Olfactory function in psychotic disorders: Insights from neuroimaging studies

Olfactory dysfunction in schizophrenia and other psychotic disorders: A comprehensive and updated meta-analysis

Olfaction plays a key role in our daily life, influencing food enjoyment, threat detection, mood and social relationships. Numerous studies have provided evidence of abnormal olfactory function in schizophrenia and other psychotic disorders. This pre-registered meta-analysis was conducted to (a) provide an updated overview of olfactory function in schizophrenia-spectrum disorders, and (b) examine the modulatory effects of demographic and clinical variables on distinct olfactory abilities. We complied with the PRISMA guidelines, searching throughout PubMed, MEDLINE, and PsycInfo, until the 12th of August 2023. A total of 73 publications were included, comprising data from 3282 patients and 3321 healthy controls. Results revealed that (a) patients performed significantly worse in higher-order olfactory tests (identification and discrimination) compared to healthy controls, while no differences were observed in odor sensitivity; (b) patients' performance in odor identification was moderated by education, as well as disease duration and negative symptoms. Our findings support the presence of olfactory impairments in schizophrenia-spectrum disorders, leading to significantly poorer performance in both odor identification and discrimination, but not sensitivity, when compared to healthy controls.

Inflammation-related pathology in the olfactory epithelium: its impact on the olfactory system in psychotic disorders

Smell deficits and neurobiological changes in the olfactory bulb (OB) and olfactory epithelium (OE) have been observed in schizophrenia and related disorders. The OE is the most peripheral olfactory system located outside the cranium, and is connected with the brain via direct neuronal projections to the OB. Nevertheless, it is unknown whether and how a disturbance of the OE affects the OB in schizophrenia and related disorders. Addressing this gap would be the first step in studying the impact of OE pathology in the disease pathophysiology in the brain. In this cross-species study, we observed that chronic, local OE inflammation with a set of upregulated genes in an inducible olfactory inflammation (IOI) mouse model led to a volume reduction, layer structure changes, and alterations of neuron functionality in the OB. Furthermore, IOI model also displayed behavioral deficits relevant to negative symptoms (avolition) in parallel to smell deficits. In first episode psychosis (FEP) patients, we observed a significant alteration in immune/inflammation-related molecular signatures in olfactory neuronal cells (ONCs) enriched from biopsied OE and a significant reduction in the OB volume, compared with those of healthy controls (HC). The increased expression of immune/inflammation-related molecules in ONCs was significantly correlated to the OB volume reduction in FEP patients, but no correlation was found in HCs. Moreover, the increased expression of human orthologues of the IOI genes in ONCs was significantly correlated with the OB volume reduction in FEP, but not in HCs. Together, our study implies a potential mechanism of the OE-OB pathology in patients with psychotic disorders (schizophrenia and related disorders). We hope that this mechanism may have a cross-disease implication, including COVID-19-elicited mental conditions that include smell deficits.

Unraveling the Link between Olfactory Deficits and Neuropsychiatric Disorders

Smell loss has caught public attention during the recent COVID-19 pandemic. Research on olfactory function in health and disease gains new momentum. Smell deficits have long been recognized as an early clinical sign associated with neuropsychiatric disorders. Here we review research on the associations between olfactory deficits and neuropathological conditions, focusing on recent progress in four areas: (1) human clinical studies of the correlations between smell deficits and neuropsychiatric disorders; (2) development of olfactory mucosa-derived tissue and cell models for studying the molecular pathologic mechanisms; (3) recent findings in brain imaging studies of structural and functional connectivity changes in olfactory pathways in neuropsychiatric disorders; and (4) application of preclinical animal models to validate and extend the findings from human subjects. Together, these studies have provided strong evidence of the link between the olfactory system and neuropsychiatric disorders, highlighting the relevance of deepening our understanding of the role of the olfactory system in pathophysiological processes. Following the lead of studies reviewed here, future research in this field may open the door to the early detection of neuropsychiatric disorders, personalized treatment approaches, and potential therapeutic interventions through nasal administration techniques, such as nasal brush or nasal spray.

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.

