Odor identification impairments in schizophrenia: relationship with demographic measures, clinical variables, and diagnostic subtypes

Association between social determinants of health and olfactory function: a scoping review

BACKGROUND

Social determinants of health (SDoH) include the socioeconomic, demographic, and social conditions that influence differences in health status among individuals and groups. The impact of these conditions on olfactory function remains poorly understood. In this scoping review, we systematically review the available literature to synthesize the association between SDoH and olfactory function.

METHODS

In accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses extension for Scoping Review (PRISMA-ScR) guidelines, we performed systematic search queries in PubMed, Embase, and Ovid databases and categorized articles according to themes that emerged regarding SDoH. The primary outcomes included self-reported and objective measurements of smell.

RESULTS

We identified 722 unique references that underwent title and abstract review by two independent reviewers, with 70 articles undergoing full-text review and 57 relevant for data extraction. Six themes emerged in our review, under which we categorized the studies and synthesized respective associations with olfactory function. These include studies exploring socioeconomic status (n = 19, 33%), education status (n = 27, 47%), occupational exposures (n = 26, 46%), racial/ethnic disparities (n = 12, 21%), and lifestyle/behavioral factors (n = 33, 58%).

CONCLUSIONS

Within the context of this scoping review, olfactory dysfunction is significantly more prevalent in patients with lower socioeconomic status, exposure to environmental and occupational toxins, and of minority race/ethnicity, whereas the associations between olfactory dysfunction and education level and lifestyle factors such as smoking and drinking seem to be much more elusive. This review highlights the importance of accounting for SDoH in observational studies examining olfactory outcomes. Given the increased awareness of olfactory loss, special consideration should be given to understanding olfactory dysfunction in the context of these factors.

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

Frontotemporal dementia (FTD) is a progressive neurodegenerative disease. Diagnosis of FTD, especially the behavioural variant, is challenging because of symptomatic overlap with psychiatric disorders (depression, schizophrenia, bipolar disorder). Olfactory dysfunction is common in both FTD and psychiatric disorders, and often appears years before symptom onset. This systematic review analysed 74 studies on olfactory function in FTD, depression, schizophrenia and bipolar disorder to identify differences in olfactory dysfunction profiles, focusing on the most common smell measures: odour identification and discrimination. Results revealed that FTD patients were severely impaired in odour identification but not discrimination; in contrast, patients diagnosed with schizophrenia showed impairments in both measures, while those diagnosed with depression showed no olfactory impairments. Findings in bipolar disorder were mixed. Therefore, testing odour identification and discrimination differentiates FTD from depression and schizophrenia, but not from bipolar disorder. Given the high prevalence of odour identification impairments in FTD, and that smell dysfunction predicts neurodegeneration in other diseases, olfactory testing seems a promising avenue towards improving diagnosis between FTD and psychiatric disorders.

Can olfactory dysfunction be a marker of trait or states of bipolar disorders? A comprehensive review

BACKGROUND

Olfactory deficits (OD) are reported as markers for a large spectrum of neuro-psychiatric disorders. Alterations can concern perception, identification, discrimination and assignment of odour's valence of olfaction process. We propose a comprehensive review to summarize which kind of OD were reported in bipolar disorders (BD) and in which phase of the disease, to know if they could be a marker of state or trait.

METHODS

A Systematic Literature Review was conducted using PRISMA guidelines to include all studies assessing olfaction with objective measures in BD.

RESULTS

9 studies were identified. All of them have assessed odour identification and 3 reported deficits mainly in patients with psychotic features or elements of illness severity in comparison to healthy subjects. There is no difference in threshold of perception between BD patients and controls and it is no possible to conclude for discrimination because only one study has assessed this dimension in comparison to control. We cannot conclude for hedonic value of odours regarding these studies.

LIMITATIONS

These studies are very incomplete because only one has evaluated all the processes involved in olfaction process.

CONCLUSIONS

In light of this review, evidence is still missing to unveil potential disturbances of olfactory process as a marker of BD. These new avenues of research could help to clarify the links between OD and BD and provide information on the pathophysiology of the disorder according to the impaired dimension.

