric disorders: A systematic review

Olfactory Dysfunction and Glaucoma

BACKGROUND

Olfactory dysfunction is a well-known phenomenon in neurological diseases with anosmia and hyposmia serving as clinical or preclinical indicators of Alzheimer's disease, Parkinson's disease, and other neurodegenerative disorders. Since glaucoma is a neurodegenerative disease of the visual system, it may also entail alterations in olfactory function, warranting investigation into potential sensory interconnections.

METHODS

A review of the current literature of the last 15 years (from 1 April 2008 to 1 April 2023) was conducted by two different authors searching for topics related to olfaction and glaucoma.

RESULTS

three papers met the selection criteria. According to these findings, patients with POAG appear to have worse olfaction than healthy subjects. Furthermore, certain predisposing conditions to glaucoma, such as pseudoexfoliation syndrome and primary vascular dysregulation, could possibly induce olfactory changes that can be measured with the Sniffin Stick test.

CONCLUSIONS

the scientific literature on this topic is very limited, and the pathogenesis of olfactory changes in glaucoma is not clear. However, if the results of these studies are confirmed by further research, olfactory testing may be a non-invasive tool to assist clinicians in the early diagnosis of glaucoma.

Gray matter and episodic memory associations with olfaction in middle-aged to older adults

BACKGROUND

Age-related declines in olfaction contribute to low quality of life and appear to occur with declines in cognitive function, including diminished episodic memory. We tested the hypothesis that low gray matter volume within cortical regions that support olfaction and episodic memory can explain age-related differences in olfactory and episodic memory functions.

METHODS

T1-weighted images, Sniffin' Sticks olfactory measures, and the NIH Toolbox-Cognition Battery were administered to 131 middle-aged to older adults (50-86 years; 66% female). Correlation was used to examine the associations between these measures. A network-based image processing approach was then used to examine the degree to which spatial patterns of gray matter variance were related to the olfactory and cognitive measures. Structural equation modeling was used to characterize the relative specificity of olfactory, cognitive, gray matter, and aging associations.

RESULTS

Olfactory threshold, discrimination, and identification exhibited small to medium effect size associations with episodic memory performance (rs = 0.27-0.42, ps < 0.002). Gray matter volume within medial temporal and orbitofrontal cortex was also related to olfactory (discrimination and identification) and episodic memory function (rs = 0.21-0.36, ps < 0.019). Age and episodic memory explained the same variance in olfaction that was explained by the medial temporal and orbitofrontal pattern of gray matter volume.

CONCLUSIONS

The results of this cross-sectional study suggest that identifying mechanisms contributing to differences in medial temporal and orbitofrontal cortex will advance our understanding of co-morbid olfactory and cognitive declines.

Olfactory status in neurodegeneration with brain iron accumulation disorders

BACKGROUND

Olfactory dysfunction has been suggested as a diagnostic and discriminative biomarker in some neurodegenerative disorders. However, there are few studies regarding the olfactory status in rare diseases including neurodegeneration with brain iron accumulation (NBIA) disorders.

METHODS

Genetically-confirmed NBIA patients were enrolled. Neurological and cognitive examinations were conducted according to the Pantothenate Kinase-Associated Neurodegeneration-Disease Rating Scale (PKAN-DRS) and the Mini-Mental State Examination (MMSE) questionnaire, respectively. Olfaction was assessed in three domains of odor threshold (OT), odor discrimination (OD), odor identification (OI), and total sum (TDI) score by the Sniffin' Sticks test. The olfactory scores were compared to a control group and a normative data set.

RESULTS

Thirty-seven patients, including 22 PKAN, 6 Kufor Rakeb syndrome, 4 Mitochondrial membrane Protein-Associated Neurodegeneration (MPAN), 5 cases of other 4 subtypes, and 37 controls were enrolled. The mean PKAN-DRS score was 51.83±24.93. Sixteen patients (55.2%) had normal cognition based on MMSE. NBIA patients had significantly lower olfactory scores compared to the controls in TDI and all three subtests, and 60% of them were hyposmic according to the normative data. Including only the cognitively-normal patients, still, OI and TDI scores were significantly lower compared to the controls. The phospholipase A2-Associated Neurodegeneration (PLAN) and MPAN patients had a significantly lower OI score compared to the cognitively-matched PKAN patients.