Primary and secondary olfactory centres in human ontogeny

The olfactory centres are the evolutionary oldest and most conservative area of the telencephalon. Olfactory deficiencies are involved in a large spectrum of neurologic disorders and neurodegenerative diseases. The growing interest in human olfaction has been also been driven by COVID-19-induced transitional anosmia. Nevertheless, recent data on the human olfactory centres concerning normal histology and morphogenesis are rare. Published data in the field are mainly restricted to classic studies with non-uniform nomenclature and varied definitions of certain olfactory areas. While the olfactory system in model animals (rats, mice, and more rarely non-human primates) has been extensively investigated, the developmental timetable of olfactory centres in both human prenatal and postnatal ontogeny are poorly understood and unsystemised, which complicates the process of analysing human material, including medical researches. The main purpose of this review is to provide and discuss relevant morphological data on the normal ontogeny of the human olfactory centres, with a focus on the timetable of maturation and developmental cytoarchitecture, and with special reference to the definitions and terminology of certain olfactory areas.

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.

Olfactory function changes and the predictive performance of the Chinese Smell Identification Test in patients with mild cognitive impairment and Alzheimer's disease

Objectives

Olfactory disorder is one of the sensory features that reflects a decline in cognitive function. However, olfactory changes and the discernibility of smell testing in the aging population have yet to be fully elucidated. Therefore, this study aimed to examine the effectiveness of the Chinese Smell Identification Test (CSIT) in distinguishing individuals with cognitive decline from those with normal aging and to determine whether the patients with MCI and AD show changes in their olfactory identification abilities.

Methods

This cross-sectional study included eligible participants aged over 50 years between October 2019 and December 2021. The participants were divided into three groups: individuals with mild cognitive impairment (MCI), individuals with Alzheimer's disease (AD), and cognitively normal controls (NCs). All participants were assessed using neuropsychiatric scales, the Activity of Daily Living scale, and the 16-odor cognitive state test (CSIT) test. The test scores and the severity of olfactory impairment were also recorded for each participant.

Results

In total, 366 eligible participants were recruited, including 188 participants with MCI, 42 patients with AD, and 136 NCs. Patients with MCI achieved a mean CSIT score of 13.06 ± 2.05, while patients with AD achieved a mean score of 11.38 ± 3.25. These scores were significantly lower than those of the NC group (14.6 ± 1.57; P < 0.001). An analysis showed that 19.9% of NCs exhibited mild olfactory impairment, while 52.7% of patients with MCI and 69% of patients with AD exhibited mild to severe olfactory impairment. The CSIT score was positively correlated with the MoCA and MMSE scores. The CIST score and the severity of olfactory impairment were identified as robust indicators for MCI and AD, even after adjusting for age, gender, and level of education. Age and educational level were identified as two important confounding factors that influence cognitive function. However, no significant interactive effects were observed between these confounders and CIST scores in determining the risk of MCI. The area under the ROC curve (AUC) generated from the ROC analysis was 0.738 and 0.813 in distinguishing patients with MCI and patients with AD from NCs based on the CIST scores, respectively. The optimal cutoff for distinguishing MCI from NCs was 13, and for distinguishing AD from NCs was 11. The AUC for distinguishing AD from MCI was 0.62.

Conclusions

The olfactory identification function is frequently affected in patients with MCI and patients with AD. CSIT is a beneficial tool for the early screening of cognitive impairment among elderly patients with cognitive or memory issues.

Olfactory impairment in psychiatric disorders: Does nasal inflammation impact disease psychophysiology?

Olfactory impairments contribute to the psychopathology of mental illnesses such as schizophrenia and depression. Recent neuroscience research has shed light on the previously underappreciated olfactory neural circuits involved in regulation of higher brain functions. Although environmental factors such as air pollutants and respiratory viral infections are known to contribute to the risk for psychiatric disorders, the role of nasal inflammation in neurobehavioral outcomes and disease pathophysiology remains poorly understood. Here, we will first provide an overview of published findings on the impact of nasal inflammation in the olfactory system. We will then summarize clinical studies on olfactory impairments in schizophrenia and depression, followed by preclinical evidence on the neurobehavioral outcomes produced by olfactory dysfunction. Lastly, we will discuss the potential impact of nasal inflammation on brain development and function, as well as how we can address the role of nasal inflammation in the pathophysiological mechanisms underlying psychiatric disorders. Considering the current outbreak of Coronavirus Disease 2019 (COVID-19), which often causes nasal inflammation and serious adverse effects for olfactory function that might result in long-lasting neuropsychiatric sequelae, this line of research is particularly critical to understanding of the potential significance of nasal inflammation in the pathophysiology of psychiatric disorders.