Sex differences in hedonic judgement of odors in schizophrenia cases and healthy controls

The neurocircuitries subserving affective and olfactory processes overlap, are sexually dimorphic, and show disruptions in schizophrenia, suggesting their intersection may be a window on the core process producing psychosis. This study investigated diagnostic and sex differences in hedonic judgments of odors and smell identification in 26 schizophrenia cases and 27 healthy controls. Associations between olfaction measures and psychiatric symptoms were also examined. Cases and controls had similar identification accuracy of unpleasant odors, but cases were significantly less accurate in naming pleasant odors. In cases, greater negative symptom severity was related to abnormal hedonic judgments; specifically, higher pleasantness ratings for unpleasant odors and higher unpleasantness ratings for pleasant odors. Greater positive symptom severity was associated with lower pleasantness ratings for neutral odors. Regarding sex differences, male cases and female controls rated pleasant odors as significantly more unpleasant than male controls. Correlations between depression severity and pleasantness ratings of neutral odors were in opposite directions in male and female cases. These results suggest that a normal sexual dimorphism in the circuitry for hedonic odor judgments may interact with schizophrenia pathology, supporting the utility of olfactory hedonics as a sex-specific biomarker of this pathology.

Overrated hedonic judgment of odors in patients with schizophrenia

AIMS

The odor identification ability and its hedonic judgment in patients with schizophrenia were evaluated in the study. The association between olfactory performance and negative symptoms and β-endorphin concentration was also analyzed.

METHODS

Study groups consisted of 23 patients with negative symptoms (PN) and 25 without predominant negative symptoms (PP) and 21 healthy individuals. The University of Pennsylvania Smell Identification Test, odor hedonic evaluation, and plasma concentrations of β-endorphin assay in all participants were performed.

RESULTS

PN perceived the poorer olfactory identification; nevertheless, they evaluated unpleasant odors as more pleasant than PP and controls. Beta-endorphin concentration was significantly higher among PN than in other study groups. No association was observed between β-endorphin and odors identification and odor hedonic judgment among all study groups.

CONCLUSIONS

There is potential relationship between increased β-endorphin concentration and severity of negative symptoms. Patients with predominant negative symptoms tend to evaluate odors as significantly more pleasant. Individuals with this subtype of schizophrenia might present specific, altered pattern of smell identification and hedonic judgment. Presumably, β-endorphin has no direct influence on olfactory identification performance and hedonic judgment in schizophrenia.

The tetrapartite synapse: a key concept in the pathophysiology of schizophrenia

Growing evidence points to synaptic pathology as a core component of the pathophysiology of schizophrenia (SZ). Significant reductions of dendritic spine density and altered expression of their structural and molecular components have been reported in several brain regions, suggesting a deficit of synaptic plasticity. Regulation of synaptic plasticity is a complex process, one that requires not only interactions between pre- and post-synaptic terminals, but also glial cells and the extracellular matrix (ECM). Together, these elements are referred to as the ‘tetrapartite synapse’, an emerging concept supported by accumulating evidence for a role of glial cells and the extracellular matrix in regulating structural and functional aspects of synaptic plasticity. In particular, chondroitin sulfate proteoglycans (CSPGs), one of the main components of the ECM, have been shown to be synthesized predominantly by glial cells, to form organized perisynaptic aggregates known as perineuronal nets (PNNs), and to modulate synaptic signaling and plasticity during postnatal development and adulthood. Notably, recent findings from our group and others have shown marked CSPG abnormalities in several brain regions of people with SZ. These abnormalities were found to affect specialized ECM structures, including PNNs, as well as glial cells expressing the corresponding CSPGs. The purpose of this review is to bring forth the hypothesis that synaptic pathology in SZ arises from a disruption of the interactions between elements of the tetrapartite synapse.