CONCLUSION

Olfactory impairment as a common finding in various subtypes of NBIA disorder can potentially be considered a discriminative biomarker. Better OI in PKAN compared to PLAN and MPAN patients may be related to the different underlying pathologies.

The anterior olfactory nucleus revisited - an emerging role for neuropathological conditions?

Olfaction is an important sensory modality for many species and greatly influences animal and human behavior. Still, much about olfactory perception remains unknown. The anterior olfactory nucleus is one of the brain's central early olfactory processing areas. Located directly posterior to the olfactory bulb in the olfactory peduncle with extensive in- and output connections and unique cellular composition, it connects olfactory processing centers of the left and right hemispheres. Almost 20 years have passed since the last comprehensive review on the anterior olfactory nucleus has been published and significant advances regarding its anatomy, function, and pathophysiology have been made in the meantime. Here we briefly summarize previous knowledge on the anterior olfactory nucleus, give detailed insights into the progress that has been made in recent years, and map out its emerging importance in translational research of neurological diseases.

Photobiomodulation therapy restores olfactory function impaired by photothrombosis in mouse olfactory bulb

An ischemic stroke typically accompanies numerous disorders ranging from somatosensory dysfunction to cognitive impairments, inflicting patients with various neurologic symptoms. Among pathologic outcomes, post-stroke olfactory dysfunctions are frequently observed. Despite the well-known prevalence, therapy options for such compromised olfaction are limited, likely due to the complexity of olfactory bulb architecture, which encompasses both the peripheral and central nervous systems. As photobiomodulation (PBM) emerged for treating ischemia-associated symptoms, the effectiveness of PBM on stroke-induced impairment of olfactory function was explored. Novel mouse models with olfactory dysfunctions were prepared using photothrombosis (PT) in the olfactory bulb on day 0. The post-PT PBM was performed daily from day 2 to day 7 by irradiating the olfactory bulb via an 808 nm laser with a fluence of 40 J/cm2 (325 mW/cm2 for 2 min per day). The buried food test (BFT) was used to score behavioral acuity in food-deprived mice to assess the olfactory function before PT, after PT, and after PBM. Histopathological examinations and cytokine assays were performed on the mouse brains harvested on day 8. The results from BFT were specific to an individual, with positive correlations between the baseline latency time measured before PT and its alteration at the ensuing stages for both the PT and PT + PBM groups. Also, the correlation analysis in both groups showed highly similar, significant positive relationships between the early and late latency time change independent of PBM, implicating a common recovery mechanism. Particularly, PBM treatment accelerated the recovery of impaired olfaction following PT by suppressing inflammatory cytokines and enhancing both glial and vascular factors (e.g., GFAP, IBA-1, and CD31). PBM therapy during the acute phase of ischemia improves the compromised olfactory function by modulating microenvironments and inflammation status of the affected tissue.

Olfactory Dysfunction in Mental Illness

PURPOSE OF REVIEW

Olfactory dysfunction contributes to the psychopathology of mental illness. In this review, we describe the neurobiology of olfaction, and the most common olfactory alterations in several mental illnesses. We also highlight the role, hitherto underestimated, that the olfactory pathways play in the regulation of higher brain functions and its involvement in the pathophysiology of psychiatric disorders, as well as the effect of inflammation on neurogenesis as a possible mechanism involved in olfactory dysfunction in psychiatric conditions.

RECENT FINDINGS

The olfactory deficits present in anxiety, depression, schizophrenia or bipolar disorder consist of specific alterations of different components of the sense of smell, mainly the identification of odours, as well as the qualifications of their hedonic valence (pleasant or unpleasant). Epidemiological findings have shown that both environmental factors, such as air pollutants, and inflammatory disease of the upper respiratory tract, can contribute to an increased risk of mental illness, at least in part, due to peripheral inflammatory mechanisms of the olfactory system. In this review, we describe the neurobiology of olfaction, and the most common olfactory function alterations in several psychiatric conditions and its role as a useful symptom for the differential diagnosis. We also highlight the effect of inflammation on neurogenesis as a possible mechanism involved in olfactory dysfunction in these psychiatric conditions.