The importance of smell and taste in everyday life: Dysfunction in COVID-19 patients

Human-to-human transmission of coronavirus (SARS-CoV-2) - COVID-19 (corona virus disease 2019) - is characterized by a pandemic exponential rate and the patients with mild to moderate infection have odor and taste problems that represent a new atypical disease. A new viral syndrome of acute anosmia or "new loss of taste or smell" without rhinitis and nasal obstruction or rhinorrhea has been placed on the list of symptoms that may occur 2 to 14 days after exposure to the COVID-19 virus. Two months after declaring the COVID-19 pandemic in May 2020, the World Health Organization (WHO) has recognized changes in the perception of smell and taste as symptoms of this disease. The described cardinal symptoms are more common in the population of young patients and able-bodied people which facilitates the spread of disease. Significantly higher prevalence of patients with COVID-19 who have lost their taste and smell is treated at home (rare hospitalization), lung damage is rare, as well as oxygen therapy with mild lymphopenia. Different scenarios of SARS-CoV-2 viral infection can be assumed: it is probable that the virus does not enter directly into olfactory sensory neurons (they do not have ACE2 and TMPRSS2 receptors), but it is localized to vascular pericytes and causes inflammatory processes and vasculopathies. On the other hand, direct infection of non-neuronal cells which contain said receptors is possible. Those are specific cell types in the olfactory epithelium such as sustentacular, horizontal basal cells, as well as Bowman's glands, which leads to massive degeneration and loss of olfactory neurons. The sense of taste is a complex sensation that is the result of the interaction of smell, taste, temperature and texture of food. The virus damages cranial nerves, epithelial receptors and blood vessels leading to taste damage (ageusia or dysgeusia). A multidisciplinary approach with epidemiological, clinical and basic research is needed to elucidate the mechanism of sensorineural odor and taste loss caused by coronavirus.

Association Between Anxious Symptom Severity and Olfactory Impairment in Young Adults with Generalized Anxiety Disorder: A Case-Control Study

OBJECTIVE

The findings of olfaction are inconsistent in anxiety disorders, and few studies have reported on the olfactory performance in generalized anxiety disorder (GAD). Therefore, we aim to investigate the olfactory function of patients with GAD and the association between olfactory function and anxiety symptoms.

METHODS

We conducted a case-control study by selecting 107 patients with GAD and 107 healthy controls matched for sex and age, to investigate olfaction deficits in GAD and the association between anxiety severity and olfaction. All patients were treatment-naïve prior to the inclusion. Anxiety severity and olfaction were assessed by the Hamilton anxiety rating scale (HAM-A) and the Sniffin' Sticks test (SST), respectively. Partial correlations were used to analyze the relationship between olfaction and anxiety severity. False discovery rate (FDR) correction was used in multi-correlation analyses.

RESULTS

Compared with healthy controls, patients in the GAD group demonstrated odor threshold, discrimination and identification deficits. In the moderate/severe anxiety subgroup, discrimination score was significantly and inversely correlated with the somatic anxiety score (γ=-0.44, q = 0.03), and identification score was significantly and inversely correlated with the HAM-A total score (γ=-0.42, q= 0.04). The TDI score (the sum of threshold, discrimination and identification scores) was significantly and inversely correlated with the somatic (γ=-0.44, q = 0.04) and psychic (γ=-0.45, q = 0.04) anxiety scores in the moderate/severe anxiety subgroup.

CONCLUSION

This study demonstrated the olfactory impairment in patients with GAD and that poorer odor discrimination was correlated with greater severity of somatic anxiety.

Difference of olfactory deficit in patients with acute episode of schizophrenia and major depressive episode

Aim Olfactory deficits are potential markers of early diagnosis, monitoring progress and predicting outcome in patients with schizophrenia and depression. We aimed to investigate differences in patterns and influencing factors of olfactory deficits between patients with acute episode of schizophrenia and major depressive episode (MDE).

METHODS

Fifty-two patients with acute episode of schizophrenia, 75 patients with unipolar MDE and 199 healthy controls were included in this retrospective study. Following a structured interview, participants underwent olfactory tests (Sniffin' Sticks), assessment of psychiatric symptoms (Positive and Negative Syndrome Scale), depressive symptoms (Hamilton Depression Rating Scale), and cognitive function (color-word test and word generation test).

RESULTS

Both patients with schizophrenia and MDE exhibited significant olfactory deficits, and MDE patients have poorer olfactory sensitivity than schizophrenia. Patients with MDE had a higher proportion of olfactory deficits (45.3% and 28%, respectively) but a better self-awareness (21.3% and 9.6%, respectively) than patients with schizophrenia. In patients with schizophrenia, PANSS scores was positively associated with olfactory sensitivity but negatively associated with olfactory identification, and olfactory discrimination was associated with word generation. In patients with MDE, olfactory discrimination was associated with word generation and age, but not disease severity. First-episode schizophrenia group showed significantly lower threshold scores than recurrent schizophrenia group, and first-episode MDE group had significantly lower threshold scores and higher discrimination scores than the recurrent MDE group.

