Neurocognitive deficits are a core feature of both schizophrenia and bipolar disorder. People with either disorder may perform poorly on cognitive tasks assessing intelligence, memory, executive functioning, language, information processing and attention. Establishing differences in these cognitive domains may assist correct diagnosis and treatment of the two disorders.

What is the evidence for cognition in bipolar disorder compared to schizophrenia?

Moderate to high quality evidence found large effects of better overall cognition, attention, and social cognition, and medium-sized effects of better speed of processing, working memory, learning, reasoning, and problem solving in people with bipolar disorder. A small effect was found of better overall cognition in people with bipolar disorder compared to people with schizoaffective disorder (particularly depressive type), which remained across different cognitive domains, bipolar disorder type (I or I and II mixed), age, sex, duration of illness, antipsychotic use or no use, and symptom severity.

Moderate to high quality evidence found a medium-sized effect of higher premorbid IQ, and moderate to low quality evidence found a medium-sized effect of higher current IQ in people with first-episode bipolar disorder compared to people with first-episode schizophrenia. There were also medium-sized effects of better verbal memory and verbal fluency, and small effects of better working memory and processing speed in people with first-episode bipolar disorder.

Compared to controls without a mental illness, moderate to high quality evidence found a small effect of poorer pre-illness-onset cognitive functioning, and a medium-sized effect of poorer post-illness-onset cognitive functioning in people with bipolar disorder. In people with schizophrenia compared to controls, there was a medium-sized effect of poorer pre-illness-onset cognitive functioning and a large effect of poorer post-illness-onset cognitive functioning. Moderate quality evidence found similar, medium to large effects of poor semantic inhibition in people with bipolar disorder and in people with schizophrenia when compared to controls.

A medium-sized effect was found of better social cognition in people with bipolar disorder on Theory of Mind and negative facial emotion recognition tasks, particularly in male patients, but no differences between bipolar disorder and schizophrenia on positive (happy) facial emotion recognition tasks.

September 2021

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Diagnostic scales are widely used within clinical practice and research settings. These scales have been extensively validated and provide a set of criteria that is used to define and diagnose an illness. Two key examples include the American Psychiatric Association’s Diagnostic and Statistical Manual (DSM) and the World Health Organization’s International Classification of Diseases (ICD). Both the DSM and ICD criteria are regularly updated, and the most recent versions are the DSM-5 and the ICD-11.

For a DSM-5 diagnosis of schizophrenia, at least two symptoms need to have been present for at least six months, and for a significant portion of time over a one-month period. Symptoms include delusions, hallucinations, disorganised speech and behaviour, and negative symptoms such as diminished emotional expression, poverty of speech, and lack of purposeful action. At least one symptom of delusions, hallucinations, or disorganised speech needs to be present, and there also needs to be significant social or occupational dysfunction.

For an ICD-11 diagnosis of schizophrenia, at least two symptoms must be present, including positive, negative, depressive, manic, psychomotor, and cognitive symptoms. Of the two symptoms, one core symptom needs to be present, such as delusions, thought insertion, thought withdrawal, hallucinations, or thought disorder. Symptoms should have been present for most of the time during a period of at least one month.

What is the evidence on schizophrenia diagnosis?

Moderate to high quality evidence finds the DSM-III, DSM-III-R, and DSM-IV diagnostic criteria assigns more males with psychosis to schizophrenia than any other psychosis. Males are also found to have more negative symptoms. The ICD-9 shows no differences in gender distribution.

Moderate quality evidence finds Black people in the United States are more likely to be diagnosed with schizophrenia than White people in the United States. This is regardless of diagnostic method (structured vs. unstructured), or DSM version (DSM-III or DSM-IV). This effect was largest in studies with more males, more White patients, more young patients, studies in hospital or military settings, and studies conducted in the Midwest, Southeast, National, or multistate USA.

Moderate to high quality evidence suggests the proportion of first-episode psychosis patients retaining a diagnosis of schizophrenia over time is around 90%, and 72% for schizoaffective disorder. Also, the rate of a schizophrenia diagnosis following a diagnosis of schizophreniform disorder is around 65% over four years. Following brief, atypical, or not otherwise specified psychoses, the rate of a schizophrenia diagnosis is around 36% over four years. Following a substance-induced psychosis, the rate of a schizophrenia diagnosis is around 25% over four years. The rates of a transition to schizophrenia were highest for cannabis-induced psychosis, hallucinogen-induced psychosis, and amphetamine-induced psychosis.

Moderate to high quality evidence suggests better reliability for a diagnosis of schizophrenia than for a diagnosis of schizoaffective disorder. There was evidence to support vector machines combined with other machine learning techniques applied to structural and functional neuroimaging data (particularly prefrontal and temporal) for assisting the clinical diagnosis of schizophrenia.

February 2022

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