Neurons, Nose, and Neurodegenerative Diseases: Olfactory Function and Cognitive Impairment

Olfactory capacity declines with aging, but increasing evidence shows that smell dysfunction is one of the early signs of prodromal neurodegenerative diseases such as Alzheimer's and Parkinson's disease. The study of olfactory ability and its role in neurodegenerative diseases arouses much interest in the scientific community. In neurology, olfactory impairment is a potential early marker for the onset of neurodegenerative diseases, but the underlying mechanism is poorly understood. The loss of smell is considered a clinical sign of early-stage disease and a marker of the disease's progression and cognitive impairment. Highlighting the importance of biological bases of smell and molecular pathways could be fundamental to improve neuroprotective and therapeutic strategies. We focused on the review articles and meta-analyses on olfactory and cognitive impairment. We depicted the neurobiology of olfaction and the most common olfactory tests in neurodegenerative diseases. In addition, we underlined the close relationship between the olfactory and cognitive deficit due to nasal neuroepithelium, which is a direct extension of the CNS in communication with the external environment. Neurons, Nose, and Neurodegenerative diseases highlights the role of olfactory dysfunction as a clinical marker for early stages of neurodegenerative diseases when it is associated with molecular, clinical, and neuropathological correlations.

Cognitive Impairment in Older Adults: Epidemiology, Diagnosis, and Treatment

Cognitive impairment and dementia affect dozens of millions of people worldwide and cause significant distress to patients and caregivers and a financial burden to families and health care systems. Careful history-taking, cognitive and physical examination, and supplemental neuroimaging and fluid-based biomarkers can accurately diagnose neurocognitive disorders. Management includes non-pharmacological and pharmacological treatments tailored to the etiology and to the individual.

Comparing the effects of olfactory-based sensory stimulation and board game training on cognition, emotion, and blood biomarkers among individuals with dementia: A pilot randomized controlled trial

Olfactory dysfunction can indicate early cognitive decline and is associated with dementia symptoms. We developed an olfactory-based sensory stimulation program and investigated its effects on cognition and emotion, and board game training were used as a comparison. In this parallel design pilot study, 30 participants with mild to moderate dementia were equal randomly assigned to the control (CONT), olfactory stimulation with cognitive training (OS), and board game (BG) groups. Two participants were withdrawn from CONT and OS groups, respectively. The intervention was a 12-week program with one 30-min session twice a week. We employed a blood-based biomarker technique and several cognitive and psychological tests to measure basal and after-intervention values. No significant differences were observed between the groups after intervention, as measured using the Mini-Mental State Examination, Loewenstein Occupational Therapy Cognitive Assessment (LOTCA), Top International Biotech Smell Identification Test, and Geriatric Depression Scale (GDS). The results showed that the OS group had a lower plasma Tau level than the other groups following intervention, whereas the CONT group had a significantly increased plasma amyloid ß1-42 level. OS participants had a lower concentration ratio of plasma Tau and amyloid Aß1-42 and showed more stable or improved cognition, olfactory function, and mood state. Both the OS and BG groups had a higher percentage of participants with stable or improved cognition and emotion. Taken together, these results suggest that olfactory-based sensory stimulation can be a beneficial intervention for patients with dementia.

International consensus statement on allergy and rhinology: Olfaction

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

Caspase Inhibition Restores NEP Expression and Rescues Olfactory Deficit in Rats Caused by Prenatal Hypoxia