CONCLUSIONS

Patterns and modulating factors of olfactory deficits in acute episode of schizophrenia and MDE are different, their differences should be considered when using olfactory deficits as marker in clinical practice.

Olfactory threshold selectively predicts positive psychometric schizotypy

Olfactory impairment might be useful as a non-invasive pre-morbid biological marker of psychosis. People with schizophrenia show consistent impairments, but an association between olfaction and schizotypy in non-clinical populations is inconclusive and has been somewhat controversial. This is important as impairment in patients may be artefacts of antipsychotic medication. Meta-analyses indicate small effect sizes in non-clinical populations, suggesting prior negative studies may have been underpowered to demonstrate them. We measured olfaction and psychometrically-defined schizotypy in a sample of 739 non-clinical volunteers [mean age 23.1]. Subsets reported whether they had a history of mental illness in the family or smoked. We used (sniffin' sticks) to measure threshold detection, discrimination and identification of odours. O-LIFE was used to measure schizotypy. Lower olfactory-threshold selectively predicted higher scores on the positive dimension, unusual experiences. This association was most evident in sub-groups reporting history of mental illness in the family and/or smoking. There was a weak trend for an association between identification and introvertive anhedonia and discrimination and cognitive disorganisation in those with a history of mental illness in the family. These data support the idea that olfaction merits further investigation as a biomarker for psychosis and that olfactory-threshold detection in particular has potential to selectively predict unusual experiences. Variability in previous studies may have been exacerbated by including different proportions of participants with history of mental illness in the family and/or smoking. We propose that non-clinical participants be stratified by these factors in future studies of olfaction and potentially any study that measures psychometric schizotypy.

Translational Significance of Selective Estrogen Receptor Modulators in Psychiatric Disorders

Accumulating data from various clinical trial studies suggests that adjuvant therapy with ovarian hormones (estrogens) could be effective in reducing cognitive deficit and psychopathological symptoms in women with psychiatric disorders. However, estrogen therapy poses serious limitations and health issues including feminization in men and increased risks of thromboembolism, hot flashes, breast hyperplasia, and endometrium hyperplasia when used for longer duration in older women (aged ≥ 60 years) or in women who have genetic predispositions. On the other hand, selective estrogen receptor modulators (SERMs), which may (or may not) carry some risks of hot flashes, thromboembolism, breast hyperplasia, and endometrial hyperplasia, are generally devoid of feminization effect. In clinical trial studies, adjuvant therapy with tamoxifen, a triphenylethylene class of SERM, has been found to reduce the frequency of manic episodes in patients with bipolar disorder, whereas addition of raloxifene, a benzothiophene class of SERM, to regular doses of antipsychotic drugs has been found to reduce cognitive deficit and psychological symptoms in men and women with schizophrenia, including women with treatment refractory psychosis. These outcomes together with potent neurocognitive, neuroprotective, and cardiometabolic properties suggest that SERMs could be the potential targets for designing effective and safer therapies for psychiatric disorders.

Developmental dynamics of prepiriform cortex in prenatal human ontogenesis

The prepiriform cortex is a part of the phylogenetically oldest pallial division (paleocortex) representing the primary olfactory cortex. While olfactory centers in laboratory animals have been extensively investigated, the developmental timetable of the human prepiriform area is poorly understood. Thus, in the present study we aim to examine the prepiriform cortex in human fetuses from eight postconceptional weeks to birth. Based on cytoarchitecture and immunohistochemistry analysis (NeuN-, SYP-, NSE-, TH-, GFAP-, MBP-) four main periods of the prepiriform cortex fetal development are suggested: the beginning of prefetal stage (the eighth week from conception), the period from the ending of prefetal stage (9-12 postconceptional weeks) to 17 weeks of gestation, 18-27 weeks of gestation and the late fetal period (29-40 gestational weeks). We found that the initial layer differentiation took place before the ninthtenth weeks from conception and by ten weeks the paleocortical plate of the prepiriform cortex was shaped. Both total cell density and NeuN-immunoreactive cell density peaked in the early fetuses and started to decrease after 17 gestational weeks, attaining intermediate values at 18-27 weeks and becoming significantly lower in the late fetuses. In contrast, the NeuN-immunoreactive cell ratio gradually increased over the whole examined period. The prepiriform cortex was defined as approaches the state at birth at 30 gestational weeks. The same developmental periods were observed with SYP- and NSE-assays. No significant distribution of TH immunoreactivity was described in the prepiriform cortex of human fetuses. The prior paleocortex development was demonstrated using glial markers: GFAPimmunoreactivity appeared in the prepiriform cortex at the middle of the early fetal period, ahead of the neocortex and insular cortex. The earlier rates of GFAP-immunoreactivity expansion in the prepiriform cortex, as compared to other pallial regions, persisted in the later fetuses. The first MBP-immunoreactive fibres within pallium were detected in the lateral olfactory tract at 30 weeks. Therefore, the prepiriform cortex approaches a level of maturation similar to that at birth already at the beginning of the late fetal period and matures prior to other pallial regions.