Olfaction in the psychosis prodrome: electrophysiological and behavioral measures of odor detection

Smell identification deficits (SIDs) are relatively specific to schizophrenia and its negative symptoms, and may predict transition to psychosis in clinical high-risk (CHR) individuals. Moreover, event-related potentials (ERPs) to odors are reduced in schizophrenia. This study examined whether CHR patients show SIDs and abnormal olfactory N1 and P2 potentials. ERPs (49 channels) were recorded from 21 CHR and 20 healthy participants (13 males/group; ages 13-27 years) during an odor detection task using three concentrations of hydrogen sulfide (H2S) or blank air presented unilaterally by a constant-flow olfactometer. Neuronal generator patterns underlying olfactory ERPs were identified and measured by principal components analysis (unrestricted Varimax) of reference-free current source densities (CSD). Replicating previous findings, CSD waveforms to H2S stimuli were characterized by an early N1 sink (345 ms, lateral-temporal) and a late P2 source (600 ms, mid-frontocentroparietal). N1 and P2 varied monotonically with odor intensity (strong > medium > weak) and did not differ across groups. Patients and controls also showed comparable odor detection and had normal odor identification and thresholds (Sniffin' Sticks). However, olfactory ERPs strongly reflected differences in odor intensity and detection in controls, but these associations were substantially weaker in patients. Moreover, severity of negative symptoms in patients was associated with reduced olfactory ERPs and poorer odor detection, identification and thresholds. Three patients who developed psychosis had poorer odor detection and thresholds, and marked reductions of N1 and P2. Thus, despite the lack of overall group differences, olfactory measures may be of utility in predicting transition to psychosis among CHR patients.

Effects of adjunctive intranasal oxytocin on olfactory identification and clinical symptoms in schizophrenia: results from a randomized double blind placebo controlled pilot study

BACKGROUND

Deficits in olfactory identification have been widely reported in patients with schizophrenia (SZ) and are associated with negative symptomatology. Adjunctive oxytocin delivered intranasally has been shown to improve some aspects of social cognition as well as positive and negative symptoms in patients with schizophrenia. Given the intranasal delivery route of oxytocin to olfactory pathways and that olfactory abnormalities are a potential endophenotype in SZ, we investigated the effect of intranasal oxytocin on olfactory identification as well as positive and negative symptoms in people with schizophrenia.

METHODS

Individuals with schizophrenia or schizoaffective disorder (n=28; 16 outpatients, 12 inpatients) were randomized to receive adjunctive intranasal oxytocin 20 IU BID or placebo for 3 weeks.

RESULTS

All 28 participants completed the clinical trial. Odor identification performance significantly improved on the University of Pennsylvania Smell Identification Test (UPSIT) total score and subscore for pleasant smells. UPSIT score (F=5.20, df=1,23, p=0.032) and subscore for pleasant smells (F=4.56, df=1,23, p=0.044), in patients treated with oxytocin were compared to placebo from baseline to endpoint. Global symptomatology as well as positive and negative symptoms were not improved by intranasal oxytocin. In fact, global symptoms, not positive or negative symptoms, improved in the placebo group. Secondary analysis shows that intranasal oxytocin improved negative symptoms in the small group of inpatients. Intranasal oxytocin was well tolerated during the three week trial.

CONCLUSION

Adjunctive intranasal oxytocin may improve olfactory identification, particularly in items of positive valence. Larger studies are needed to determine the effects of oxytocin on negative symptoms in SZ. (NCT00884897; http://www.clinicaltrials.gov).

MicroRNA-382 expression is elevated in the olfactory neuroepithelium of schizophrenia patients