Development of the olfactory system begins early in embryogenesis and is important for the survival of new-borns in postnatal life. Olfactory malfunction in early life disrupts development of behavioural patterns while with ageing manifests development of neurodegenerative disorders. Previously, we have shown that prenatal hypoxia in rats leads to impaired olfaction in the offspring and correlates with reduced expression of a neuropeptidase neprilysin (NEP) in the brain structures involved in processing of the olfactory stimuli. Prenatal hypoxia also resulted in an increased activity of caspases in rat brain and its inhibition restored NEP content in the brain tissue and improved rat memory. In this study, we have analysed effects of intraventricular administration of a caspase inhibitor Ac-DEVD-CHO on NEP mRNA expression, the number of dendritic spines and olfactory function of rats subjected to prenatal hypoxia on E14. The data obtained demonstrated that a single injection of the inhibitor on P20 restored NEP mRNA levels and number of dendritic spines in the entorhinal and parietal cortices, hippocampus and rescued rat olfactory function in food search and odour preference tests. The data obtained suggest that caspase activation caused by prenatal hypoxia contributes to the olfactory dysfunction in developing animals and that caspase inhibition restores the olfactory deficit via upregulating NEP expression and neuronal networking. Because NEP is a major amyloid-degrading enzyme, any decrease in its expression and activity not only impairs brain functions but also predisposes to accumulation of the amyloid-β peptide and development of neurodegeneration characteristic of Alzheimer's disease.

Interactive Effects of Agitation and Cognitive Impairment on Odor Identification in Patients With Late-Life Depression

BACKGROUND

Late-life depression (LLD) is a risk factor for cognitive decline in older adults, and odor identification (OI) deficits are an early indicator of cognitive decline with LLD. However, neuropsychiatric symptoms (NPSs) are common in LLD and are associated with OI deficits. In subjects with LLD, when OI deficits forecast cognitive decline, whether and how NPS affects the relationship between OI and cognition still must be further explored.

OBJECTIVE

To comprehensively explore the potential effects of various NPSs on the relationship between OI and cognition in participants with LLD.

METHODS

There were 167 patients with LLD and 105 normal elderly (NE) participants. The odor identification test (Sniffin' Sticks), cognitive function assessments (global cognition, memory, executive function, attention, language, visual space), and an NPS assessment (the neuropsychiatric inventory questionnaire) were performed on the subjects. In patients with LLD, the relationship among OI, cognition and NPSs was examined using correlation analysis and moderation analysis.

RESULTS

In patients with LLD, OI was positively correlated with cognition (global cognition, memory, executive function, attention, language) and negatively associated with NPSs (agitation and aberrant motor behavior). In NE group, OI was correlated with executive function. Moderation analysis showed that there was an interactive effect of agitation and cognitive impairment (language deficit or attention deficit) on OI in patients with LLD.

CONCLUSION

The coexistence of agitation and language or attention deficit was associated with worse OI in subjects with LLD. Agitation should be considered since OI predicts cognitive decline in patients with LLD.

The Olfactory System as Marker of Neurodegeneration in Aging, Neurological and Neuropsychiatric Disorders

Research studies that focus on understanding the onset of neurodegenerative pathology and therapeutic interventions to inhibit its causative factors, have shown a crucial role of olfactory bulb neurons as they transmit and propagate nerve impulses to higher cortical and limbic structures. In rodent models, removal of the olfactory bulb results in pathology of the frontal cortex that shows striking similarity with frontal cortex features of patients diagnosed with neurodegenerative disorders. Widely different approaches involving behavioral symptom analysis, histopathological and molecular alterations, genetic and environmental influences, along with age-related alterations in cellular pathways, indicate a strong correlation of olfactory dysfunction and neurodegeneration. Indeed, declining olfactory acuity and olfactory deficits emerge either as the very first symptoms or as prodromal symptoms of progressing neurodegeneration of classical conditions. Olfactory dysfunction has been associated with most neurodegenerative, neuropsychiatric, and communication disorders. Evidence revealing the dual molecular function of the olfactory receptor neurons at dendritic and axonal ends indicates the significance of olfactory processing pathways that come under environmental pressure right from the onset. Here, we review findings that olfactory bulb neuronal processing serves as a marker of neuropsychiatric and neurodegenerative disorders.