Neurocognitive impairment in deficit and non-deficit schizophrenia: a meta-analysis

BACKGROUND

Most studies suggested that patients with deficit schizophrenia have more severe impairment compared with patients with non-deficit schizophrenia. However, it is not clear whether deficit and non-deficit schizophrenia are associated with differential neurocognitive profiles.

METHODS

The aim of this meta-analytic review was to compare cognitive performances of deficit and non-deficit patients with each other and with healthy controls. In the current meta-analysis, differences in cognitive abilities between 897 deficit and 1636 non-deficit patients with schizophrenia were examined. Cognitive performances of 899 healthy controls were also compared with 350 patients with deficit and 592 non-deficit schizophrenia.

RESULTS

Both deficit (d = 1.04-1.53) and non-deficit (d = 0.68-1.19) schizophrenia were associated with significant deficits in all cognitive domains. Deficit patients underperformed non-deficit patients in all cognitive domains (d = 0.24-0.84) and individual tasks (d = 0.39-0.93). The relationship between deficit syndrome and impairment in olfaction, social cognition, verbal fluency, and speed-based cognitive tasks were relatively stronger.

CONCLUSIONS

Our findings suggest that there is consistent evidence for a significant relationship between deficit syndrome and more severe cognitive impairment in schizophrenia.

Prognosis of Olfactory Dysfunction according to Etiology and Timing of Treatment

Objective We evaluated the severity of olfactory impairment according to risk factors, compared responses with risk factors and treatment timing, and investigated prognosis according to treatments. Study design Case series with chart review. Setting Tertiary referral center. Subjects and Methods We retrospectively reviewed medical records of patients complaining of loss of their sense of smell between January 2006 and May 2016. In total, 491 patients were included. We evaluated olfactory function using the Connecticut Chemosensory Clinical Research Center test (threshold test) and Cross-cultural Smell Identification Test. Results Post-upper respiratory infection patients showed better results than those with other risk factors (59.6% recovered). Patients with head trauma (12.5% recovered) and congenital olfactory dysfunction (0% recovered) showed poorer results. Earlier treatment showed better olfactory recovery outcomes for post-upper respiratory infection ( P = .001), head trauma ( P = .022), and nasal/sinus surgery ( P = .009). Xerostomia ( P = .73) and idiopathy ( P = .365) showed no significant difference in terms of treatment timing. The threshold test better reflected subjective recovery than the identification test. The systemic + topical steroid group and the systemic steroid treatment group both showed better smell recovery outcomes than the group with topical treatment alone (both, P < .001). However, there was no significant difference between the systemic treatment group and the systemic + topical treatment group ( P = .978). Conclusions Our findings suggest that the duration of smell loss is important for better olfactory outcomes with most etiologies. Also, the effects of systemic steroids were better than those of topical steroids, regardless of combined treatment.

Neurocognitive, Neuroprotective, and Cardiometabolic Effects of Raloxifene: Potential for Improving Therapeutic Outcomes in Schizophrenia

Raloxifene is a selective estrogen receptor modulator that has been approved for treating osteoporosis and breast cancer in high-risk postmenopausal women. However, recent evidence suggests that raloxifene adjunct therapy improves cognition and reduces symptom severity in men and women with schizophrenia. In animal models, raloxifene increases forebrain neurogenesis and enhances working memory and synaptic plasticity. It may consequently repair the neuronal and synaptic connectivity that is disrupted in schizophrenia. It also reduces oxidative stress and neuroinflammation, which are potent etiological factors in the neuropathology of schizophrenia. Furthermore, in postmenopausal women, raloxifene reduces the risks for atherosclerosis, diabetes mellitus, and weight gain, which are serious adverse effects associated with long-term antipsychotic treatment in schizophrenia; therefore, it may improve the safety and efficacy of antipsychotic drugs. In this review, recent insights into the neurocognitive, neuroprotective, and cardiometabolic effects of raloxifene in relation to therapeutic outcomes in schizophrenia are discussed.