Olfactory perception in schizophrenia: the rating range for hedonic judgements is increased during acute episodes

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 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.

Impaired olfactory identification and hedonic judgment in schizophrenia patients with prominent negative symptoms

Introduction: Evidence suggests that schizophrenia patients have olfactory dysfunctions, but the relationship between olfactory identification, hedonic judgement, and negative symptomatology remains unclear. Few studies have investigated whether co-activation of pleasant and unpleasant emotions are more prevalent in schizophrenia patients.Methods: Thirty schizophrenia outpatients with prominent negative symptoms (PNS), 30 outpatients without PNS, and 30 controls completed the University of Pennsylvania Smell Identification Test, and were asked to identify the odourants and to rate their emotions. The effects of gender and medications on olfactory function were examined.Results: Schizophrenia patients exhibited olfactory identification impairments, even after accounting for gender and medication effects. Patients with PNS demonstrated larger magnitude of deficit than those without. Patients with PNS reported less pleasure to positive-valenced odourants, and less unpleasantness to negative-valenced odourants than controls. Olfactory anhedonia in patients with PNS disappeared after controlling for medication effect. Schizophrenia patients do not exhibit affective ambivalence in olfaction.Conclusions: Schizophrenia patients with PNS exhibit deficits in olfactory identification and hedonic judgement, even after controlling for gender and medication effects. Our findings support the close relationship between olfactory dysfunctions and negative symptoms. Further studies should investigate the effect of dopamine-blocking agents on the olfactory hedonic judgment in schizophrenia patients.

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.

Deletion of collapsin response mediator protein 4 results in abnormal layer thickness and elongation of mitral cell apical dendrites in the neonatal olfactory bulb

Collapsin response mediator protein 4 (CRMP4), a member of the CRMP family, is involved in the pathogenesis of neurodevelopmental disorders such as schizophrenia and autism. Here, we first compared layer thickness of the olfactory bulb between wild-type (WT) and CRMP4-knockout (KO) mice. The mitral cell layer (MCL) was significantly thinner, whereas the external plexiform layer (EPL) was significantly thicker in CRMP4-KO mice at postnatal day 0 (PD0) compared with WTs. However, differences in layer thickness disappeared by PD14. No apoptotic cells were found in the MCL, and the number of mitral cells (MCs) identified with a specific marker (i.e. Tbx21 antibody) did not change in CRMP4-KO neonates. However, DiI-tracing showed that the length of mitral cell apical dendrites was greater in CRMP4-KO neonates than in WTs. In addition, expression of CRMP4 mRNA in WT mice was most abundant in the MCL at PD0 and decreased afterward. These results suggest that CRMP4 contributes to dendritic elongation. Our in vitro studies showed that deletion or knockdown of CRMP4 resulted in enhanced growth of MAP2-positive neurites, whereas overexpression of CRMP4 reduced their growth, suggesting a new role for CRMP4 as a suppressor of dendritic elongation. Overall, our data suggest that disruption of CRMP4 produces a temporary alteration in EPL thickness, which is constituted mainly of mitral cell apical dendrites, through the enhanced growth of these dendrites.

Olfactory function in psychotic disorders: Insights from neuroimaging studies

Olfactory deficits on measures of identification, familiarity, and memory are consistently noted in patients with psychotic disorders relative to age-matched controls. Olfactory intensity ratings, however, appear to remain intact while the data on hedonics and detection threshold are inconsistent. Despite the behavioral abnormalities noted, no specific regional brain hypoactivity has been identified in psychosis patients, for any of the olfactory domains. However, an intriguing finding emerged from this review in that the amygdala and pirifom cortices were not noted to be abnormal in hedonic processing (nor was the amygdala identified abnormal in any study) in psychotic disorders. This finding is in contrast to the literature in healthy individuals, in that this brain region is strongly implicated in olfactory processing (particularly for unpleasant odorants). Secondary olfactory cortex (orbitofrontal cortices, thalamus, and insula) was abnormally activated in the studies examined, particularly for hedonic processing. Further research, using consistent methodology, is required for better understanding the neurobiology of olfactory deficits. The authors suggest taking age and sex differences into consideration and further contrasting olfactory subgroups (impaired vs intact) to better our understanding of the heterogeneity of psychotic disorders.

Neurology issues in schizophrenia

Schizophrenia ranks among the leading causes of disability worldwide. The presence of neurological signs co-occurring with the psychiatric symptoms is indicative of an organic brain pathology. In the present article, we review the current literature on neurology issues in schizophrenia. Firstly, common neurological signs found in patients with schizophrenia (neurological soft signs and smell abnormalities) and their association with imaging findings are reviewed. Secondly, the significant association of schizophrenia with epilepsy and stroke is described as well as the absent association with other organic brain diseases such as multiple sclerosis. Thirdly, we discuss the potential role of NMDA receptor antibodies in schizophrenia. Fourthly, neurological side effects of antipsychotic drugs and their treatment are reviewed; and lastly, we discuss neurocognitive deficits in patients with schizophrenia and their treatment. The focus of the review remains on articles with relevance to the clinician.