Schizophrenia is a common neuropsychiatric disorder that has a strong genetic component. MicroRNAs (miRNAs) have been implicated in neurodevelopmental and psychiatric disorders including schizophrenia, as indicated by their dysregulation in post-mortem brain tissues and in peripheral blood of schizophrenia patients. The olfactory epithelium (OE) is one of the few accessible neural tissues that contain neurons and their stem cells. Previous studies showed that OE-derived tissues and cells can be safely and easily collected from live human subjects and may provide a "window" into neuronal processes involved in disorders such as schizophrenia, while avoiding the limitations of using postmortem brain samples or non-neuronal tissues. In this study, we found that the brain-enriched miR-382 (miR-382-5p) expression was elevated in in vitro cultured olfactory cells, in a cohort of seven schizophrenia patients compared with seven non-schizophrenic controls. MiR-382 elevation was further confirmed in laser-capture microdissected OE neuronal tissue (LCM-OE), enriched for mature olfactory neurons, in a cohort of 18 schizophrenia patients and 18 non-schizophrenic controls. In sharp contrast, miR-382 expression could not be detected in lymphoblastoid cell lines generated from schizophrenic or non-schizophrenic individuals. We further found that miR-382 directly regulates the expression of two genes, FGFR1 and SPRY4, which are downregulated in both the cultured olfactory cells and LCM-OE derived from schizophrenia patients. These genes are involved in the fibroblast growth factor (FGF) signaling pathway, while impairment of this pathway may underlie abnormal brain development and function associated with schizophrenia. Our data suggest that miR-382 elevation detected in patients' OE-derived samples might serve to strengthen current biomarker studies in schizophrenia. This study also illustrates the potential utility of OE-derived tissues and cells as surrogate samples for the brain.

Olfaction in affective and anxiety disorders: a review of the literature

BACKGROUND

Olfaction and its relation to mental health is an area of growing interest. Brain areas linked to olfaction partially overlap with brain areas involved in psychiatric disorders; consequently, the study of olfactory function allows us to explore the integrity of these brain areas with a non-invasive and effective method. Accordingly, the aim of this paper is to review olfactory function in affective and anxiety disorders.

METHODS

For this purpose, an extensive literature review of English-language studies on olfactory function in patients with the aforementioned pathologies was performed using several online databases. A manual search of relevant journals and books as well as reference lists from selected papers was also performed.

RESULTS

The available data show that depressed patients are usually characterised by preserved olfactory function, except for detection threshold, where contrasting reports have been found. Bipolar disorder has been studied to a lesser extent, but the findings have shown a lack of impairment in most cases. Research on seasonal affective disorders is scant, and future studies are needed to make conclusions. Anxiety disorders have been scarcely approached, but the results note identification deficits in obsessive-compulsive and posttraumatic stress disorders.

CONCLUSIONS

Olfactory assessment appears to be a complementary, valuable research tool in the study of psychiatric disorders. However, further investigation is needed to improve our understanding of olfactory function in these disorders.

Olfactory processing, sex effects and heterogeneity in schizophrenia

INTRODUCTION

Smell identification deficits are associated with negative symptoms in schizophrenia, particularly in males. Far less information is known about the relationship of odor detection sensitivity (acuity) and negative symptoms in schizophrenia, and currently there is a dearth in sex-stratified research specifically examining odor sensitivity and smell identification.

METHODS

Fifty-eight individuals with schizophrenia and 42 healthy comparison subjects were assessed on tests of odor sensitivity, smell identification and cognition. Negative symptoms were assessed with the Positive and Negative Syndrome Scale and the Schedule for the Deficit Syndrome.

RESULTS

In healthy males, increased odor detection sensitivity predicted better smell identification scores. In contrast, male schizophrenia patients showed a significant inverse relationship, in which increased odor sensitivity predicted lower smell identification scores. Odor sensitivity and smell identification were unrelated in both schizophrenia and healthy females. Olfactory processing was strongly linked to negative symptoms, but the relationships differed by sex. Emotional expression deficits were related to odor detection hypersensitivity in female patients, whereas smell identification deficits predicted these emotional deficits in male cases.

CONCLUSION

Sex differences in olfactory functioning were identified in healthy subjects and in schizophrenia patients. Smell identification was related to negative symptoms in males with schizophrenia, whereas odor detection sensitivity predicted these features in females. Sex differences should be considered in future analyses that employ odor stimuli for neuropsychiatric research.

Olfactory acuity is associated with mood and function in a pilot study of stable bipolar disorder patients

OBJECTIVES

Olfactory dysfunction is described in several neuropsychiatric disorders but there is little research on olfactory processing in bipolar disorder.