Olfaction and anosmia: From ancient times to COVID-19

Olfaction, one of our five main qualitative sensory abilities, is the action of smelling or the capacity to smell. Olfactory impairment can be a sign of a medical problem, from a benign nasal/sinus problem up to a potentially serious brain injury. However, although clinicians (neurologists or not) usually test the olfactory nerves in specific clinical situations (for example, when a neurodegenerative disorder is suspected), they may omit such tests in many other situations. With the recent COVID-19 pandemic, the resurgence of anosmia has reminded us of the importance of testing this sensorineural function. We retrace here the main historical steps and discoveries concerning olfaction and anosmia.

Development of the Spanish Version of Sniffin's Sticks Olfactory Identification Test: Normative Data and Validity of Parallel Measures

The Sniffin' Sticks Olfactory Identification Test is a tool for measurement of olfactory performance developed in Germany and validated in several countries. This research aims to develop the Spanish version of the Sniffin' Sticks Olfactory Identification Test and obtain normative values for the Spanish population. The parameters are free recall and subjective intensity of odorants are included. The influence of possible demographic covariates such as sex, age, smoking, or educational level are analyzed, and the items that best discriminate are studied. In addition, the internal structure validity of the blue and purple versions is studied as a parallel measure, and a cultural adaptation of the purple version is carried out. For this, three independent samples of normosmic healthy volunteers were studied. To obtain normative values, the sample was of 417 participants (18-89 years). For the internal structure validity study of both versions, the sample was 226 (18-70 years), and for familiarity of the purple version, the sample was 75 participants (21-79 years). Results indicated that men and women and smokers and non-smokers perform equally. However, differences were found as age progresses, being more pronounced after 60 years old in all three measurements of the identification test. This research also provides the items that best discriminate in the blue version and a cultural adaptation for the purple version. In conclusion, the Sniffin' Sticks Odor Identification Test is a suitable tool for olfactory assessment in the Spanish population. The instrument has been expanded with two new scores, and normative data as a function of age are provided. Its parallel version also seems appropriate for testing, as items have been culturally adapted and evidence of internal structure validity for both versions is reported.

Relationship between depression and olfactory sensory function: a review

Links between olfactory sensory function and effect have been well established. A robust literature exists in both humans and animals showing that disrupting olfaction sensory function can elicit disordered mood state, including serve as a model of depression. Despite this, considerably less is known regarding the directionality and neural basis of this relationship, e.g. whether disruptions in sensory function precede and contribute to altered mood or if altered mood state precipitates changes in olfactory perception. Further, the neural basis of altered olfactory function in depression remains unclear. In conjunction with clinical studies, animal models represent a valuable tool to understand the relationship between altered mood and olfactory sensory function. Here, we review the relevant literature assessing olfactory performance in depression in humans and in rodent models of depressive-like behavioral states. Rodents allow for detailed characterization of alterations in olfactory perception, manipulation of experiential events that elicit depressive-like phenotypes, and allow for interrogation of potential predictive markers of disease and the cellular basis of olfactory impairments associated with depressive-like phenotypes. We synthesize these findings to identify paths forward to investigate and understand the complex interplay between depression and olfactory sensory function.

Spanish Validation for Olfactory Function Testing Using the Sniffin' Sticks Olfactory Test: Threshold, Discrimination, and Identification

The assessment of olfactory function is becoming increasingly relevant, especially in cases of cognitive decline (i.e., neurodegenerative diseases), where olfactory alterations may be relevant as potential early biomarkers. The Sniffin' Sticks Olfactory Test, developed in Germany and validated in several countries, is an objective measure of olfactory performance. This study aims to validate this test in a Spanish sample. This study included 209 healthy normosmic volunteers (154 females and 55 males) aged between 20 to 79 years (mean age = 50.11 ± 15.18 years) as the normative sample. From this group, 22 participants were retested in order to obtain test-retest reliability evidence. Odor familiarity for descriptors in the olfactory identification test was also studied on an independent healthy sample (n = 69), and required cultural modifications were applied. Results indicate that men and women, as well as smokers and non-smokers, performed equally in every test. However, significant differences were found between age groups in every score. The general trend is that olfactory function progressively decreases as a function of age, the elderly group (+60 years) being the one with the lowest scores. In conclusion, this normative data, in addition to the test's cultural modifications, allows the Sniffin' Sticks Olfactory Test to be administered on a Spanish population.