Olfactory processes may be dysfunctional in people with bipolar disorder. These processes are hierarchical and involve lower-order processing (detection of the stimulus) and higher-order processing (discrimination and identification of the stimulus). Measures of olfactory acuity include odor detection, identification, discrimination, intensity, and quality. Odour detection occurs at the lowest chemical concentration needed to register an odourant. Odour identification draws on a person’s knowledge and memory to correctly label the smell. Odour discrimination involves comparing the differences between multiple stimuli, judging odours as pleasant or unpleasant, or comparing the relative concentration of odours.

What is the evidence for olfactory dysfunction in people with bipolar disorder?

Moderate to low quality evidence suggests poorer olfactory identification in people with bipolar disorder than in people without a mental illness, and better olfactory identification compared to people with psychosis. Olfactory hallucinations were particularly associated with depressive episodes in people with bipolar disorder.

September 2021

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The olfactory system is the sensory system used to interpret and perceive smell, which may be dysfunctional in people with schizophrenia. Olfactory functioning is hierarchical and involves lower-order processing (detection of the stimulus) and higher-order processing (discrimination and identification of the stimulus). Odour detection occurs at the lowest chemical concentration needed to register an odourant. Odour discrimination involves comparing the differences between multiple stimuli, judging odours as pleasant or unpleasant, or comparing the relative concentration of odours. Odour identification draws on a person’s knowledge and memory to correctly label the smell.

What is the evidence for altered olfactory functioning in people with schizophrenia?

Moderate to high quality evidence suggests a medium to large effect of impaired odour detection, identification, and discrimination in people with schizophrenia compared to people without schizophrenia. A longer duration of illness, taking first generation rather than second generation antipsychotics, and older age were all associated with more impairment in patients. Being male or a smoker were related to less impairment in patients.

Moderate to high quality evidence suggests impaired olfactory identification, but not acuity, in people at high clinical or high familial risk of schizophrenia. Moderate quality evidence shows no differences in olfactory functioning between people at clinical high-risk of psychosis who made the transition to psychosis compared to people at clinical high-risk of psychosis who did not make the transition to psychosis.

March 2022

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