METHODS

We assessed odor detection threshold (sensitivity) and smell identification test scores, along with symptoms, cognition, and social function in 20 DSM-IV bipolar disorder patients and 44 control subjects.

RESULTS

The patient and control groups had similar demographic measures, intelligence, and mean olfaction scores, but significantly differed in social domains, including adjustment, function, and anxiety. Odor detection sensitivity showed significantly opposite correlations for the depressive and manic mood domains in bipolar disorder (r to z = 2.83, p = 0.005). Depressive symptoms were related to increased sensitivity (the ability to detect odors at a lower concentration) and mania symptoms were related to decreased sensitivity for odor detection. Increased sensitivity for odor detection also predicted significantly better employment (r = -0.642, p = 0.024), whereas less sensitivity was associated with social avoidance (r = 0.702, p =0.024) and social fear (r = 0.610, p = 0.046).

CONCLUSIONS

Diminished odor detection sensitivity predicted mania and social avoidance, whereas more sensitive odor detection predicted more depressive symptoms but better employment functioning in bipolar disorder patients. Odor acuity may be an illness state marker of mood syndromes in bipolar disorder. Alternatively, differences in odor acuity may identify heterogeneous subgroups within the bipolar spectrum. Longitudinal assessments in a large, sex-stratified sample are needed to understand the implications of odor sensitivity in patients with bipolar disorder.

Negative symptoms of schizophrenia correlate with impairment on the University of Pennsylvania smell identification test

Deficits in odor identification have been most frequently described in schizophrenia (SZ). A relationship between dysfunction in odor identification and negative symptoms of SZ has also been reported. Furthermore, deficit SZ (a subtype of the illness with primary, enduring negative symptoms) has been found to be associated with a particularly poor performance on odor identification tests indicating that deficits in smell identification could be differentially expressed in some subtypes of SZ. We describe correlations of performance on smell identification with positive and negative symptoms of SZ. Patients with SZ (n=15) and normal controls (n=19) were tested by the University of Pennsylvania Smell Identification Test (UPSIT). Psychopathology was assessed with the Scales for the Assessment of Positive and Negative Symptoms (SAPS and SANS). SZ patients performed more poorly on the UPSIT test than did normal controls. Consistent with previous findings, we observed a correlation of SANS with UPSIT performance. In particular, specific subdomains of SANS, such as blunted affect, apathy and anhedonia, were associated with odor identification deficits. Furthermore, UPSIT score predicts these subdomains of negative symptoms. No correlation was observed between positive symptom and odor identification deficits. Our study further reinforces a relation between olfactory identification deficit and negative symptoms in SZ and suggests that smell identification could be a candidate endophenotype relevant to negative symptoms of SZ.

Olfaction: a potential cognitive marker of psychiatric disorders

Cognitive deficits are well documented in psychiatric disorders, particularly in schizophrenia and depression. Cognitive activity roots in perceptions. However, research on sensorial alterations in psychiatric conditions has mainly focused on visual or auditory processes and less on olfaction. Here, we examine data on olfactory deficits in psychiatric patients using a systematic review of recent publications. Schizophrenic patients are mainly characterized by no reliable change in odour sensitivity and by a deficit in odour identification, recognition and discrimination. Depressed patients principally exhibit a deficit in the hedonic aspects of this perception, even if, in some case, alterations in sensitivity or identification are also found. Changes in odour perception are also found in dementia and in some neurodegenerative disease, but in this case alterations concern all aspects of the sensorial experience (detection threshold, identification and recognition). Taken together, these data indicate that olfactory abnormalities might be a marker of psychiatric conditions, with a specific pattern for each disease.

Increased processing speed for emotionally negative odors in schizophrenia

Although olfactory identification deficits in schizophrenia have been repeatedly demonstrated, some studies indicate that schizophrenia patients may also show an olfactory hypersensitivity. In addition, recent evidence points to impaired odor hedonics in schizophrenia. It was the aim of the present study to investigate olfactory information processing in schizophrenia patients with positive symptoms, by means of chemosensory event-related potential (CSERP) analysis. To control for modality-specific effects, these data were compared to visual event-related potentials (VERP), and in order to control for disorder-specific effects, the schizophrenia patients were compared to healthy controls as well as to patients with major depressive disorder (MDD). In each group, nine subjects were investigated. During EEG recording (32 scalp locations) two odors (positive valence: phenyl-ethylalcohol, negative valence: isobutyraldehyde) were presented using the constant flow method. Afterwards, two colors (red, yellow) were presented. The schizophrenia patients responded to both odors with shorter CSERP latencies than healthy controls and MDD-patients. This effect was most pronounced for the early processing (N1 component) of the emotionally negative odor. However, in response to colors, schizophrenia patients showed increased N1 latencies and a reduced P3 amplitude. It will be discussed whether schizophrenia patients with positive symptoms show a general processing advantage for emotionally negative (threat-related) information.

Olfactory deficits, cognition and negative symptoms in early onset psychosis

BACKGROUND

Smell identification deficits (SID) are common in adult schizophrenia, where they are associated with negative symptoms and lower intelligence. However, smell identification has not been examined in adolescents with early onset psychosis, wherein diagnosis is often obscure, and there are few prognostic predictors.

METHOD

We examined smell identification, diagnosis, neuropsychological performance and symptoms in 26 well characterized adolescents with early onset psychosis, age 11-17 years.

RESULTS

SID existed in the sample and were more common in patients with schizophrenia and psychotic depression than in patients with psychosis NOS and bipolar disorder. As in adults, SID were significantly associated with greater negative symptoms and lower verbal IQ. However, the associations of verbal IQ (and other verbal tasks) to smell identification in this pediatric sample were explained by the relation of both of these types of variables to negative symptoms.

CONCLUSIONS

SID existed across this sample of youths with psychotic disorder, and were specifically related to typical characteristics of schizophrenia, such as negative symptoms and lower intelligence, but not to features of bipolar disorder, such as grandiosity. SID is a characteristic of early onset psychosis that may be useful for prognostic purposes.

Trail making and olfaction in schizophrenia: implications for processing speed

BACKGROUND

Previous research has established a relationship between smell identification deficits (SID) and particular aspects of cognitive function among patients with schizophrenia.

OBJECTIVE

To expand the extant literature, we examined the relationship between SID and the Trail Making Test to determine if processing speed is related to SID.

METHODS

Our sample included 60 inpatients from the New York State Psychiatric Institute's Schizophrenia Research Unit. We considered age, deficit syndrome, verbal intelligence quotient, and education in our analyses due to their documented relationship to smell identification ability.

RESULTS

Trails A errors and Trails A seconds accounted for a significant amount of the variance in University of Pennsylvania Smell Identification Test scores in a regression analysis (R2=.10, P=.008 and R2=.05, P=.04).

CONCLUSION

Linking neurocognition to smell identification deficits may prove to be an essential marker for schizophrenia research.

A brief smell identification test discriminates between deficit and non-deficit schizophrenia

Evidence is accumulating that smell identification deficits (SID) and social dysfunction in schizophrenia may share a common pathophysiology. While most schizophrenia studies utilize the lengthy 40-item University of Pennsylvania Smell Identification Test (UPSIT) to assess smell identification ability, a brief 12-item smell identification test (B-SIT) has recently been constructed as a culturally neutral substitute for the UPSIT. By selecting the 12 items of the UPSIT from which the B-SIT was originally derived, we constructed a proxy for the B-SIT and compared the performance of 83 patients with schizophrenia to 69 normal subjects. We examined select properties of the B-SIT proxy in relation to the UPSIT to determine its efficacy for use in psychiatric populations. We considered the sensitivity of the B-SIT proxy and evaluated a cutoff score for identifying deficit syndrome schizophrenia (DS). The UPSIT and B-SIT proxy were significantly related in the patients (n=83, r=0.85, P=0.01) and in comparison subjects (n=69, r=0.83, P=0.01), and both measures similarly distinguished DS from non-deficit syndrome (non-DS) patients. The results of this study support the utility of the B-SIT for schizophrenia research and highlight the robustness of the relationship between SID and social dysfunction in schizophrenia.